COMPaRS: a stand-alone program for map comparison using quantile rank scaling

The usual metrics for comparison of two crystallographic or cryoEM maps, for example the overall map correlation coefficient, measure the similarity of two sets of values with no consideration of their position in space. In contrast, when analyzing the maps visually it is the positions of sets of points with map values equal to or greater than some cutoff level that is of interest. An intrinsic and scale-invariant characteristic of such a set is the quantile rank defining the fraction of grid nodes (or of the unit-cell volume) with values less than this cutoff level. Comparison of the quantile ranks associated with the same point in the two maps is very similar to a comparison of the isosurfaces. The programCOMPaRSuses new metrics for map comparison based on this idea: this gives quantitative results that agree with the qualitative results obtained from a visual analysis.