[gdal-dev] Setting up GDAL Custom Transformation from ESRI GTF File

[Derek Stuart]

I've discovered that the current implementation of RAS Mapper only allows
an ESRI Prj file to be read in. It uses the GDAL routines, but in a limited
way.  I suspect that I should be able to get RAS Mapper to access a custom
transformation if I create a dummy ESPG value with the desired parameters
in the GDAL files listed in my original email.

Derek

On Sun, Apr 3, 2016 at 8:56 PM, Andre Joost <andre+joost at nurfuerspam.de>
wrote:

> Am 03.04.2016 um 14:05 schrieb Derek Stuart:
>
>> Andre: The only Gulshan coordinate system I see with EPSG 4682 is a
>> geographic coordinate system.  I am using a projected coordinate
>> system called BTM. This is a fairly new coordinate system, but it is
>> the standard here in Bangladesh.
>>
>>
>> PROJCS["BTM",GEOGCS["GCS_Everest_Bangladesh",DATUM["D_Everest_Bangladesh",SPHEROID["Everest_Adjustment_1937",6377276.345,300.8017]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",-2000000.0],PARAMETER["Central_Meridian",90.0],PARAMETER["Scale_Factor",0.9996],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]
>>
>>  There is a projected coordinate system called Gulshan 303 Bangladesh
>> TM that uses the Gulshan datum, but it has EPSG 3106, not 4682.
>>
>>
>> PROJCS["Gulshan_303_Bangladesh_TM",GEOGCS["GCS_Gulshan_303",DATUM["D_Gulshan_303",SPHEROID["Everest_Adjustment_1937",6377276.345,300.8017]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",90.0],PARAMETER["Scale_Factor",0.9996],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0],AUTHORITY["EPSG",3106]]
>>
>>  This has very similar parameters to BTM, except that Gulshan 303 has
>> a false northing of 0 and BTM has -2000000.0.  I asked some other
>> GIS professionals locally and was told that here isn't a EPSG code
>> that matches BTM.  Unless I want to reproject my data and work in
>> UTM, I don't see a way to avoid adding a custom transformation to
>> GDAL.
>>
>
> Your first post did not include information on the projected coordinate
> system. Since it has a different false Northing, you really have to put up
> a custom CRS. But you have to take the +towgs84 parameter set to shift from
> the Everest ellipsoid to WGS84. The WKT definitions you give do not include
> that, because the ARCGIS world handles datum transformations separately.
>
> You can put the WKT definition above in a text file, and run gdalsrsinfo
> on that file. It will report:
>
> +proj=tmerc +lat_0=0 +lon_0=90 +k=0.9996 +x_0=500000 +y_0=-2000000
> +a=6377276.345 +b=6356075.41314024 +units=m +no_defs
>
> As mentioned above, add the +towgs84 set for Gulshan, which you find in
> EPSG:4682:
>
> +proj=tmerc +lat_0=0 +lon_0=90 +k=0.9996 +x_0=500000 +y_0=-2000000
> +a=6377276.345 +b=6356075.41314024 +towgs84=283.7,735.9,261.1,0,0,0,0
> +units=m +no_defs
>
>
> HTH,
> André Joost
>
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-- 
Derek
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