[Proj] How grids work in PROJ4

[Frank Warmerdam]

On Fri, Jan 13, 2012 at 7:49 AM, J Luis <jmfluis at gmail.com> wrote:
> 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,

I don't agree with this change amoung other reasons because it
is different than the horizontal datum case.  Note that despite
the +geoidgrids parameter name, the system does not know
anything about geoid's.  Such grids can also be used to express
offsets from other kinds of vertical datums from the ellipsoidal
height.


Best regards,
-- 
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