GME

Geospatial Modelling Environment

isopleth (Isopleths)

Generares isopleth lines based on a raster representing a probability surface

Description

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 higher-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.

Two types of polygon outputs are available: full polygons or donut polygons. For full polygons the smaller isopleth polygons will overlap (and be contained by) the polygons for larger isopleth values. In the case of donut polygons, only the portion of the polygon representing the interval between two isopleths is retained, and there are no overalapping polygons. Always specify different output polygon datasets for the poly and donut parameters (never write both to the same dataset).

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.

Syntax

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

inthe input integer raster data source
outthe output line data source
quantilesthe 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)
[donut]an output polygon data source to which the donut polygons are written (the isopleths are converted to polygons; refer to full documentation for details)

Example

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)));


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