[Proj] Dutch correction grid
molnar at sas.elte.hu
molnar at sas.elte.hu
Sat Apr 9 08:25:45 EST 2011
> I see. So this would be a floating point grid with two bands, the first
> for the x (lon) offset, the second for the y (lat) offset. All
> measurements in arcseconds.
I can share my experiences creating a grid shift binary (gsb) file for
Hungary (between "Hungarian Datum 1972 (HD72)" and WGS84). As NTv2
standard was invented on the Western Hemisphere, some parameters might
seem strange for those, who learned geodesy east from Greenwich.
1: You should create a four column table. First and second columns should
be the latitude and longitude shift on the first datum grid points,
starting from the bottom right (!) corner of the grid in westward
Third and fourth column should be the standard deviation of the latitude
and longitude shift value
2: As longitude grows westward for the standard, longitude shifts should
be opposite sign as calculated (not lon(2nd datum)-lon(1st datum) but the
3: the delimitation character for this table in ascii format should not be
'tab' but 'white space'. (At least it worked for me...)
4: you should add an ascii header and a one line footer to your table. The
header should you like this: (see my comments after # signs)
> It's not meant for a datum shift, I've got two targets:
> - maps in epgs:28992 (Dutch coordinates, with corrections on a regular
> grid up to 25 cm)
> - maps in the 19th century Bonne-based projection, with corrections for
> about 1000 triangulation points (mostly church towers), up to 60 meters.
> These should have to be gridded.
> Am I correct that I can create NTv2 files this way and use them in the
> PROJ-parameter list as @nadgrids=...NTV2?
> On 8-4-2011 18:29, Frank Warmerdam wrote:
>> On 11-04-08 11:28 AM, Jan Hartmann wrote:
>>> I don't quite understand, Frank. I have a spreadsheet with four
>>> columns: x, y,
>>> correction for x and correction for y. What kind of raster file do I
>>> need to
>>> create with these data?
>> As input we would require an input raster with two "bands"
>> representing the
>> offset in longitude and latitude. I'm not sure off hand what the units
>> are - possibly arcseconds? The input file would need to be
>> in latitude and longitude.
>> If your input points are sparse and irregular you could likely use
>> gdal_grid to interpolate a grid from the point data.
>> My point was mainly that you don't necessarily have to write an NTv2
>> "writer" - it is sufficient to get the datum shift data into any GDAL
>> support grid format.
>> Best regards,
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