[GRASS-user] towgs84 datum parameters for "Hong Kong 1980"

[Otto Dassau]

Hi, 

I need some help to add support for "Hong Kong 1980" datum in GRASS. I
have a wkt file with following information:

PROJCS["Hong_Kong_1980_Grid",GEOGCS["GCS_Hong_Kong_1980",DATUM["D_Hong_Kong_198
0",SPHEROID["International_1924",6378388.0,297.0]],PRIMEM["Greenwich",0.0],UNIT
["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["Fal
se_Easting",836694.05],PARAMETER["False_Northing",819069.8],PARAMETER["Central_
Meridian",114.1785555555556],PARAMETER["Scale_Factor",1.0],PARAMETER["Latitude_
Of_Origin",22.31213333333334],UNIT["Meter",1.0]]

The datum D_Hong_Kong_1980 does not exist in GRASS yet. I found following
parameters and added them to the datum.table file:

# Hong Kong Datum 1980
hk80     "Hong_Kong_1980"   international    dx=-156    dy=-270    dz=-189

Now I am missing the Datum Transformation Parameters to add to the
datumtransform.table. 

I hope that someone has experience with "Hong_Kong_1980_Grid", can confirm,
what I found so far and maybe add the missing datum transformation parameters
or knows where I can find more about it?

thanks a lot
 Otto