A method of determining topographical variation across a substrate on which one or more patterns have been applied. The method includes obtaining measured topography data representing a topographical variation across a substrate on which one or more patterns have been applied by a lithographic process; and combining the measured topography data with knowledge relating to intra-die topology to obtain derived topography data having a resolution greater than the resolution of the measured topography data. Also disclosed is a corresponding level sensor apparatus and lithographic apparatus having such a level sensor apparatus, and a more general method of determining variation of a physical parameter from first measurement data of variation of the physical parameter across the substrate and intra-die measurement data of higher resolution than the first measurement data and combining these.