[GRASS-user] exotic datum creation

[Hamish]

Wolfgang wrote:

> datum_code=	6678 (Lao National Datum 1997)
> datum_name=	Lao National Datum 1997
> datum_type=	geodetic
> origin_description=	Fundamental point: Vientiane (Nongteng) Astro 
> Pillar. Latitude: 18 deg 01 min 31.3480 sec N; Longitude: 102 deg 30
> min  57.1376 sec E (of Greenwich).
> realization_epoch=	1997
> ellipsoid_code=	7024 (Krassowsky 1940)
> prime_meridian_code=	8901 (Greenwich)
> area_of_use_code=	1138 (Lao People's Democratic Republic (Laos).)
> datum_scope=	Cadastre, topographic mapping, engineering survey.
> remarks=	Supersedes Lao 1993.
> information_source=	National Geographic Department
> data_source=	EPSG
> revision_date=	2004-04-27
> change_id=	
> deprecated=	0

EPSG is a very good sign for finding an answer.

> I found some dx,dy,dz values on the net. But now I have an other 
> question: I want to calibrate a scanned map. The grid on the map is in
> this Laos Datum, but finally I want to have it wgs84. Do I really have
> to go the way: xy->lao97->wgs84?

project/transform your grid coordinates to WGS84 using a tool like
PROJ's cs2cs or GRASS 6.1's m.proj before the i.points step. This may or
may not be an appropriate method depending on the map projection(s)
involved.

Expert advice on projections can by found on the PROJ.4 mailing list.
http://proj.maptools.org/

Be aware that there is a new georectification module in the GRASS 6.1
GUI which could use some testing (File->Georectify [pull into target
location from xy]). Also the old i.vpoints module has had many
improvements in the last few weeks. (i.rectify [push to target location
from xy])



Hamish