## License Information

Use of this function requires a license for Whitebox Workflows for Python Professional (WbW-Pro). Please visit www.whiteboxgeo.com to purchase a license.

## Description

This tool creates a slope vs. aspect plot for an input digital elevation model, or DEM (`input`). Similar to a slope vs. elevation analysis (`SlopeVsElevationPlot`), the slope-aspect relation can reveal the basic topographic character of a site. The output of this analysis is an HTML document (`output`) that contains the slope-aspect chart, which is a radial line plot. The plot displays the median and interquartile range of slope values for the range of aspect values from 0 - 360 degrees. In reality, the aspect range is binned and the user must specify the bin size (`bin_size`). As slopes becomes quite shallow, the numerical instability in aspect becomes apparent, due to the relatively small signal-to-noise ratio in these areas of the input DEM. These shallow-gradient grid cells can have an out-sized impact on the shape of the slope-aspect relation. Therefore, users can specify to ignore slopes less than a certain threshold minimum slope (`min_slope`).

In interpreting the slope-aspect plots output by this tool, users should take note of asymmetries in polygonal paths taken by the percentile slope values, asymmetries in the range of slopes (i.e. the interquartile range), and anisotropy patterns (i.e. non-circularity or oval-shaped patterns). For example, asymmetries in the patterns may be indicative of landscape processes of interest, such as the differential energy balances experienced by north- and south-facing slopes at high latitudes. Increased rates of weathering on slopes with more direct sunlight at higher latitudes can result in flatter hillslopes. Asymmetry in the slope-aspect relation may also be indicative of DEM error and can be used as a quality control procedure, particularly for InSAR DEMs. Anisotropy in the slope-aspect relation may indicate a characteristic of the bedrock geology or the drainage structure of the landscape. The tool will also output the elongation ratio, a measure of anisotropy, of the mapped percentile polygons in a table.

The following are some examples of the output plots. In actuality, the outputs of the tool are interactive plots.

You may wish to smooth your DEM prior to analysis with this tool, in order to emphasis longer-scale patterns in the landscape. We would recommend using a method such as the feature_preserving_smoothing tool for this purpose.

The Z conversion factor (`z_factor`) is only important when the vertical and horizontal units are not the same in the DEM. When this is the case, the algorithm will multiply each elevation in the DEM by the Z conversion factor. If the DEM is in the geographic coordinate system (latitude and longitude), the following equation is used:

> zfactor = 1.0 / (111320.0 x cos(mid_lat))

where mid_lat is the latitude of the centre of each raster row, in radians.

## See Also

`SlopeVsElevationPlot`, feature_preserving_smoothing

## Project Links

WbW Homepage User Manual Support WbW