I am trying to convert a DEM file from EPSG:3035 (ETRS89-extended / LAEA Europe) to EPSG:4326 (WGS 84) with gdalwarp. EPSG:3035 uses the European Terrestrial Reference System 1989 ensemble datum where as EPSG:4326 uses World Geodetic System 1984 ensemble datum.
I assume that as the datums are different there should be some height difference in the DEM after conversion.
As per my search, it can be done with gdalwarp with proj string (link1, link2)
gdalwarp documentation also states that,
Starting with GDAL 2.2, if the SRS has an explicit vertical datum that points to a PROJ.4 geoidgrids, and the input dataset is a single band dataset, a vertical correction will be applied to the values of the dataset.
Here there is a mention of geogrid (egm96_15.gtx for EGM96 elevation as described in this link).
gdalwarp -s_srs "+proj=longlat +datum=WGS84 +no_def" -t_srs "+proj=longlat +datum=WGS84 +no_defs +geoidgrids=egm96_15.gtx" cartosat.tif cartosat_orthometric.tif
But I couldn't find the gtx file that should be used for EPSG:3035.
So, I have done the transformation as follows
gdalwarp -s_srs "+proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs +type=crs" -t_srs "+proj=longlat +datum=WGS84 +no_defs +type=crs" input.tif output.tif
I have used PROJ.4 string of EPSG:3035 in -s_srs and PROJ.4 string of EPSG:4326 in -t_srs.
With this, the output.tif does show some difference in height (around 1.7m in some area while none in other).
Is my approach to vertical transformation correct, or I am missing something here?
Edit-1: Even if I don't use proj.4 string and do the reprojection as follows, I am getting the same output as with the proj.4 string.
gdalwarp -t_srs EPSG:4326 input.tif output.tif
