[Proj] Dutch correction grid

[Jan Hartmann]

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.

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?

Jan






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?
>
> Jan,
>
> 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 
> georeferenced
> 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,
>