[OSRS-PROJ] Datum transformation - more Sphere to WGS84...

[Duncan Agnew]

Sverre:

	You do not need PROJ. The Sandwell-Smith gravity data are on a
Mercator grid, the Mercator projection being for a spherical Earth. If
you use the program img2xyz that comes with the grid it will convert the
x-y grid coordinates back to geographical lat and long--not, despite
the name, to spherical XYZ (perhaps the source of confusion). So there
is no need to transform to any other latitude.

	As far as datums go, this is all marine data, so the sources for the
positions are:

	1. Satellite radar-ranging to the sea surface: this will be on a DoD
global datum (different ones at different times), the same as WGS84 to the 
relevant precision.

	2. Shipboard gravity:
		A) Celestial navigation: astronomical lat and long
		B) Satellite naviagation: same as (1)
		C) Loran--may use a regional datum
	Most of the high-seas data will be (B).

Since the grid is averaged to 1-minute spacing, none of the differences
matter anyway.

Duncan Agnew
Scripps Institution of Oceanography
dagnew at ucsd.edu

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