A series of repeated first order leveling surveys across The Geysers geothermal field were carried out during the 1970ís. The results revealed that the region was apparently subsiding. Between 1973 and 1977 a
maximum subsidence of some 0.19 m was observed at, what was then, the centre of steam production activities. During 1996 many of the leveling monuments were reoccupied using GPS receivers and their locations measured to a typical accuracy of UH x 0.006 m, ITV x 0.02 m, where IS is one standard deviation and the subscript refers to horizontal, H, or
vertical, VI measurements. Comparison of GPS to leveling heights is complicated by the fact that the GPS measurements are located within an ellipsoidal reference frame, in this case the WGS84 model, whilst
leveling heights are relative to a geoid based reference frame, in this case NGVD29. The data were transformed to the same co-ordinate system using a high precision geoid model, GEOID96, plus some further datum corrections. These transformations add approximately 0.03 m of uncertainty to the results but allow direct comparison of measured heights over a 20
year period. Subsidence is clearly observed between the 1977 and 1996 surveys, throughout The Geysers, with a maximum of 0.9 f 0.05 m. The subsidence can be closely modeled using a small number of simple di-
latational point sources (Mogi sources). We note that the location of the best fitting sources corresponds to the mapped steam pressure lows within the reservoir.

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