[Proj] How grids work in PROJ4

[J Luis]

>
>
> Yes, but I would express it: ... that should be added to the orthometric
> height to get height above ellipsoid.
>
> The term "ellipsoidal height" almost always means "height of something
> above
> the ellipsoid", so that's a harmless term.
>    But the term "geoid height" or "geoidal height" sometimes means "height
> of geoid surface above ellipsoid surface", and sometimes means "height of
> something above the geoid".
>    So "geoid undulation" is a more unambiguous term for the first meaning,
> and "orthometric height" is a more unambiguous term for the second meaning.
>
> So in summary, I think everyone is (almost) right!
>

Hi,

It happens that I just tested with our Portuguese geoid with following
command

cs2cs +proj=latlong +ellps=WGS84 +towgs84=0,0,0 +geoidgrids=GeodPT08.gtx
+to +proj=latlong
+ellps=WGS84 +towgs84=0,0,0
-8 37 0
8dW     37dN 51.910

which is fine and means that the left side altitude was interpreted as
ellipsoidal height and the output is the geoid undulation at that point.
Because this matter can easily be confusing, I propose that 2 extra options
are added to cs2cs so that we can unambiguously tell the program what type
of altitudes are provided in input. And those would be

+H -> means input height are ellipsoidal heights
+h -> means input are Orthometric (plain 'altitudes')

and the output is determined according with the relation h = H - N

Joaquim
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