[GRASS-dev] custom CRS

[Paulo van Breugel]

On 19/03/2017 12:27, Benjamin Ducke wrote:
> On 18/03/17 12:15, Paulo van Breugel wrote:
>>
>> On 18-03-17 10:06, Paulo van Breugel wrote:
>>> 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]]
> [SNIP]
>
>>> +proj=longlat +a=6377301.243 +b=6356100.2284
>>> +towgs84=295,736,257,0,0,0,0 +no_defs
> Are you sure that your "+towgs84" parameters are correct?
> It seems to me that you intend to apply a three parameter
> transformation, but you are specifying a seven parameter
> transform with the scaling (last parameter) set to "0",
> which might have unintended consequences.
Good question. I am not sure, I have copied that from 
https://github.com/klokantech/epsg.io/issues/49 (the same is given in 
[2]). This is not my area of expertise, so any tips (for dummies) how I 
can test or find out more about this would be great. I have just send an 
email forwarding your question to the person that proposed the proj 
definition, so hopefully I'll get some useful feedback.

>   
> To specify a three parameter datum transform:
>
> +towgs84=295,736,257
>
> If you really want to specify all seven parameters, but
> want only the first three to take effect:
>
> +towgs84=295,736,257,0,0,0,1
>
> Best,
>
> Ben
>
>>> 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? Or can / should
>>> I add this to the datum information files in $GISBASE/etc/proj/ogr_csv?
>>>
>>>
>>> Regards,
>>>
>>> Paulo
>>>
>>>
>>> [1] https://epsg.io/37203
>>> [2]
>>> https://disqus.com/home/discussion/qgistutorials/georeferencing_topo_sheets_and_scanned_maps/#comment-1426454689
>> A link that provides further relevant information
>>
>> https://github.com/klokantech/epsg.io/issues/49
>>
>>
>>
>>
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>
>