USGS

Venus Mapping White Paper

Open-File Report 95-519


VI. Interpretation of RMS Slope Values

The rms slope value derived from the Hagfors model is an estimate of the slope distribution of the surface at scales of tens to hundreds of meters. Comparisons of lunar photogrammetric data and radar echoes show some degree of correlation between the two methods of measuring slopes, but the baseline over which the radar results apply is often difficult to quantify [Moore et al., 1980]. For much of Venus, the Magellan rms slope values follow the trend of the SAR echoes, with rougher surfaces having higher slopes and higher . Comparing these data with terrestrial analog surfaces has not been simple, and numerous questions as to the derivation and interpretation of such slope estimates remain [Simpson and Tyler, 1982; McCollom and Jakosky, 1992; Campbell and Garvin, 1993]. Published maps produced by the Stanford research group [Tyler et al., 1992], using different techniques than those used to make the current GsDR dataset [Ford and Pettengill, 1992], suggest that the choice of data reduction technique may strongly affect the estimate of rms slope in rough areas. Rms slope is nonetheless a valuable measure of angular spreading in the near-nadir radar echo, which can be a useful diagnostic of inhomogeneity within the footprint or of possible porous terrain units [Campbell and Rogers, 1994]. The average value and range of this parameter, which are part of the anc_data output, should be included in the map unit description.