Datums (spheroid) definitions in Grass

Gerald_I_Evenden · October 6, 1994, 2:41am

Date: Wed, 05 Oct 1994 21:45:29 +0100
From: anabel@asterix.fi.upm.es (Ana Isabel Donadios Algarra)
Subject: Datums (spheroid) definitions in Grass
To: grassp-list@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@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@charon.er.usgs.gov
voice: (508)563-6766 Postal: P.O. Box 1027
fax: (508)457-2310 N.Falmouth, MA 02556-1027