[Proj] Geoid models egm96 et egm08 in PROJ4

[Howard Butler]

Charles,

I have updated the files at http://download.osgeo.org/proj/vdatum/ with your set and renamed the old ones with a scary filename so that they can stay around for posterity and other archaeological operations if necessary.  

https://trac.osgeo.org/proj/ticket/126

Thanks for digging through this. Sorry it took so long to attend to it.

Howard

On Feb 14, 2014, at 9:36 AM, Charles Karney <charles.karney at sri.com> wrote:

> Corrected versions of egm96_15.gtx and egm08_25.gtx are available at
> 
>   https://sarnoff.sharefile.com/d/sb23d64908da48ffb
> 
> These were produced directly from the NGA spherical harmonic expansions
> using the GeoidToGTX program (a sample program distributed with
> GeographicLib).  These files correct the following issues with the PROJ4
> files:
> 
> (1) +/- 5mm quantization errors in egm96.
> (2) incorrect origins in egm96 and egm08
> 
> GeoidToGTX faithfully reproduces the grids given by the harmonic
> synthesis programs (in Fortran) provided by NGA.
> 
> (The link above should be valid "indefinitely".  However, I can't be
> sure that it will work beyond about a year.)
> 
>   --Charles
> 
> On 2014-02-12 08:29, Alain Orsoni wrote:
>> Hi,
>> 
>> We think there could be a problem in the egm96_15.gtx and egm08_25.gtx files provided on ftp://ftp.remotesensing.org/proj/vdatum/
>> 
>> We noticed a difference between the values we used to get and the values provided by PROJ4 using these gtx files.
>> It seems that both files have a shift (although it is different for each of the files.
>> 
>> Probably the reason is related to the egm08togtx.py and egm96togtx.py :
>> 
>> =============   in egm96togtx.py the line
>> ds_gtx.SetGeoTransform( (-180.25,0.25,0,90.125,0,-0.25) )
>> should probably become
>> ds_gtx.SetGeoTransform( (-180.125,0.25,0,90.125,0,-0.25) )
>> assuming that the grid value is given for the center of a cell (0.25°x0.25° cell).
>> 
>> This hypothesis seems to be confirmed by the following verification:
>> on the web site http://www.unavco.org/community_science/science-support/geoid/
>> which is a geoid height calculator, if you enter 0 0 and 0 values for long lat and height you will get a -17.162 orthometric height
>> 
>> If you use the following command
>> 
>> cs2cs +init=epsg:4326 +to +proj=longlat +datum=WGS84 +geoidgrids=egm96_15.gtx +units=m +no_defs
>> you will get the following result
>> 0 0 0
>> 0dE	0dN -17.120
>> while if you shift your value of -0.125
>> -0.125 0 0
>> 0d7'30"W	0dN -17.160
>> 
>> 
>> =============  in egm08togtx.py the line
>>     ps_25 = 2.5 / 60.0
>>     ds_gtx.SetGeoTransform( (-180 - ps_25,ps_25,0,90 + ps_25,0,-1 * ps_25) )
>> 
>> should become
>> 
>>     ds_gtx.SetGeoTransform( (-180 - (ps_25/2),ps_25,0,90 + (ps_25/2),0,-1 * ps_25) )
>> 
>> =================
>> 
>> If some one as encountered the same problem and confirm or could unconfirm this hypothesis, it would be helpful.
>> 
>> Alain ORSONI
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