Guidelines for the use of GNSS in surveying and mapping

Table C: Positioning guidelines
Guideline Explanation Pt. 2 ref.
Use a national DGNSS reference station for
positioning.
Many countries provide both real-time base stations and/or data from base stations which can be postprocessed.
If the data or the real-time corrections are available for a positioning survey, it is best practice to
use these, as the base stations are already positioned accurately in the ITRS89 co-ordinate system.
7.3.2
Use an accurate ITRS89 co-ordinate when
using your own DGNSS reference station.
If a national differential GNSS (DGNSS) reference station is not available, then a suitable base station should
be established for the project. It is best practice to survey this point and to obtain a sub-metre ITRS89
position, preferably from a GNSS control survey. This will ensure that co-ordinates generated from the
survey will be precise in ITRS89 and can be used with data from other surveys.
7.3.5
Use real-time systems for most asset-capture
applications.
It is best practice to use a real-time system for data capture in positioning surveys. This allows the operator
to ensure that sufficient data has been captured to the required precision. It also removes the need for
costly post-processing of data.
4.4.2
Do not move the captured data to fit the map. When capturing features or assets for a geographical information system (GIS), these are usually displayed
with a background map. It is best practice not to move the features to the correct position on the map, as in
some cases the mapping may be in error. If such mapping is later updated and loaded to the GIS, the
features will then be shown in the correct position.
4.4.2
Position some of the map features in asset
surveys.
When capturing features or assets that will be presented on a map background, survey some of the actual
map features such as road junctions, boundaries and other well-defined features. These can then show up
in the final data as check points to assess the map accuracy.
4.4.2
For accuracy requirements of around 5–10m,
use a stand-alone GNSS receiver.
As of May 2000, selective availability (S/A), the intentional degradation of the C/A code, on GPS, was set to
zero, but it is technically possible for the US authorities to turn it back on again. When set to zero this
allows stand-alone GNSS receivers working on the C/A code to obtain positions with an accuracy of around
5–10m. For asset data capture however, a surveying rather than stand-alone navigation receiver should be
used.
4.4.2
High-quality ionospheric correction methods. Satellite-based augmentation systems (SBAS) such as EGNOS (European Geostationary Navigation Overlay
Service), MSAS (Multi-Functional Satellite Augmentation System) and WAAS (Wide Area Augmentation
System) are satellite-based differential GNSS system (DGPS). The major potential inaccuracy within current
GNSS surveying is related to ionospheric corrections; the geostationary SBAS create another ‘layer’ of
receiver data that users can utilise for higher accuracy survey solutions.
4.4.2
10 | GUIDELINES FOR THE USE OF GNSS IN LAND SURVEYING AND MAPPING