Reid et al.'s (1990) method for interpreting magnetic survey data in grid form to give source locations and depths uses Euler's homogeneity relation. The relation is formulated in terms of a 'structural index' by which geological constraints are imposed - the index represents the rate of fall-off of the field with distance, which can be calculated for a given source type (thin dyke, finite step, etc.). Reid et al. (1990) used a least squares technique to solve for the source position (lateral coordinates and depth) and regional field value at each grid point, using a window of surrounding values at which the field gradients were known, and the structural index specified. Solutions were accepted or rejected using a criterion which compared the calculated source depth to its standard deviation.