Just a quick teaser post for our forestry/ecology readers out there.  I have a methodology developed for calculating McNab indices that directly corresponds with the field technique (unlike, as far as I know, any previous GIS-based techniques– which are probably adequate proxies).

What is a McNab index?  Well there are two kinds, the minor landforms and mesoscale landforms that are field-measured topographic position or terrain shape indices inform the location of ecological processes across the landscape.  So, for example, some plant forest types like ridges, and some like ravines.  The question is, quantitatively, how raviney or ridgey should it be for a given species, association, or alliance, and how is it measured?  Basically either average angle to horizon, or average angle to the local landscape, e.g. 10m away are the two McNab indices.  See i.e. http://www.treesearch.fs.fed.us/pubs/1150 and http://www.treesearch.fs.fed.us/pubs/24472.

So here’s the output (more to come), including code.  The darker the shade, the lower the relative position, the lighter the shade, the higher the valley, e.g. ridges and planes.  I know, it just looks like a hillshade, but there’s deeper stuff happening here: