Datums (spheroid) definitions in Grass

[Gerald I. Evenden]

>Date: Wed, 05 Oct 1994 21:45:29 +0100
>From: anabel at asterix.fi.upm.es (Ana Isabel Donadios Algarra)
>Subject: Datums (spheroid) definitions in Grass
>To: grassp-list at max.cecer.army.mil

	-------- WARNING SEE FOLLOWING NOTES -------
>Hello everybody:
> 
>    We are working with in a program similar to m.datum.shift. We need this program calculates itself the shift values. 
>    We are going to use the datum's definitions that we have found on datum.c file:
> 
>    File $GISBASE/src/libes/coorcnv/datum.c:
>    ---------------------------------------------------  
> 
>    "wgs72", "World Geodetic System 1972",         0.0,    0.0,    0.0,
	...
>    "sa69",  "South American 1969",              -77.0,    3.0,  -45.0
> 
>    ----------------------------------------------------
>    
>    We also need to add other datums. So, our new datum's table is the next:
> 
>     DATUM        dx          dy          dz          ELLIPSOID
>     =====        ==          ==          ==          =========
> 
>     grs80        ?           ?           ?           grs80
>     wgs84        0           0           0           wgs84
	...
>     sa69         ?           ?           ?           s. america
>     ?            ?           ?           ?           new international 
> 
>    We would like TO COMFIRM if our table is right. Also we need to complete it because we don't know the shitf values of the next datums (character ? in the table ): 
>    grs80: ellipsoid grs80
>    arc50: ellipsoid Clarke 80
>    indian: ellipsoid Everest 30
>    afgooye: ellipsoid Krassovsky
>    sa69: ellipsoid South America 69
> 
>    Could any one help us, please?
>    Thanks in advance
> 
>    Anabel Donadios
>    Email: anabel at asterix.fi.upm.es
>    Spain

First, PLEASE, this is a critical warning!  DO NOT confuse ellipsoids
with datums.  Yes, datums do refer to an ellipsoid, but the same
ellipsoid may be used with different datums in different parts of
the world.

The above factors are for the Molodensky method of datum shift
which is highly regional in application and usually only good to
5-10 meters.  For example, the Conterminous US has two sets of Molodensky
coefficeints: one for each side of the Mississippi River.  There
are two more for Canada and Mexico (both also NAD27).  And on, and on.

Any list of Molodensky correction factors must be based upon
not only target datums but also to the region that apply.  I have
a table of more than 200 such regions but cannot distribute it
at the moment because I am lacking a good qualification of their
area of extent.

Gerald (Jerry) I. Evenden   Internet: gie at charon.er.usgs.gov
voice: (508)563-6766          Postal: P.O. Box 1027
  fax: (508)457-2310                  N.Falmouth, MA 02556-1027