An NTv2 file contains one or more quasi-rectangular coordinate grids which are defined in the NTv2 headers. Instead of a rectangular area only an embedded polygonal area should and must be covered, for example, a state territory within the country borders. How can outside of a polygonal administrative area located coordinates be excluded from the coordinate transformations and reference system transitions (datum shifts) and thereby localization errors be suppressed?
The polygonal validity scope is specified by the developer while creating a NTv2 file or subsequently. Therefore the developer inserts so-called exopolygonal entries into the shift or accuracy values of the outside of the required polygon lying grid meshes. The evaluating software then must be so constituted that it can identify the exopolygonal entries and declare them as void. KilletSoft’s geodetic software developer kit GeoDLL from version 16.00 on and their coordinate transformation program TRANSDAT from version 19.00 on already support polygonal validity scopes. Both tools detect exopolygonal entries in shifts and accuracies of grid meshes. On hits the software generates a message, that the coordinate is located outside of the polygonal validity scope of the NTv2 grid file. Common NTv2 files without polygonal validity scopes still work as usual.
It depends on GIS developers and GIS users whether polygonal validity scopes will be implemented in NTv2 files in the future. A few if-statements and an additional loop structure in the source code of programs with NTv2 support are sufficient for implementing polygonal validity scopes. KilletSoft would like to support developers who are interested in this idea.
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