isopleth (Isopleths)

This tool creates isopleth lines based on a raster dataset representing a probability surface. Isopleths represent the boundary lines that contain a specified volume of a surface. For instance, the 0.95 isopleth represents the line containing 95% of the volume of the surface. The isopleths are specified as 'quantiles' that can be expressed either as a proportion, or as a percentage. If the list of quantiles contains any values greater than 1, the list is interpreted as containing percentages. If all the values are in the range 0-1, the quantiles are interpreted as proportions.

This algorithm produces high-quality isopleth lines at the expense of processing time, and is the same algorithm that is used in the contour command. It interprets the raster as a triangular regular network and performs a plane intersection to determine the paths of the contour lines. It does not perform any sort of smoothing on the line.

While lines are always output from this command, you can also optionally specify that polygons should be created. This does not, however, guarantee that a polygon will be created for every isopleth. For a polygon to be created, the line must form a complete loop. For the larger quantiles the isopleths may not form complete loops if the raster has been generated over too small an extent. It would be inappropriate for a tool to close those isopleths based on the boundary of the raster extent, as these boundaries do not represent the true position of those isopleths. Rather than close isopleth lines inappropriately, the tool only converts the complete loops to polygons and reports a warning message that some lines were not closed. If you receive that message it is recommended that you regenerate the raster surface over a larger extent. I have added an 'edgeinflation' parameter to the kde command to assist with this problem.

Although a 'band' option is included so that users can identify which band to process in the case of multiband images, it is likely that this tool will most often be run with continuous, single-band raster data.

Updates: in GME 0.5.3 the isopleth command has been improved by increasing the accuracy of the isopleth point estimates.

isopleth(in, out, quantiles, [band], [poly]);

in		the input integer raster data source
out		the output line data source
quantiles		the quantiles (expressed as a proportion or percentage) at which to generate isopleths, e.g. 0.95 or c(0.5, 0.9, 0.95)
[band]		the input band (default=1)
[poly]		an output polygon data source (the isopleths are converted to polygons; refer to full documentation for details)

isopleth(in="C:\data\kde", out="C:\data\isopleths.shp", quantiles=c(0.5, 0.9, 0.95));

isopleth(in="C:\data\kde", out="C:\data\isopleths.shp", poly="C:\data\isopleths_poly.shp", quantiles=0.99);

isopleth(in="C:\data\kde", out="C:\data\isopleths.shp", quantiles=r.eval(seq(0,100,10)));