[Proj] OSTN02 question: GRS80 instead of Airy.

[Roger Oberholtzer]

On Mon, 2006-10-30 at 16:04 +0000, Paul Kelly wrote:
> 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.

I did get the file from you a while back. Again, many thanks. While I
have your attention, what is the size of the grid in the file?

> >> 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.

I thought OSTN02 required BOTH the GRS80 ellipsoid and the grid shift.

I am confused how, if I set the ellipsoid in both statements to GRS80,
that the library will know that my source data are WGS84?

-- 
Roger Oberholtzer

OPQ Systems AB
Ramböll Sverige AB
Kapellgränd 7
P.O. Box 4205
SE-102 65 Stockholm, Sweden

Tel: Int +46 8-615 60 20
Fax: Int +46 8-31 42 23