MAGNETIC SUPERGRIDS Ontario Airborne ... - Geology Ontario

MAGNETIC SUPERGRIDS 

Ontario Airborne Geophysical Surveys 

Magnetic Data 

Geophysical Data Set 1037 

Ontario Geological Survey 

Ministry of Northern Development and Mines 

Willet Green Miller Centre 

933 Ramsey Lake Road 

Sudbury, Ontario, P3E 6B5 

Canada 

Report on Magnetic Supergrids 

Geophysical Data Set 1037

TABLE OF CONTENTS 

CREDITS .....................................................................................................................................................................3 

DISCLAIMER .............................................................................................................................................................3 

CITATION...................................................................................................................................................................3 

1) INTRODUCTION..............................................................................................................................................4 

2) PREPARATION OF MAGNETIC SUPERGRIDS........................................................................................9 

3) SPECIFIC DETAILS: MAGNETIC SUPERGRIDS ...................................................................................14 

APPENDIX A 

ARCHIVE DEFINITION........................................................................................................19 

APPENDIX B MERGING OF GRIDS BY GRIDKNIT TM METHOD ........................................................23 


Geophysical Data Set 1037Rev. 

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CREDITS 

The following is a list of the organizations that have participated in this project, and their various 

responsibilities: 

• The overall project management, scientific authority and quality control was provided by 

Desmond Rainsford, Ontario Geological Survey, Sudbury. 

• Paterson, Grant & Watson Limited, Toronto - Prime contractor 

- project management 

- magnetic data processing 

DISCLAIMER 

To enable the rapid dissemination of information, this digital data has not received a technical 

edit. Every possible effort has been made to ensure the accuracy of the information provided; 

however, the Ontario Ministry of Northern Development and Mines does not assume any 

liability or responsibility for errors that may occur. Users may wish to verify critical 

information. 

CITATION 

Information from this publication may be quoted if credit is given. It is recommended that 

reference be made in the following form: 

Ontario Geological Survey 2003. Magnetic Supergrids, Ontario Airborne Geophysical Surveys; 

magnetic data, grid data; Ontario Geological Survey, Geophysical Data Set 1037. 



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1) INTRODUCTION 

Aeromagnetic Surveys Acquired Before 1999 

High resolution airborne magnetic and electromagnetic surveys, over major greenstone belts, 

were initiated in 1975 by the Ontario Department of Mines (currently known as the Ontario 

Geological Survey) to aid geological mapping and mineral exploration. Between the period 

1975 to 1992, thirty-two airborne surveys were flown and processed by different survey 

contractors and subcontractors. The two earlier surveys, Matachewan and Bamaji-Fry Lakes, 

were acquired in analog form and the remaining thirty surveys were recorded digitally. The 

surveys were flown at a nominal flight line spacing of 200 metres with the exception of James 

Bay Cretaceous Basin survey, which was flown at a flight line spacing of 1000 metres. The 

flight directions for the surveys, including individual survey blocks, were chosen to transect the 

predominant regional structural trends of the underlying rocks. The results of the surveys were 

published on 1:20,000 semi-controlled photo mosaic paper maps, showing total magnetic field 

contours onto which picked electromagnetic conductor anomalies were superimposed in symbol 

form. 

There are significant differences in quality of data acquisition, and original processing, of older 

and newer surveys. The surveys flown recently were designed and flown based on the state-ofthe-art 

specifications, equipment and technology, while some of the older surveys, though 

conducted to the then industry prevailing standards, were poorly processed. In many cases, on 

older surveys, the local map coordinates were registered in map inches of uncontrolled to semicontrolled 

photo mosaics. The vast amount of digital data, collected by the survey contractors, 

was archived on 9-track tapes of different sizes and densities, in numerous incompatible data 

formats and file structures, many of which were difficult to access. For this reason the archival 

digital data largely remained inaccessible to the mining industry. 

To alleviate many of these problems, and to bring the archival data set of all thirty-two airborne 

magnetic and electromagnetic (AMEM) surveys to modern data storage, digital processing and 

interpretation standards, the present recompilation and reprocessing project was initiated under 

the Northern Ontario Development Agreement (NODA). This includes approximately 450 000 

line-km of AMEM data, which was recompiled and reprocessed to correct any errors in the 

original data sets, to compute new derived products and to produce a revised electromagnetic 

anomaly database, using state-of-the-art geophysical data processing and imaging techniques. 

The major objectives of this project were to: 

1) Provide a single, well-defined, common data format for all thirty-two AMEM survey data 

sets on CD-ROM, allowing easy access to the data on a PC platform. 

2) Digitize survey data acquired in analog form, obtain missing or bad data from digital or 

paper archives, and check the validity of all data including units, conversion factors, etc. 



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3) Analyze and correct errors in the flight path to account for distortions in the 

photomosaics and provide a digital flight path referenced to Universal Transverse 

Mercator (UTM) coordinates. 

4) Link and level all thirty-two surveys total magnetic field data to the Single Master 

Aeromagnetic Grid for Ontario, which was prepared from the Geological Survey of 

Canada's magnetic database under an earlier project. 

5) Reprocess the magnetic data to provide image quality total magnetic field grids and 

profiles for each survey. 

6) Create image quality second vertical derivative grids of the total magnetic field. 

7) Compute image quality apparent resistivities from a selected single frequency of the 

frequency-domain electromagnetic data, which have been fully corrected for signal to 

noise enhancement and accurately levelled. 

8) Compute image quality decay constant and resistivity values from the corrected and 

levelled data channels of time-domain electromagnetic data (INPUT, GEOTEM I and 

GEOTEM II systems). Provide de-herringboned (i.e. correct for directional effect) decay 

constant and resistivity grids for all time-domain surveys. 

9) Re-pick electromagnetic anomalies from the surveys in a consistent manner, and store 

anomaly parameters, including a unique identifier, in a new digital anomaly database. 

10) Prepare seamless supergrids of total magnetic field and its second vertical derivative, for 

all surveys that share a common boundary or overlap. 

The resulting profile and grid data in digital form, along with the second vertical derivative of 

the total magnetic field, the apparent resistivity and decay constant values, and a comprehensive 

EM anomaly database, will become valuable tools for orebody detection, and enhanced 

lithological and structural mapping of the geology. The original profile data are also included in 

the profile database for reference. 

A complete description of the magnetic processing techniques applied for all AMEM surveys, 

and the specific details concerning each individual survey, are provided in the survey report 

included on the CD-ROM issued for each survey (e.g. Geophysical Data Set CD-ROM 1027, 

Armstrong-Caribou Lake Area). 



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Aeromagnetic Surveys Acquired After 1999 

Recognising the value of geoscience data in reducing private sector exploration risk and 

investment attraction, the Ontario Government embarked on “Operation Treasure Hunt” (OTH). 

The OTH initiative comprises a three-year, $29 million program that commenced April 1, 1999. 

It incorporates: 

• airborne geophysics (high-resolution magnetic/electromagnetic surveys, including the 

purchase of proprietary data sets) 

• surficial geochemistry (lake sediments and indicator minerals) 

• bedrock map compilation 

• methods development (e.g. electro-geochemical modelling applied to exploration and 3- 

D geological/geophysical modelling) 

• delivery of digital data products. 

The OGS was charged with the responsibility to manage OTH. The OGS sought advice about the 

mineral industry needs and priorities from its OGS Advisory Board – a stakeholder board 

including representatives from the Ontario Mining Association, Ontario Prospectors Association, 

Prospectors and Developers Association, Aggregate Producers Association of Ontario, Chairs of 

Ontario University Geology Departments, Canadian Mining Industry Research Organisation and 

Geological Survey of Canada. The OGS Advisory Board mandated a Technical Committee to 

advise the OGS on geographic areas of interest within Ontario where collection of new data 

would make the greatest impact on reducing exploration risk. Various criteria were assessed, 

including: 

• commodities and deposit types sought 

• prospectivity of the geology 

• state of the local mining industry and infrastructure 

• existing, available data 

• mineral property status. 

The geophysical component of the OTH program involved: 

• acquiring from the private sector existing proprietary airborne geophysical data that met 

Ontario Geological Survey (OGS) quality standards and objectives of Operation Treasure 

Hunt; 

• flying new airborne magnetic and electromagnetic surveys over various greenstone belts; 

• disseminating these geophysical data sets and their daughter products to clients in digital 

and hardcopy formats. 

In late 1999, MNDM commenced an ambitious program of airborne magnetic and 

electromagnetic surveys as part of OTH geoscience initiative. The project involved four survey 

contractors, five different electromagnetic systems and more than 105,000 line-km of data 

acquisition. 

The airborne survey contracts were awarded through a Request for Proposal and Contractor 

Selection process. The system and contractor selected for each survey area were judged on many 



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criteria, including the following: 

• applicability of the proposed system to the local geology and potential deposit types 

• aircraft capabilities and safety plan 

• experience with similar surveys 

• QA/QC plan 

• capacity to acquire the data and prepare final products in the allotted time 

• price-performance. 

In August 1999, Paterson, Grant & Watson Limited (PGW) was retained by MNDM to provide 

Geophysicist Project Management and quality assurance (QA)/quality control (QC) inspection 

services for the airborne geophysical survey component of OTH. One of PGW’s roles as the 

OTH Geophysicist was to seek out, and recommend for purchase by MNDM, proprietary 

airborne geophysical data that would complement the acquisition of new data being undertaken 

by OTH. PGW commenced the search process in September 1999. 

Ranking and valuation of submitted airborne geophysical survey data sets were based on the 

following criteria: 

• date of survey – recent surveys were favoured over older surveys because of improved 

acquisition technology, greater data density and improved final products. 

• survey method – magnetometer surveys, without supplementary radiometrics or VLF, 

were given the lowest rating in this category; AEM and magnetometer were given the 

highest; the objective was to acquire data that complements what is already available in 

the public domain, with emphasis on exploration rather than mapping. 

• location of area 

• highest value was accorded to data sets lying within areas identified by the Ontario 

Geological Survey Advisory Board as being of special commodity interest and 

worthy of airborne geophysical coverage (Figure 1). 

• data sets occurring within areas already surveyed or scheduled for survey under 

Operation Treasure Hunt were only selected if they added significantly to the 

acquired data sets, 

• proximity or coincidence of the survey block with areas having restricted land use 

designations affected the value assigned to that survey, 

• consideration was given to data sets that were collected in remote areas where 

logistical costs are very high. 

• line spacing - not normally a significant factor in the valuation of an airborne 

geophysical survey; however, in the case of OTH, points were assigned according to 

how well the line spacing met the desired exploration requirements; detailed surveys 

were normally accorded a higher rating than reconnaissance surveys. 

• quality of data - data quality, processed products, and adherence to correct survey 

specifications had to be up to normal industry standards. 

• survey size - data sets comprising less than 1000 line-km were selected only if they 

fell in very strategic locations. 

• other criteria - factors such as apparent mineral significance, previous exploration 



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activity and land availability were also considered in making the final selection. 

Figure 1 

In February 2002, Paterson, Grant & Watson Limited (PGW) was retained by MNDM to 

microlevel and level to a common datum the aeromagnetic surveys of OTH. Adjacent surveys 

were also to be merged together with existing AMEM surveys into supergrids. PGW 

commenced this project in March 2002. 



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2) PREPARATION OF MAGNETIC SUPERGRIDS 

Six supergrids were produced in 1999 using 22 of the 32 AMEM high resolution grids flown 

during the period 1975 to 1992. Aeromagnetic data acquired since 1999 through Operation 

Treasure Hunt were microlevelled, levelled to a common datum and added to the existing 

supergrids. Where large overlaps existed between surveys, the survey with the higher quality 

data took precedence. 

The following table lists the final supergrids and the grids used in their construction: 

Table 1 

Supergrid 

Abitibi Supergrid 

Bamaji - Vickers Supergrid 

Stormy Supergrid 

Schreiber Supergrid 

Red Lake Supergrid 

Fort Hope - James Bay – 

Attawapiskat Supergrid 

Component Grids 

Abitibi (previous supergrid), Wawa (previous supergrid), 

Temagami, Matheson, Kirkland Lake, Cochrane, Oba - 

Kapuskasing, Kapuskasing - Chapleau, Tyrell, Amyot - 

Browning, Gowan - Evelyn, Cobalt, Temagami South, 

Temiskaming, Robb - Jamieson, Turnbull - Godfrey, 

Kenabeek, Latchford, Redwater 

Pickle Lake (previous supergrid), Vickers, Gitche Lake 

Dryden (previous supergrid), Stormy Lake, Docker, 

Kakagi Lake 

Schreiber, GECO Manitouwadge, Hemlo 

Red Lake (previous supergrid), Troutlake River, Pakwash 

Lake 

Fort Hope, Albany River-James Bay, Attawapiskat, 

Albany – Atikameg - Attawapiskat Rivers, Nagagami – 

Squirrel -Wakashi Rivers, Kenogami River, Kapiskau 

River East 

Creation of Supergrids from Aeromagnetic Surveys Acquired Before 1999 

Six magnetic supergrids were constructed for the AMEM project, in areas where two or more 

individual surveys share a common boundary and/or survey data overlap. This applies to 22 of 

the 32 magnetic surveys that were reprocessed. The goal was to provide, wherever possible, a 

regional picture of the total magnetic field and its second vertical derivative that encompasses a 

number of individual surveys, where geological trends can be reliably followed across survey 

boundaries. The transition between individual aeromagnetic surveys is seamless on the total 

magnetic and its second vertical derivative supergrids. The integrity of the individual grids has 

been maintained (except occasionally in the overlapping areas – see below “Preparation of the 

Magnetic Supergrids”) and therefore detailed magnetic interpretation can be undertaken, and is 



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considered to be as reliable as on the individual magnetic grids. 

Each individual grid was constructed with a grid cell size of 40 metres, and the resulting 

supergrids have the same grid cell size of 40 metres. The grids of the total magnetic field 

supplied on the Geophysical Data Set (GDS) CD-ROMs were utilised as the starting point for 

preparation of the magnetic supergrids. These are: 

I. The Abitibi Supergrid (9 individual surveys merged) GDS CD-ROM # 

1. Black River - Matheson 1001 

2. Blake River Syncline 1006 

3. Detour – Burntbush - Abitibi 1007-1008 

4. Kirkland Lake 1022 

5. Matachewan 1014 

6. North Swayze - Montcalm 1005 

7. Shining Tree 1003 

8. Swayze 1015 

9. Timmins 1004 

II. The Dryden supergrid (4 individual surveys merged) GDS CD-ROM # 

1. Dryden 1016 

2. Manitou-Stormy Lakes 1019 

3. Sioux Lookout 1023 

4. Sturgeon Savant Lake 1033 

III. The Geraldton supergrid (2 individual surveys merged) GDS CD-ROM # 

1. Armstrong - Caribou Lake 1027 

2. Geraldton - Tashota (north-west and south-east) 1031, 1032 

IV. The Pickle Lake supergrid (2 individual surveys merged) GDS CD-ROM # 

1. Bamaji - Fry Lakes 1013 

2. Pickle Lake 1012 

V. The Red Lake supergrid (3 individual surveys merged) GDS CD-ROM # 

1. Birch - Confederation Lakes 1025 

2. Red Lake 1028 

3. Uchi - Bruce Lakes 1026 

VI. The Wawa supergrid (2 individual surveys merged) GDS CD-ROM # 

1. Micipicoten 1010 

2. Wawa 1009 

If aeromagnetic measurements were perfect, there would be no need for any further adjustments 

to data or grids in order to get a seamless fit between survey boundaries. In theory, one should be 

able to measure the magnetic field at the same location above the earth, but at different times, 



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and obtain the same magnetic value after all corrections have been applied. In aeromagnetic 

survey practice, it is common that data mismatches occur where surveys overlap. The possible 

contributing factors include: 

• accuracy of survey navigation and flightpath recovery; 

• type and noise envelope of the magnetometer; 

• differing flightline directions; 

• variations in magnetic sensor height and orientation; 

• magnetometer sampling rate; 

• techniques used to measure errors and correct the data (e.g. diurnal, IGRF, levelling); 

• variations in the diurnal activity; and 

• changes in the ground conditions between surveys (e.g. cultural effects) 

All AMEM surveys were levelled to a common magnetic datum provided by the Single Master 

Aeromagnetic Grid for Ontario. One effect of this procedure is that any medium to long 

wavelength mismatch (i.e. > ~1000 metres) between surveys is removed. In fact, one QC/QA 

check employed during the AMEM project was to verify that the total magnetic field grids 

matched well along common boundaries. 

There are two typical magnetic sensor elevations for the AMEM project: 120 metres for fixed 

wing surveys, and 45 metres for helicopter surveys. All magnetic surveys have been brought to a 

common altitude (level of observation) prior to the creation of the supergrids. They were either 

downward or upward continued (data mathematically projected to a level surface above or 

below the original datum). In general, surveys flown at a higher altitude were downward 

continued to the level of the lower altitude surveys. This has the effect that every detail in the 

grid flown at a lesser height was preserved and the details on the grid flown at a greater height 

were enhanced. 

Of the 22 surveys incorporated in the supergrids, four required downward continuation and one 

required upward continuation (see Table 2: the continuation value represents the number of 

meters the grid was upward (cf. 75 metres) or downward (cf. –75 metres) continued; where no 

continuation was applied, the value is 0). The continuation process was applied using standard 

2-D Fourier-domain filtering. Downward continuation may result in amplification of any noise 

contained in a grid, as well as of geological signal. Consequently, the survey-specific optimum 

Wiener filter, originally designed for computation of the second vertical derivative (see 

individual survey reports on Geophysical Data Set CD ROM’s), was incorporated in the 

downward continuation. This resulted in the enhancement of the geological signal in preference 

to the noise (by the very design of the Wiener optimum filter; see any individual report: e.g. 

Armstrong-Caribou Lake Area Geophysical Data Set CD-ROM # 1027). 

The suture method of the Geosoft GridKnit TM routine was used to merge the grids, since it 

offers more flexibility and control in the process of merging grids than the blending routine. The 

suture method defines a line at which to join the two grids, which lies completely within the 

overlapping area (see Appendix B). The automatic option of the suture path definition was 

GridKnit TM : Trademark of Geosoft Inc. 



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usually chosen. The automatic option produces smoother paths, by using the edges from both 

grids simultaneously and finding the midpoints between them (see Appendix B – Locating a 

Suture Path). When values in the grids differ along the suture path, they must be corrected by 

adjusting the grids on either side of the path (see Appendix B – Multi-Frequency Suturing 

Algorithm). 

Table 2 

SURVEY 

GEOPHYSICAL 

DATA SET # 

SURVEY HEIGHT 

(metres) 

Armstrong - Caribou Lake 1027 45 0 

Bamaji - Fry Lakes 1013 120 0 

Black River - Matheson 1001 120 0 

Blake River Syncline 1006 45 75 

Birch - Confederation Lakes 1025 120 -75 

Detour - Burntbush - Abitibi 1007-1008 120 0 

Dryden 1016 120 -75 

Geraldton - Tashota 1031-1032 45 0 

Kirkland Lake 1022 120 0 

Manitou - Stormy Lakes 1019 45 0 

Matachewan 1014 120 0 

Micipicoten 1010 30 0 

North Swayze - Montcalm 1005 120 0 

Pickle Lake 1012 120 0 

Red Lake 1028 120 -75 

Shining Tree 1003 120 0 

Sioux Lookout 1023 45 0 

Sturgeon Savant Lake 1033 45 0 

Swayze 1015 120 0 

Timmins 1004 120 0 

Uchi Bruce Lakes 1026 45 0 

Wawa 1009 45 -15 

Thus, for the six supergrids the final continuation height was, as follows: 

Table 3 

Supergrid 


Dryden Supergrid 

Geraldton Supergrid 

Pickle Lake Supergrid 


Final Continuation Height 

120 metres 

45 metres 

45 metres 

120 metres 

45 metres 

CONTINUATION 

(metres) 



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Wawa Supergrid 

30 metres 

As all grids were already levelled to the Ontario Master Grid, no trend removal or base level shift 

was required. The only adjustments to grid values were made in the overlapping areas in order 

to achieve a seamless link, affected the short wavelength features. The adjustments may reach 

several hundred nanoTeslas in areas of high amplitude, high gradient anomalies. Wherever the 

overlapping area between two surveys was reasonable in extent (a couple of kilometres), and the 

quality of the data was at a similar level, both grids were adjusted equally (see Appendix B – 

Suture Weighting). Wherever there were large overlaps and/or different levels of data quality, 

the lower quality grid (usually the older dataset) was adjusted to the higher quality one (usually 

the most recent dataset) which remained unmodified. The adjustments were made only within 

the overlapping areas. In all other areas, the grids have not been modified at all. 

Creation of Supergrids from Aeromagnetic Surveys Acquired After 1999 

Five of the six supergrids created in 1999 were incorporated within the final supergrids as part of 

Geophysical Data Set 1037. The procedure to link the individual surveys into one contiguous 

supergrid is similar that that of the previous phase of supergrid creation. 

To summarize: 

• all grids to be included within the supergrid are reprojected to the same UTM projection. 

The chosen UTM zone is the zone for the majority of the surveys. 

• all grids are upward or downward continued to their final continuation height. The value 

of the final continuation height is chosen to be that of the most recent survey. This 

usually refers a survey flown for “Operation Treasure Hunt”. 

• all grids are regridded to a 40 metre grid cell size, however grids used to construct the 

Fort Hope - James Bay - Attawapiskat supergrid were regridded to 80 metres. 

• all supergrids were created using Geosoft’s GridKnit TM routine. The total magnetic 

field grids were merged using the suturing method, while the 2vd grids were merged 

using the blending method. 

The residual magnetic field supergrids were created from the individual residual magnetic field 

grids. The second vertical derivative supergrids were created from the individual second vertical 

derivative grids. 

Two different grid stitching methods were used for the total magnetic field and the second 

vertical derivative grids. The total magnetic field grids have been levelled to the Ontario Master 

Grid but there still may be a static shift or trend between neighbouring grids. The suture method 

is effective at removing these differences. By definition, the second vertical derivative grids 

have had a trend and static shift removed so that the mean value of the grid is approximately 

zero. Thus, further static shifts and trends would not be applied by using the blending method. 



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3) SPECIFIC DETAILS: MAGNETIC SUPERGRIDS 

Thirty-two airborne magnetic surveys, in five geographical regions, containing 40 metre by 40 

metre cell size grids of the total magnetic field and its second vertical derivative were merged to 

create five 40 metre by 40 metre cell size supergrids of the total magnetic field and five 

supergrids of the second vertical derivative of the total magnetic field. Ten airborne magnetic 

surveys, in one geographical region, containing a 80 metre by 80 metre cell size grid of the total 

magnetic field and its second vertical derivative were merged to create a 80 metre by 80 metre 

cell size supergrid of the total magnetic field and a supergrid of the second vertical derivative of 

the total magnetic field. The supergrids were merged using the suture method of the Geosoft 

GridKnit TM software. 

Table 4 Final Continuation and grid cell size of the supergrids 

Survey 

Final 

Continuation 

Height (metres) 

Final Grid Cell Size 

(metres) 

Abitibi Supergrid 70 40 UTM 17N 

Bamaji - Vickers 30 40 UTM 15N 

Supergrid 

Stormy Supergrid 75 40 UTM 15N 

Schreiber Supergrid 45 40 UTM 16N 

Red Lake Supergrid 45 40 UTM 15N 

Fort Hope - James Bay - 

Attawapiskat Supergrid 

80 80 UTM 16N 

Final Projection 

(NAD27 NTv2) 

I. The Abitibi magnetic supergrid 

The magnetic sensor heights of the OTH surveys in the Abitibi area varied from 70 to 75 metres. 

As well, many of the proprietary surveys were at 70 metres, so that a final continuation height 

was chosen to be 70 metres. 

Table 5 Abitibi Supergrid 

Survey 

Magnetic 

Sensor Height 

(metres) 

Continuation 

(metres) 

Original Grid 

Cell Size 

(metres) 

Projection 

(NAD27 

NTv2) 

Kirkland Lake 75 -5 40 UTM 17N 1 

Matheson 72 -2 40 UTM 17N 2 

Temiskaming 100 -30 25 UTM 17N 3 

Temagami 70 0 40 UTM 17N 4 

Order of 

Merging 



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Temagami 70 0 40 UTM 17N 5 

South 

Gowan - Evelyn 45 +25 20 UTM 17N 6 

Cobalt 70 0 60 UTM 17N 7 

Abitibi 120 -50 40 UTM 17N 8 

Supergrid 

Robb - Jamieson 70 0 20 UTM 17N 9 

Turnbull – 75 -5 40 UTM 17N 10 

Godfrey 

Cochrane 70 0 40 UTM 17N 11 

Tyrell 90 -20 30 UTM 17N 12 

Amyot - 122 -52 30 UTM 17N 13 

Browning 

Kapuskasing - 100 -30 40 UTM 17N 14 

Chapleau 

Wawa Supergrid 30 +40 40 UTM 16N 15 

Latchford - 100 -30 50 UTM 17N 16 

Redwater 

Kenabeek 100 -30 40 UTM 17N 17 

Oba - 

Kapuskasing 

45 +25 40 UTM 17N 18 

The OTH surveys Kirkland Lake and Matheson and the OTH proprietary surveys Gowan – 

Evelyn, Turnbull – Godfrey, and Robb - Jamieson almost entirely overlaps the older Abitibi 

supergrid. Since the quality and resolution of the newer magnetic data is superior to that of the 

older Abitibi supergrid data, only the Kirkland Lake, Matheson, Gowan – Evelyn, Turnbull – 

Godfrey, and Robb - Jamieson data were used in the overlapping area. 

The Latchford-Redwater survey was originally one contiguous survey, but it overlapped with the 

Temagami survey. Thus the survey was windowed into two separate surveys with approximately 

1 kilometre of overlap with the Temagami survey. 

II. 

The Bamaji – Vickers magnetic supergrid 

The magnetic sensor height of the OTH survey Vickers was flown at 30 metres. Since the 

downward continuation of the Bamaji supergrid and Gitche Lake grids did not increase its noise 

significantly, the final continuation height was chosen to be at 30 metres. 



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Table 6 Bamaji - Vickers Supergrid 

Survey 

Magnetic 

Sensor Height 

(metres) 

Continuation 

(metres) 

Original Grid 

Cell Size 

(metres) 

Projection 

(NAD27 

NTv2) 

Bamaji 120 -90 40 UTM 15N 1 

Supergrid 

Vickers 30 0 40 UTM 15N 2 

Gitche Lake 45 -15 20 UTM 15N 3 

Order of 

Merging 

III. 

The Stormy magnetic supergrid 

The magnetic sensor height of the OTH survey Stormy Lake was flown at 75 metres. Since the 

upward continuation of the Dryden supergrid, Docker and Kakagi Lake grids does not increase 

the noise, the final continuation height was chosen to be at 75 metres. 

Table 7 Stormy Supergrid 

Survey 

Magnetic 

Sensor Height 

(metres) 

Continuation 

(metres) 

Original Grid 

Cell Size 

(metres) 

Projection 

(NAD27 

NTv2) 

Stormy Lake 75 0 40 UTM 15N 1 

Supergrid 

Dryden 45 +30 40 UTM 15N 2 

Supergrid 

Docker 45 +30 25 UTM 15N 3 

Kakagi Lake 73 +2 40 UTM 15N 4 

Order of 

Merging 

The OTH survey Stormy Lake overlaps the Dryden supergrid. As the quality and resolution of 

the Stormy Lake magnetic data is superior to that of the Dryden supergrid data, only the Stormy 

Lake data were used in the overlapping area. 

IV. 

The Schreiber magnetic supergrid 

The magnetic sensor height of the OTH grid Schreiber was not chosen as the final continuation 

height. The three surveys that make up the Schreiber supergrid (Schreiber, Manitouwadge and 

Hemlo) were all flown at low terrain clearances. If the Schreiber height of 30 metres was chosen 

as the final continuation height, the Hemlo survey needed to be downward continued by 25 

metres. Such a large downward continuation of the Hemlo data increased the amount of noise to 

an unacceptable degree. A compromise height was chosen so that intermediate height of 45 

metres of the Manitouwadge survey was used as the final continuation height. 



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Table 8 Schreiber Supergrid 

Survey 

Magnetic 

Sensor Height 

(metres) 

Continuation 

(metres) 

Original Grid 

Cell Size 

(metres) 

Projection 

(NAD27 

NTv2) 

Schreiber 30 +15 40 UTM 16N 1 

GECO 

45 0 30 UTM 16N 2 

Manitouwadge 

Hemlo 55 -10 20 UTM 16N 3 

Order of 

Merging 

The OTH survey Schreiber overlaps the GECO Manitouwadge grid. As the quality and 

resolution of the Schreiber magnetic data is superior to that of the GECO Manitouwadge data, 

only the Schreiber data were used in the overlapping area. 

V. The Red Lake magnetic supergrid 

There were no OTH surveys flown in the Red Lake supergrid area. Thus the final continuation 

height was chosen to be the same as the existing Red Lake supergrid, i.e., a final continuation 

height of 45 metres. The downward continuation of the Troutlake River and Pakwash Lake 

surveys did not increase its noise significantly. 

Table 9 Red Lake Supergrid 

Survey 

Magnetic 

Sensor Height 

(metres) 

Continuation 

(metres) 

Original Grid 

Cell Size 

(metres) 

Projection 

(NAD27 

NTv2) 

Red Lake 45 0 40 UTM 15N 1 

Supergrid 

Troutlake River 73 -28 40 UTM 15N 2 

Pakwash Lake 65 -20 50 UTM 15N 3 

Order of 

Merging 

The OTH proprietary surveys Troutlake River, and Pakwash Lake overlap the Red Lake 

supergrid. As the quality and resolution of the Troutlake River, and Pakwash Lake magnetic 

data is much superior to those of the Red Lake supergrid data, only the Troutlake River, and 

Pakwash Lake data were used in the overlapping area. 

VI. 

The Fort Hope - James Bay – Attawapiskat magnetic supergrid 

Fort Hope, Albany River – James Bay and Attawapiskat are the three large grids that comprise 

most of the Fort Hope - James Bay – Attawapiskat supergrid. The James Bay and Attawapiskat 

surveys were flown with a sensor height of 80 metres and this height was chosen as the final 

continuation height. Only one grid, the Kapiskau River East survey, had to be downward 

continued, but did not increase its noise significantly. 



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Table 10 Fort Hope - James Bay - Attawapiskat Supergrid 

Survey 

Magnetic 

Sensor Height 

(metres) 

Continuation 

(metres) 

Original Grid 

Cell Size 

(metres) 

Projection 

(NAD27 

NTv2) 

Attawapiskat 80 0 40 and 80 UTM 16N 1 

Albany River - 80 0 80 UTM 16N 2 

James Bay 

Kenogami River 65 +15 50 UTM 16N 3 

Nagagami – 

Squirrel - 

Wakashi Rivers 

56 +24 40 UTM 16N 4 

Albany – 

Atikameg - 

Attawapiskat 

Rivers 

80 0 40 and 80 UTM 16N 5 

Kapiskau River 100 -20 50 UTM 17N 6 

East 

Fort Hope 75 +5 40 UTM 16N 7 

The Nagagami – Squirrel - Wakashi Rivers survey consisted of three separate aeromagnetic 

surveys. The Albany – Atikameg - Attawapiskat Rivers survey consisted of two separate 

aeromagnetic surveys. 

Order of 

Merging 

Grids that had a grid cell size less than 80 metres were regridded to 80 metres in the GridKnit TM 

process. 



18

APPENDIX A 

ARCHIVE DEFINITION 

Data File Layout 

The files are archived on two Geophysical Data Set CD’s 1037a and 1037b which contain ASCII 

and Geosoft ® binary formatted data respectively. The content of the ASCII and binary products 

is identical. They are provided in both forms to suit the user’s available software. 

Coordinate Systems 

The gridded data are provided in the two UTM coordinate systems: 


• Universal Transverse Mercator (UTM) projection, Zone 17N, NAD27 datum, Canada 

NTv2 (20 min) local datum 

• Universal Transverse Mercator (UTM) projection, Zone 17N, NAD83 datum, North 

American local datum 

Bamaji - Vickers Supergrid 





Stormy Supergrid 





Schreiber Supergrid 












19

Fort Hope - James Bay - Attawapiskat Supergrid 





The original profile, electromagnetic and gridded data were compiled in UTM NAD27 

projection. No documentation could be found that specified a local datum. A local datum of 

Canada NTv2 (20 min) was assumed and used to create the reprojected data in UTM NAD83, 

North American local datum. 



20

Gridded Data 

The gridded data are provided in two formats, one ASCII and one binary: 

• *.gxf - ASCII Grid eXchange Format (revision 3.0) 

• *.grd - Geosoft OASIS montaj binary grid file (no compression) 

• *.gi - binary file that defines the coordinate system for the *.grd file 

The grids are summarized as follows: 

thabmagsup27.grd/.gxf 

thabmagsup83.grd/.gxf 

thab2vdsup27.grd/.gxf 

thab2vdsup83.grd/.gxf 

thbamagsup27.grd/.gxf 

thbamagsup83.grd/.gxf 

thba2vdsup27.grd/.gxf 

thba2vdsup83.grd/.gxf 

thjafhmagsup27.grd/.gxf 

thjafhmagsup83.grd/.gxf 

thjafh2vdsup27.grd/.gxf 

thjafh2vdsup83.grd/.gxf 

thrlmagsup27.grd/.gxf 

thrlmagsup83.grd/.gxf 

thrl2vdsup27.grd/.gxf 

thrl2vdsup83.grd/.gxf 

thshmagsup27.grd/.gxf 

thshmagsup83.grd/.gxf 

thsh2vdsup27.grd/.gxf 

Abitibi supergrid of the GSC levelled magnetic field in nanoteslas 

(UTM coordinates, NAD27 datum) 

Abitibi supergrid of the GSC levelled magnetic field in nanoteslas 


Abitibi supergrid of the calculated second vertical derivative of total magnetic field in 

nanoteslas per metre 2 (UTM coordinates, NAD27 datum) 

Abitibi supergrid of the calculated second vertical derivative of total magnetic field in 


Bamaji – Vickers supergrid of the GSC levelled magnetic field in nanoteslas 


Bamaji – Vickers supergrid of the GSC levelled magnetic field in nanoteslas 


Bamaji – Vickers supergrid of the calculated second vertical derivative of total magnetic 

field in nanoteslas per metre 2 (UTM coordinates, NAD27 datum) 

Bamaji – Vickers supergrid of the calculated second vertical derivative of total magnetic 

field in nanoteslas per metre 2 (UTM coordinates, NAD83 datum) 

Fort Hope - James Bay - Attawapiskat supergrid of the GSC levelled magnetic field in 

nanoteslas (UTM coordinates, NAD27 datum) 

Fort Hope - James Bay - Attawapiskat supergrid of the GSC levelled magnetic field in 

nanoteslas (UTM coordinates, NAD83 datum) 

Fort Hope - James Bay - Attawapiskat supergrid of the calculated second vertical 

derivative of total magnetic field in nanoteslas per metre 2 (UTM coordinates, NAD27 

datum) 

Fort Hope - James Bay - Attawapiskat supergrid of the calculated second vertical 

derivative of total magnetic field in nanoteslas per metre 2 (UTM coordinates, NAD83 

datum) 

Red Lake supergrid of the GSC levelled magnetic field in nanoteslas 


Red Lake supergrid of the GSC levelled magnetic field in nanoteslas 


Red Lake supergrid of the calculated second vertical derivative of total magnetic field in 


Red Lake supergrid of the calculated second vertical derivative of total magnetic field in 


Schreiber supergrid of the GSC levelled magnetic field in nanoteslas 


Schreiber supergrid of the GSC levelled magnetic field in nanoteslas 


Schreiber supergrid of the calculated second vertical derivative of total magnetic field in 




21

thsh2vdsup83.grd/.gxf 

thskmagsup27.grd/.gxf 

thskmagsup83.grd/.gxf 

thsk2vdsup27.grd/.gxf 

thsk2vdsup83.grd/.gxf 

Schreiber supergrid of the calculated second vertical derivative of total magnetic field in 


Stormy Lake supergrid of the GSC levelled magnetic field in nanoteslas 


Stormy Lake supergrid of the GSC levelled magnetic field in nanoteslas 


Stormy Lake supergrid of the calculated second vertical derivative of total magnetic field 

in nanoteslas per metre 2 (UTM coordinates, NAD27 datum) 

Stormy Lake supergrid of the calculated second vertical derivative of total magnetic field 

in nanoteslas per metre 2 (UTM coordinates, NAD83 datum) 



22

APPENDIX B 

MERGING OF GRIDS BY GRIDKNIT TM METHOD 

(Documentation supplied by Geosoft Inc.) 

Stitch Method 

Geosoft’s GridKnit TM enables you to stitch together any two grids, even if the grids have 

different cell sizes and/or transforms. The GridKnit TM tool delivers two advanced methods that 

produce fast, high-quality results for virtually any type of geophysical grid. The blending 

method provides a quick way of merging two grids via standard smoothing functions. The 

suturing method enables you to automatically or manually define a suture path for estimating 

errors and then applies a proprietary multi-frequency correction along the path. This method is 

used for joining grids that have a narrow overlap area and relatively small anomalies. Suturing 

provides more accurate results and greater control than blending. 

Blending Method 

The Blending method uses a blending function over the area of overlap so that transition from 

one grid to the other is smooth. Except for the optional removal of a static offset or trend, the 

grids beyond the overlap regions remain unchanged. 

The blending function determines the 

weighting of one grid against the other inside 

the overlap region. The function works by 

calculating the distance to the edge of each 

grid for each data point. The diagram on the 

right shows the distance to the edge of Grid 1 

(from a single data point ) as D 1 and the 

distance to Grid 2 as D 2 . The blending 

function uses a “cosine” function which varies 

smoothly from 0 to 1 – the function takes on 

the value of 0.5 at positions midway between 

the two grids, and whose derivative approaches 

0 at both ends. If a position is equidistance 

between both edges, its value is the average of 

the grid values found at that point. The 

diagram (right) shows the value (on the cosine 

curve) for Grid 1 is around 0.7 and the value 

for Grid 2 is 0.3. This means that when the 

blending function calculates the value for the 

data point, it uses 0.7 of the value of the point 

on Grid 1 and 0.3 of the value of the point on 

Grid 2. 



23

Note: Where the edges of Grid 1 and Grid 2 cross at any single point, the blending scheme 

breaks down, since by definition both grids are full owners of that point. In this case the 

average of the two grid values at that point will be used. 

Suture Method 

The suture method defines a suture line to determine where the grids will be joined. The line 

must lie completely within the overlapping area of the two grids. The area outside of the overlap 

remains unchanged. 

Along the suture line, a mismatch in the grid values is corrected by adjusting the grids on either 

side of the path. For instance, for a point on the suture path where the value for Grid #1 is larger 

than the value for Grid #2, the average value is used to eliminate discrepancies. Points adjacent 

to the path point might then be adjusted to produce a smooth transition between the two grids. 

The suture method uses a multi-frequency approach to spread corrections over the two grids in 

proportion to the wavelength of the mismatch found along the suture path. This ensures that the 

transition from one grid to the other remains smooth, regardless of the amplitude and wavelength 

of the features that the suture path crosses. 

Locating a Suture Path 

The suture line must be located inside the 

overlap area between the two grids. 

GridKnit provides four methods of 

determining the position of the suture line 

in the overlap area between the two grids. 

• Automatic 

• Interactive 

• Grid 1 edge 

• Grid 2 edge 

The diagram on the right illustrates where 

the suture path is located using each 

method. The automatic suture path is 

located equidistant between the two grid 

edges. The interactive suture line is 

defined by the user and can be located 

anywhere in the overlap area. The grid 1 

suture option uses the edge of Grid 1 as 

the suture path. Likewise, the Grid 2 

option uses the edge of grid 2 as the suture 

path. 



24

Automatic Suture Path 

If the automatic option is selected, the suture line will bisect the overlap region; every point 

along the line will be at approximately equal distance from the borders (grid edges) of the 

overlap region. Use this method when you want to place the suture line in the middle of the two 

grids and use both grids equally to determine the values of the output grid. 

Interactive Suture Path Definition 

If the interactive option is selected, the user must define a path using the current map as a 

reference. (If there is no current map, one will be requested). 

All suture paths must, by definition, lie within regions of overlap. The start and end points of the 

path within these regions is pre-determined by the geometry of the overlap, and cannot be 

altered. Those parts of the user-path not intersecting the overlapping grid sections are ignored. If 

the "start" and/or "end" points are not present in the user-path, the user-path is automatically 

altered to include them. It does this by joining the first and last points of the user-path within the 

overlap to the fixed start and end points. The resulting path may not cross any dummy positions, 

or any non-overlapping region of the data, or an error occurs. 

A single user-path may be used to define the path through two or more separate overlap sections 

by including at least one "outside" point as a space indicator between each path section through 

an overlap area. (All "outside" points are ignored when calculating the suture path from a userpath.) 

Postage Stamp Stitching (Grid Edge Path) 

"Postage stamp" stitching is used to incorporate a smaller grid into a larger grid; this is 

accomplished using the Grid Edge path selections. Selecting the Grid Edge path corresponding 

to that grid may preserve the contents of the entire smaller grid – the suture path will entirely 

enclose the smaller grid. 

Note: The path creation algorithm may run into difficulty when the Grid Edge path is selected if 

the boundaries of the grid are not sufficiently smooth. It may be necessary to smooth 

jagged edges, remove "peninsulas" or fill in "bays" using interpolation (see Preprocessing 

Algorithms) in order for the path to be determined unambiguously. The 

automatic option produces smoother paths due to the averaging effects of using edges 

from both grids simultaneously, and finding the midpoints between them. 

Suture Weighting 

A value in the range 0 to 1 determines how corrections are apportioned between the grids. A 

cosine function is used to weight the grids. The figure below shows how the weight function 

works. The numbers of the side of the chart below represent the proportion of weighting for each 

grid. For example, to calculate the suture point referring to the grid value (dot) closest to the 

bottom, the weighting function would use 0.25 of grid 1 and 0.75 of grid 2. Similarly, if a value 

of the “Proportion of Grid 1” is 0, then all corrections are applied to Grid 1. A value of 0 

indicates that all corrections are applied to Grid 2, and value of 0.5 (dot in the centre) means 

corrections are shared equally between both grids. 



25

By default, the weighting is even between the grids. However, by forcing all the corrections 

toward one grid or the other, the user can keep one grid unaltered up to and including the suture 

path. 

Suture Weighting 

1 

0 

0.75 

0.25 

Proportion 

of Grid 1 

0.5 

0.5 

Proportion 

of Grid 2 

0.25 

0.75 

0 

Multi-Frequency Suturing Algorithm (Fast Fourier Transform) 

1 

To ensure that the suturing process creates the smoothest possible transition between grids, 

GridKnit, uses a Multi-Frequency Fourier Transform (FFT) function. A Fourier transform 

breaks a single, complex curve into a family of 'sine' type curves, each with a unique frequency 

and amplitude. 

The diagram below illustrates how the function works in suturing. GridKnit first selects the 

grid values along the suture line for each grid. The grid values (frequency) for the points from 

each grid along the suture path are plotted. In the diagram below, Grid 1 values along the suture 

line are represented by the red line and Grid 2 values by the blue line. The system calculates the 

difference (purple line) between the values of Grid 1 and Grid 2. 

Grid 

Values 

(V ) 

3 

2.5 

2 

1.5 

1 

0.5 

0 

-0.5 

Grid 2 

Grid 1 

Difference 

Distance 

(D) 

Grid 1 

Grid 2 

Difference 

The difference curve is split using a Fourier transform function, into many curves, each 

representing a unique frequency (or wavelength). Next, a cascading "smooth" correction surface 

is applied to each unique frequency (or wavelength) curve. The correction surface (grid) applied 

for each wavelength is proportional to the wavelength. This proportional correction means that 

narrow features on a grid are given a narrow correction and wider features are given a wider 

correction. The correction surface for each frequency is added to the existing overlapping grid 

area so that the grids join perfectly at the edges. 



26

Pre-processing Tools 

Grid Cell Size 

If you have two grids with different cell sizes, the cell size and transform of the first grid have 

primacy. GridKnit will automatically convert the grid cell size of the second grid to match that 

of the first. Also, the offsets are adjusted (even to the primary grid) to ensure that grid lines in the 

output overlie the zero axes. If you wish to check or modify the cell size of one of your grids 

manually, follow the procedure below. 

TO CHECK GRID CELL SIZE 

1 To determine the cell size of a grid, select the Grid|Grid Info… menu. 

2 Specify the grid name and press the button. The system will display the grid information. 

3 The two numbers in the X Element Separation and Y Element Separation fields represent the grid cell size. 

These numbers must be the same for both grids. 

TO CHANGE GRID CELL SIZE 

1 To change the cell size of a grid, select the Grid|Gridding|Re-grid a grid… menu item. 

2 Specify the input and output name of the grid. 

3 Modify the cell size so that it matches the cell size of the other grid. Press the button. 

Trend Order 

The Trend Removal dialog box enables you to remove a static shift or trend from one or both 

grids before stitching. The static or zero-order correction adds or removes a single average value 

from the entire grid. Use this trend when you want to increase or decrease all the values on a grid 

by a constant amount. The slope or first-order correction removes the best-fitting plane. If none 

is specified for grid #1, and a trend is specified for grid #2, then grid #2 takes on the specified 

trend of grid #1 (i.e. grid #2 is brought to grid #1). However, if a trend is specified for both grids, 

the specified trends are removed individually from both grids. 

The trend options enable you to set a different trend order for each grid. This option is useful if 

you are connecting a small grid to a master grid and want to adjust the trend of the smaller grid 

to the master grid. In this case, you would select none for the master grid and another trending 

method (static or slope) for the smaller grid. 

Point Usage 

The calculation for the Trend Order correction may be done in one of three ways: using all the 

points in the grid, using the edge points, or using only the points that overlap with the other grid. 

It is not recommended to use the overlap point selection for the first-order (slope) correction in 

cases where the overlap region is small, or thin in one dimension. In these situations, the trend in 

a small region of the grid may be quite different from that found in the remainder, and removing 

it may seriously alter the appearance of the full grid. 

Note: If "edge points" is selected, and there is more than one data area in the grid data, then 

only the first section that the algorithm encounters is used in the calculation. 



27

Interpolation Options 

The Interpolation Options dialog box in GridKnit enables the user to set the spline method to 

use during interpolation and the maximum gap to interpolate across. Interpolation is used to fill 

gaps between points in individual grids prior to grid stitching. 

Spline Method 

The spline method determines how values are predicted in areas without data. Three methods are 

available: Linear, Cubic, and Akima. The linear spline assumes a simple linear variation between 

the values bounding the gap. The cubic spline is a minimum-curvature method. In cases where 

the data contains rapid changes in the gradient, the minimum curvature spline may produce 

undesirable highs or lows in the gap. The Akima spline method does not suffer as much from this 

problem, although it does tend to produce sharper corners around actual data points and the 

resulting grid tends to be less smooth. 

Gap Size 

A gap refers to a hole in the gridded data or a nil or dummy value within the dataset. If the gap 

size is set to a value "x", then all gaps in the data smaller than size "x" will be interpolated prior 

to stitching. If the gap size is set to 0.0, all gaps will be interpolated. If the gap size is not 

specified, no interpolation is performed prior to stitching. 

Grids may have gaps or holes inside the overlap region. The presence of gaps will affect the 

behaviour of the stitching methods. If the blending method finds a gap in the overlapping area of 

a grid, it fills in the gap by using the values from the other grid. 

In the suture method the suture path will tend to avoid the gaps, because it chooses the path 

midway between the edges of the two grids. Furthermore, a long-wavelength feature lying along 

the suture path may be interpreted as two or more short-wavelength features, with less than 

optimal results. These effects may be reduced or eliminated by choosing to interpolate the gaps 

prior to the stitching process. 

The interpolated values are used only during stitching to alter or improve performance of the 

stitching algorithm. When the final stitched grid is constructed, it is done on the basis of the 

original, un-interpolated grids. For instance, if both grids have a gap in the same location in the 

overlap area, then the gap will remain in the combined grids. In the same manner, gaps outside 

the blending area always remain in the output grid. If you wish to remove gaps from the final 

combined grid, either interpolate the grids separately prior to running the grid stitching routine, 

or interpolate the resulting, combined grid afterwards. 



28

MAGNETIC SUPERGRIDS Ontario Airborne ... - Geology Ontario

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