Bouguer gravity surveys have long been used to investigate sub-surface density contrasts. The main sources of error in previous surveys have been the determination of relative elevations of stations and the effect of topography (removed via the terrain correction). The availability of high precision Kinematic GPS data now facilitates generation of high-resolution Digital Elevation Models that can help to improve the accuracy of relative elevation determination and the terrain correction. Here we describe a high-resolution gravity survey at Po¿s volcano, Costa Rica. Our gravity modelling (i) identifies small pockets of magma at shallow depths which relate to successive magma intrusion through time and (ii) shows that the persistent degassing in the eastern part of the crater is related to local deformation at the top of the volcano and changes in the fracture network, rather than to the presence of a shallow magma intrusion.