[Proj] *SOLVED*: Creating a nadgrid shift file with GDAL

[Jan Hartmann]

This is the solution, thanks Andre.


On 01/20/2013 07:28 PM, Andre Joost wrote:


> dx values inverted:

To summarize: creating an ntv2 file with gdal utilities:

Let's say you have a list of coordinates in a non-wgs84 projection, and 
a list of the same locations in wgs84. You want to create an ntv2-file 
to shift from one to the other.

1) transfer the original coordinates to latlon, without a wgs84 shift
2) compute for each point the distance in degrees between the wgs84 
location and the original latlon-location
3) separate the distance in its lon and lat component, and multiply both 
numbers by 3600
4) invert the lon-value (<==== this was the problem)
5) create two point files, one for the lons and one for the lats. The 
coordinates should be the wgs84 values, and the z-value the lon or lat. 
Look on the gdal_grid page how to create a vrt-version, it's also 
possible with postgis or any other ogr-format. Create also a point-file 
with the same coordinates and all z-values set to zero
6) create for these three point-files each a gridded raster with 
gdal_grid. Set reasonable boundaries in wgs84 with -txe and -ty, and 
choose output format Float32
7) merge the grids into a four-band geotif: gdal_merge -separate -o 
<shiftgrid> <latgrid> <longrid> <zerogrid> <zerogrid>. Mind the order: 
lat first
8) invert the y-direction in <shiftgrid>: gdalwarp <shiftgrid> 
<new_shiftgrid> (<=== this was another problem. Is this a bug?)
9) convert to ntv2: gdal_translate <new_shiftgrid> <ntv2_grid>

I would appreciate if someone would test the procedure on their own 
data, and if it works, to add it to the documentation.

Thanks for all your help!

Jan

PS I fully agree with Gerald Evenden on the reversal of the lon-values 
in the ntv2-format

http://osgeo-org.1560.n6.nabble.com/NTv2-info-td3842825.html

I seem to get the idea that the Canadians developed this format---comments
please---and if they did then the designers deserve to be flogged for 8 
days
with wet noodles for "positive east longitude."  I can't believe it!!!! 
  What
is unbelievable arrogance by whoever had that stupid---yes, *VERY*
stupid---idea.


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