By discretizing the above expression, we are able to relate the discrete data observations on the surface of the Earth to the discrete model parameters (density) in the subsurface that we wish to know more about. For example, consider the case where we have measurements carried out at 5 locations on the surface of the Earth. In this case, our data vector, formula_32 is a column vector of dimension (5x1): its formula_37th component is associated with the formula_37th observation location. We also know that we only have five unknown masses formula_39 in the subsurface (unrealistic but used to demonstrate the concept) with known location: we denote by formula_40 the distance between the formula_37th observation location and the formula_42th mass. Thus, we can construct the linear system relating the five unknown masses to the five data points as follows: