Many of the surface phenomena are driven by elastic energy and elastic interactions. Despite the fact that there are many microscopic techniques with nm and atomic resolution, an established technique to study the distribution of strain on the surface is still lacking. We present a study on the Gd(0001)/W(110) system, in which undulations in the Gd layer are detected by STM. This creates a heterogeneous surface with reduced strains, due to relaxation, on the crests of the waved surface and elevated strains in the troughs. An additional part of the strain is released through Stransky-Krastanov growth of Gd islands. Utilizing a strain-relief model, we show that the island size and shape reflect the strain variations on the surface. Strain maps were calculated, using the island as nanoprobes, with good correlation to the surface topography.