An experimental approach for producing relative charge density maps of biological surfaces using the atomic force microscope is presented. This approach, called D minus D (D-D) mapping, uses isoforce surfaces collected at different salt concentrations to remove topography and isolate electrostatic contributions to the tip-sample interaction force. This approach is quantitative for surface potentials below 25 mV, and does not require prior knowledge of the cantilever spring constant, tip radius, or tip charge. In addition, D-D mapping does not require tip-sample contact. The performance of D-D mapping is demonstrated on surfaces of constant charge and varying topography (mechanically roughened mica and stacked bilayers of dipalmitolphosphatidylserine), a surface of varying charge and varying topography (patches of dipalmitolphosphatidylcholine on mica), and bacteriorhopsin membranes adsorbed to mica.