A digital elevation model (DEM) of an ice cap in Greenland has been generated from airborne SAR interferometry data, calibrated with a new algorithm, and compared with airborne laser altimetry profiles and carrier-phase differential GPS measurements of radar reflectors deployed on the ice cap. The accuracy of the DEM is found to be 1.5 m and it is demonstrated that surface penetration is an important issue. Penetration depths up to 13 m are measured in the percolation zone. The effective penetration i.e. the bias of the interferometric height, has not previously been measured directly via comparison with GPS data and calibrated laser data.