The State of Arsenic in Nepal - 2003 - Harvard University ...

6.2.2. Preparation of digital spatial data layers
Spatial data layers acquired from different sources for twenty
districts were in ESRI’s Arcinfo coverage format. Data layers
for each district were checked, cleaned, and converted into
geo-referenced raster images, using any standard GIS to
produce different maps as per his/her requirement.
6.3. Map preparation
It has already been discussed about the importance of maps
to present the state of arsenic in different aspects. Integrated
database of arsenic tested tube wells, results of statistical
analysis and spatial data layers were three major components
in producing maps. Different types of statistical analysis were
carried out to know the state of arsenic in different aspects,
which are discussed in following chapters.
Figure.6.4.
ArcView shape files. Geographic coordinates for these data
layers were in the UTM system. They were transformed into
WGS84 with the assistance of a standard coordinate
transformation system in order to make these data sets
compatible to global coordinate system. This world standard
coordinate system will help to integrate these data sets
comfortably to other regional or global level data sets.
Integrated data sets at the regional level can be used for
different kinds of analysis in order to describe the state of
arsenic at that level.
Wells location data layers for fourteen districts were prepared
with the help of ArcView. Certain portions of location data
were in UTM and MUTM (national standard coordinate
system of Nepal) formats, so that they were transformed into
WGS84 prior to preparing these layers. Spatial model of data
layers with all four features for a district were as in Figure 6.4.
Raster images of integrated data layers of VDC boundary,
roads and rivers for each district are also provided in the CD
included with this report. Corresponding files to georeference
the raster image of each district are also provided together
with digital images. Shape files of three data layers - boundary,
roads and rivers- could not be shared due to copyright
conditions of the source organization. The user can integrate
existing or new tube wells’ location data in WGS84 with these
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Figure.6.5.
All these three components were integrated with the help of
GIS in order to produce different types of maps (Figure 6.5).
There are three levels of maps: national, district and VDC,
and they are presented in chapters 7 and 8. All these maps
are also provided in CD in digital image format.
6.4. Accuracy of spatial data
Two types of errors found in the spatial data layers may
affect slightly the accuracy of maps produced in this report.
Typographical errors obviously occurred in transcription of
geographic coordinates from GPS unit to data form or from
data form to database. Many such errors have been found
and corrected by comparing the plotted well position with
the location political description. Certainly some such errors
in the well location layer remain where the mistakes are small
enough to remain inconspicuous. Data that plot within the
proper VDC and lack other obvious error are considered valid.