[GRASS-dev] custom CRS

[Paulo van Breugel]

On 19-03-17 21:24, Benjamin Ducke wrote:
> On 19/03/17 20:11, Paulo van Breugel wrote:
>>
>> 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.
> Because your source system does not use the WGS84
> datum, you should specify a datum transform. This is
> assuming that you wish to reproject your data at some
> point from the Indian Grid System into something more
> modern, such as UTM with WGS84 datum. If you will
> never need to reproject, then you can just ignore all
> of the following.
>
> Re. the datum transformation:
> You'll know there is a problem when you get a result
> with rather lousy accuracy after reprojection. I would
> expect a simple three parameter datum transform to give
> you ca. 5m of result accuracy.
>
> Simple datum transformations have three parameters:
>
> 1-3: x/y/z translations
>
> ... or an additional four (more accurate):
>
> 4-6:  x/y/z rotations
> 7: scaling
>
> It's a common mistake to set the seventh parameter
> to "0", believing that this means "skip this"
> (and by "common" I mean: I've done this myself before).
> This is true for parameters 1-6 (translating or rotating
> by "0" does "nothing"). But it is not true for 7!
> Scaling a number by "0" sets its value to "0". Scaling
> by "1" is the operation that "does nothing".
>
> To be on the safe side, specify only the three parameters
> that your datum transform actually has:
>
> +towgs84=295,736,257
>
>> 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.
> This seems to be a rather nice page:
>
> http://kanchu_deep.tripod.com/igintro.htm
>
> It has the three parameters for your datum transform
> plus a bunch of very useful literature at the bottom
> of the page.
This is very useful information, thank you!
>
> Best,
>
> Ben
>
>>>    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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>>>>
>>>
>
>