[Proj] How grids work in PROJ4

[Michal Seidl]

Hello Frank,
>
> Only one horizontal datum transformation can be associated with a
> coordinate system.  So you should not currently mix +towgs84 and
> +nadgrids.

I got it, do not mix +towgs84 and +nadgrids. Thanks

>
>> b) if there is +geoidgrids on the LEFT SIDE PROJ thinks there is orthometric
>> height on input so firstly it changes H by dh from grid to get ellipsoid
>> height before 3D transformation
>
> Right
>

It seems to me as cs2cs (trunk version) works exactly in an opposite 
(unreasonable?) manner. If there is +geoidgrids on the LEFT side I would 
suppose cs2cs SUBTRACTs geoid heigth above ellipsoid from GTX file but 
cs2cs ADDs it.

As I know it is usual there is positive value in geoid data set in case 
geoid is above ellipsoid. Also egm96_15.gtx is done like this. Should it 
be inverted in case of proj library?

Example:

PROJ_LIB=nad PROJ_DEBUG=ON  src/cs2cs +proj=latlong +ellps=WGS84 
+towgs84=0,0,0 +geoidgrids=egm96_15.gtx +to +proj=latlong +ellps=WGS84 
+towgs84=0,0,0

15d 50d 0
15dE    50dN 44.690

PROJ_LIB=nad PROJ_DEBUG=ON  src/cs2cs +proj=latlong +ellps=WGS84 
+towgs84=0,0,0 +to +geoidgrids=egm96_15.gtx +proj=latlong +ellps=WGS84 
+towgs84=0,0,0
15d 50d 0
15dE    50dN -44.690

Best regards, Michal