[Proj] Dutch correction grid

[Jan Hartmann]

Thanks Gabor, this will certainly save me hours of work!

Jan

On 9-4-2011 16:36, molnar at sas.elte.hu wrote:
> Dear Jan,
>
>> 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...
>
> 0: If you have a list of coordinates of the identical points in your
> starting and target geodetic systems in ellipsoid coordinates, you should
> create two grids from this pointcloud.
> (I used surfer for this) First grid should contain the latitude shifts,
> and the second should contain the longitude shifts. I save my results in
> .dat format. (Use an interpolation method, which calculates grid
> node-point values outside the convex hull of your listed coordinates, or
> add some virtual points near the corners of the created grid to ensure,
> that you should have a valid transformation outside the web of your
> control points, but inside the border your country.)
> 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
> direction. (I used excel to sort the list of points in the .dat files)
> Third and fourth column should be the standard deviation of the latitude
> and longitude shift value. (I think these values are never ever used in
> the calculations. I used dummy values like 0.001000)
> 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
> opposite).
> 3: the delimitation character for this table should not be 'tab' but
> 'white space'. (At least it worked for me...)
> 4: you should add a header and a one line footer to your table in a text
> editor. The final ascii file should look like this: (see my comments after
> # signs!)
>
> NUM_OREC 11        # number of lines for the general header
> NUM_SREC 11        # number of lines for the sub-header
> NUM_FILE  1        #I had only one dataset (so only 1 sub-header is expected)
> GS_TYPE SECONDS    #all angels are in arcseconds
> VERSION NTv2.0     #
> DATUM_F HD72        #name of your local datum, 1st datum
> DATUM_T WGS84       #name of your 2nd datum
> MAJOR_F 6378160.000  #1st datum ellipsoid half major axis in meter
> MINOR_F 6356774.516  #1st ... minor axis
> MAJOR_T 6378137.000  #2nd ... major ...
> MINOR_T 6356752.314  #2nd ... minor ...
> SUB_NAMEGGPSH95      #sub header begins: name of grid shift sub-dataset
> PARENT  NONE         # ??? (do not change)
> CREATED 04-10-10     #
> UPDATED 04-10-10     #
> S_LAT     164520.000000  # southernmost latitude in seconds
> N_LAT     174960.000000  #northernmost ...
> E_LONG    -82800.000000  #easternmost longitude in seconds. Mind the sign!
> W_LONG    -57600.000000  #westernmost ... Mind the sign!
> LAT_INC      360.000000  #latitude increment in seconds
> LONG_INC     360.000000  #longitude increment in seconds
> GS_COUNT 2130   #number of samples in the grid shift sub-dataset
>
> # her come the data like:
> -0.864000	 4.032000	0.001000	0.001000
> -0.866400	 4.029600	0.001000	0.001000
> -0.868800	 4.027200	0.001000	0.001000
> -0.871200	 4.024800	0.001000	0.001000
> ...
> ...
> # end line:
> END  .33E+33
>
> 5. Save the file as something.asc
> 6. Use GDAit for converting the ascii file above to and .gsb file.
> Download GDAit, install it, start it, and File->Import Grid file choose
> Files of type: NTv2 ASCII grid (*.asc). Make this file as the default grid
> shift file.
> 6. Create a .gsb grid using GDAit: File->Export Grid file and choose the
> .gsb type.
> 7. Now you can copy and use this .gsb file for conversion in gdal
> environment with nadgrids=... option.
>
>> 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.
> I suggest to create two separate .gsb files. One between the epsg:28992
> system and WGS84 (or ETRS89) and one another file between the old dutch
> grid and the WGS84 (or ETRS89) system.
>
> Good luck!
>
> Gabor Molnar
>
>
>
>
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