Hi. Following this thread (http://osgeo-org.1560.x6.nabble.com/How-to-offset-height-above-ellipsoid-td5195058.html ), i suggest an improvement for being able to specify a constant vertical shift with the geoidgrids parameter. I've created a new ticket here http://trac.osgeo.org/proj/ticket/270 with patch and more description. Regards. Paul.