[Proj] How grids work in PROJ4

[Thomas Knudsen]

2012/1/14 Michal Seidl <michal.seidl at gmail.com>

> - Mikael Rittri explanation helps me a lot, I was really for a moment
> confused by Frank's one
>

Mikael's clarification was, indeed, excellent and useful


> - Joaquim formula h = H-N is OK. But as Frank said +geoidgrids is only
> name and can be used for other height correction which is exactly my
> situation.
>

But please note that Wikipedia and leading geodetic textbooks
(Hoffmann-Wellenhof & Moritz: Physical Geodesy, as well as Torge: Geodesy)
use a different notation:

h: height above ellipsoid
H: orthometric height (which to an excellent approximation is equivalent to
the height above the geoid)

Hence: h = N + H

So following Joaquim's suggestion of +H and +h options is quite risky. Out
of context you can never be sure what is meant by H and h.

/Thomas
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