[Proj] OSTN02 question: GRS80 instead of Airy.

[Paul Kelly]

On Mon, 30 Oct 2006, Roger Oberholtzer wrote:

> No takers on this one?
>
> On Wed, 2006-10-25 at 12:58 +0200, Roger Oberholtzer wrote:
>> I use the C API to do transformations. I am converting from GPS values
>> in WGS84 to OSTN02 for the United Kingdom. My current code is:
>>
>>        fromProj = pj_init_plus(
>>                 "+proj=latlong "
>>                 "+ellps=WGS84 "
>>                 "+towgs84=0,0,0 "
>>                 "+nodefs")
>>
>>         toProj   = pj_init_plus(
>>                 "+proj=tmerc "
>>                 "+lat_0=49 "
>>                 "+lon_0=-2 "
>>                 "+k=0.9996012717 "
>>                 "+x_0=400000 "
>>                 "+y_0=-100000 "
>>                 "+ellps=airy "
>>                 "+nadgrids=gb.gsb "
>>                 "+units=m "
>>                 "+no_defs")
>>
>> Note that I do a grid shift, using data provided by a kind member of
>> this list.

Hmm, that might have been me. The file was huge though - around 20MB IIRC? 
I would need to look into using different resolutions for the latitude and 
longitude before making it more viable for distribution I think.

>> My question is, how can I use the GRS80 ellipsoid instead of Airy?

Well if both projections are on the same ellipsoid then there is no need 
for any of the datum transform stuff. So just delete all mention of 
towgs84 and nadgrids parameters and set both ellipsoids to GRS80.

Paul