Chris, Thank you for your valuable explications. Unfortunately it doesn’t work. In fact, our watershed covers five MTM projection zones. So whenever we change the projection parameters to set those of the new zone, the previous imported DEMs files disappear. Also, when we tried to merge imported DEMs, a gap appeared between layers that should be adjacent and whose projection zones are different. MTM is Modified Mercator Transverse associated to projection system of Eastern Canada. Thanks, Fadoua