[Proj] ostn02: northings and eastings to lats and longs

[Roger Oberholtzer]

I am happily using proj to convert WGS84 lats and longs into OSTN02
eastings and northings. My C API code (seen in these parts before...)
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 "

"+towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894 "
                "+nadgrids=gb.gsb "
                "+units=m "
                "+no_defs")

Now, how would I specify these items if I wanted to go in the opposite
direction - from OSTN02 northings and eastings to WGS84 lats and longs?
Surely it is not just to turn around the fromProj and the toProj. Note
also that I am doing a grid shift via the  "+nadgrids=gb.gsb " option.

I have a number of such conversions that I want to do, but one example
will get me on my way.


-- 
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