A method of measuring land deformation over time using interferograms derived from synthetic aperture radar data. The method includes: acquiring radar images covering an area at different points in time; deriving interferograms from pairs of the images, each interferogram measuring phase difference between pixels of a respective pair of images; for each pixel of the interferograms: determining an average coherence value over all of the interferograms; and if the average value is less than a threshold, determining an adjusted average coherence value equal to or above the threshold by excluding one or more of the interferograms below the threshold, provided the number of remaining interferograms is not less than a preset minimum number for each pixel of each interferogram for which the average or adjusted average coherence value is above the threshold, deriving vertical movement from the phase difference; and deriving the map of land deformation from the vertical movement.