[GRASS-dev] custom CRS

[Paulo van Breugel]

On 18-03-17 12:26, Moritz Lennert wrote:
>
> Le 18 mars 2017 10:06:09 GMT+01:00, Paulo van Breugel <p.vanbreugel at gmail.com> a écrit :
>> Dear all,
>>
>> I have a shapefile which is in CRS ESRI:37203 [1], which is defined in
>> the prj file as
>>
>> GEOGCS["GCS_Everest_India_Nepal",DATUM["D_Everest_India_Nepal",SPHEROID["Everest_Definition_1962",6377301.243,300.8017255]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]
>>
>>
>> The corresponding proj.4 definition (as defined in the esri epsg-codes
>> file in /usr/share/proj/)
>>
>> +proj=longlat +a=6377301.243 +b=6356100.230165384 +no_defs
>>
>> And when I create a location based on this shapefile, the projection
>> information is:
>>
>> -PROJ_INFO-------------------------------------------------
>> name       : GCS_Everest_India_Nepal
>> a          : 6377301.243
>> es         : 0.006637846068423442
>> proj       : ll
>> no_defs    : defined
>> -PROJ_UNITS------------------------------------------------
>> unit       : degree
>> units      : degrees
>> meters     : 1.0
>>
>> Question 1: Now, I can import a shapefile or raster layer in
>> latlon/WGS84 (EPGS: 4326) without reprojection. GRASS thus assumes the
>> projection is identical, even if they are not? Why is that?
>>
>> The proj.4 definition above is missing the shift w.r.t. the WGS84.
> That would be why: if no towgs84 info is given, the projection is assumed as using wgs84 as datum.
Ah, yes. Clearly I am not fully understanding how this projecting is 
handled. I was looking at the ellipsoid parameters, a and es which are 
different from EPSG 4326.
>
>
>> The
>> correct proj.4 definition for EPGS 37203 would be [2]
>>
>> +proj=longlat +a=6377301.243 +b=6356100.2284
>> +towgs84=295,736,257,0,0,0,0 +no_defs
>>
>> Question 2: In QGIS it is possible to define a custom CRS using the
>> above. Can I do this in GRASS as well, and if so, how?
> The location creation wizard allows you to define a location using a proj.4 string.
Of course, overlooked that in the rush of the moment. Thanks.
>
>> Or can / should
>> I
>> add this to the datum information files in $GISBASE/etc/proj/ogr_csv?
> Not sure about that file, but you could also add it directly in the /use/share/project/epsg or equivalent.
Ah ok, thanks. Will this work properly when reprojecting layers from 
other locations with datum transformation? I am asking because in the 
help file of r.proj it is mentioned that it supports general datum 
transformations, making use of the PROJ.4 co-ordinate system translation 
library. If I understand correctly the datum.table and 
datumtransform.table files in the  $GISBASE/etc/proj/ogr_csv folder? And 
those tables do not include the concerning datum / transformation 
information.

>
> Moritz
>
>>
>> Regards,
>>
>> Paulo
>>
>>
>> [1] https://epsg.io/37203
>> [2]
>> https://disqus.com/home/discussion/qgistutorials/georeferencing_topo_sheets_and_scanned_maps/#comment-1426454689
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