[GRASSLIST:3818] Projection lat/lon (gps) to local system

[Morten Sickel]

Hi,

I was using grass a bit a couple of years ago, but I have been away for a 
while. Now I'm back and have started looking at 5.7, quite impressive, kudos 
to the developers. 

But..

I need to project some gps data to use them together with some raster maps in 
a local (Norwegian) projection :

GRASS 5.7.cvs:~/NinaSather/Kart >g.proj -p
-PROJ_INFO-------------------------------------------------
name       : Transverse Mercator
datum      : ngo48
towgs84    : 278.3,93.0,474.5,7.889,0.050,-6.610,6.21
proj       : tmerc
ellps      : modif_bessel
a          : 6377492.0175999999
es         : 0.0066743722
f          : 299.1528128500
lat_0      : 58.0000000000
lon_0      : 8.3895830000
k_0        : 1.0000000000
x_0        : 0.0000000000
y_0        : 0.0000000000

As fas as I can see, those values are correct, but anyhow, the gps data ends 
up about 300 meters off from where they should be :-( (see 
http://sickel.net/gps.html for an illustration) It looks like some kind of 
datum / ellipse error, but I have no idea how to fix that (Except for 
identifying a few pass points and write a transformation script) I have a 
table of dx,dy,.dz values for my local projection and I have a feeling that 
those are somehow connected to the towgs84 parameter, but I don't see how to 
set those. 

Btw, if anybody among you are reading 'Landscape and Urban planning' the maps 
in 
Hanne Sickel, Margareta Ihse, Ann Norderhaug and Morten A. K. Sickel: How to 
monitor semi-natural key habitats in relation to grazing preferences of 
cattle in mountain summer farming areas: An aerial photo and GPS method study  
Landscape and Urban Planning, Volume 67, Issues 1-4, 15 March 2004, Pages 
67-77  
are made using grass. Each map made using two outputs using ps.map (5.0.0) and 
combined in the gimp, making the upper layer semi-transparent. Feel free to  
contact me if anybody have any questions regarding the production line.

-- 
Morten Sickel
http://sickel.net/
Drøbak, Norway