john.smart at caris.com
Fri Jan 13 10:28:10 EST 2006
>> Differences with Ordance Survey results must be attributed to lack of
> Is that the main thing that OSTN02 adds to all this?
OSTN02 is a grid-shifts file with grid resolution = 1km. The shifts are from
GB National Grid (i.e. a TM projection) on ETRS89 to the same projection on
OSGB36. Bilinear interpolation is used between grid nodes. The stated
accuracy is something like 0.1m at 1 sigma. The grid-shifts file has a nice
feature that it has dummy values outside of a 10km buffer around GB
coastline. The file also has orthometric height differences (OSGM02) at the
same grid points.
The 7 parameter is stated as being good for applications needing accuracy
less good than +/- 5m.
Could be nice if Proj would support this grid shift file, but what is really
needed (IMHO) is some standardization on such file formats so that a
plethora of different formats would not be needed. Ntv2 is something of a
standard but relates to lat/long differences not projection grid.
More information about the Proj