DEM/DTM file formats

[Maki]

Hi all,



I have 5m DTMs stored as GeoTIFF that are much better than the common SRTM/ASTER DEMs we routinely use in Oruxmaps. Ideally I'd reduce them to 15 meters and then convert for use in OM, but I struggle to find the proper way to do the conversion, if at all possible. Anybody did already do this  and is willing to share the procedure? QGIS/GDAL preferred but I can evaluate other software.



Thanks