We describe the vertical displacement field of an ice shelf floating on a subglacial lake, Grímsvötn, located underneath the Vatnajökull ice cap (Iceland). The uplift is measured using the correlation of two satellite optical SPOT5 images acquired 5 days apart with similar, non-vertical incidence angles. This is the first time correlation of optical images has been used to measure vertical displacements. Our technique is suitable for mapping short-term elevation changes of glaciers. If the surface features are preserved, vertical displacements can be measured every 25 m with an accuracy of about 0.5 m.

The uplift map of Grímsvötn shows that 10.9 (±1) km**2 of ice was floating between 11 and 16 August 2004. The ice shelf rose by 1.7 (±0.6) m indicating that the volume of liquid water in the lake increased by 0.018 (±0.007) km**3. Our field observations show that surface melting due to meteorological processes contributed 70% of the accumulated water, hence, the rest originated from ice melted by the subglacial geothermal activity. The power required to melt 0.005 km**3 (water equivalent) of basal ice in 5 days is 4000 MW.

The applicability of the technique can be extended to volcanology and seismology, and even landslides or subsidence, when finer-resolution optical images become available. Applied to two pairs of images, it could solve for the 3-dimensional displacements of the Earth's surface.