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

[Paul Kelly]

On Thu, 13 Dec 2007, Otto Dassau wrote:

> 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.

Just to note that you do not necessarily need to add parameters to both 
datum.table and datumtransform.table. If dx=-156    dy=-270    dz=-189 are 
the only parameters you have, that is fine and they will be the only ones 
used. An entry in datumtransform.table is only needed if you have multiple 
sets of parameters for the datum, or if the only set you have available is 
not a 3-parameter set.

Paul