[Proj] converting WGS84/ETRS89 to ITM/OSGM15

[Alain Orsoni]

Hi,

Trying to answer with my knowledge :
On my computer I can do this transform in this way

echo -6.6 53 21 | cs2cs +init=epsg:4326 +to +proj=tmerc +lat_0=53.5 +lon_0=-8 +k=0.99982 +x_0=600000 +y_0=750000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +geoidgrids=egm08_25_rg.gtx +units=m +no_defs
693972.63 695281.52 -35.37

The result is a little bit different from what you expect. Maybe because I use egm08 and you expect another Geoid model.
Doing the same way you try this (with proj program) I can’t get the correct use of the GeoidGrid. Why????

echo -6.6 53 21 | proj +proj=tmerc +lat_0=53.5 +lon_0=-8 +k=0.99982 +x_0=600000 +y_0=750000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +geoidgrids=egm08_25_rg.gtx +units=m +no_defs
693972.63 695281.52 21

But your problem in that you seem to enter with an ellipsoid elevation and want to get geoid elevation at the end of the process. For this you need to install the geoidgrids on your PROJ installation, and use it as indicated here but with cs2cs program.

Alain


Le 22 août 2018 à 11:19, Elmir Jagudin <elmir.jagudin at gmail.com<mailto:elmir.jagudin at gmail.com>> a écrit :

Hi

I'm trying to convert a WGS84/ETRS89 positions in Ireland into a ITM (Irish Transverse Mercator) easting northings and OSGM15 elevations.

Using the this commands:

  echo -6.6 53 21 | /proj \
     +proj=tmerc +lat_0=53.5 +lon_0=-8 +k=0.99982 +x_0=600000 +y_0=750000 +ellps=GRS80
     +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

gives me:

  693972.63    695281.52 21

The easting and northing are correct, however the elevation is not, the correct value is -35.15.

Is it possible to use proj4 to calculate OSGM15 elevations? What should the proj-string be in that case?

Or do i need to create a .gtx file for OSGM15?

Not sure how the elevations work in proj4, is there any documentation i could read?


Thanks in advance,
Elmir
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