[Proj] How grids work in PROJ4

[J Luis]

On Fri, Jan 13, 2012 at 6:37 PM, Frank Warmerdam <warmerdam at pobox.com>wrote:

> 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.



Right, but as I said, I think it will be a bit confusing as this thread
proves.

Joaquim
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