Topographic Position Index and Ecological Land Type (warning completely unrefined not quite Geologic dribble– with bad maps :) …)
Posted by smathermather on January 10, 2010
Warning. What follows is somewhat informed, but I’m no geologist. I just play one on wordpress.
Understanding the basic underlying geology and associated topography plus site history helps us achieve a basic understanding of a sites ecological potential. At the most basic level, we expect different wildlife and vegetation dynamics in a floodplain vs. a mountain ridge. Classification of digital elevation models can be done with a concept called topographic position index (of which Jensen Enterprises has a pretty good explanation).
I’ve been playing with TPI for two areas in the Allegheny Plateau in Ohio– one in an unglaciated portion, and one in a glaciated portion. The Allegheny Plateau developed as the once very tall Appalachians wore down to the nubs they are today. That plateau has since eroded into little hill-like remains, with stream valleys redefining a hill and hollow landscape, and most of the ridge-tops being near the same elevation. In the glaciated portion, this is overlayed with glacial till, a re-flattening of these “hilltops”, and the definition of gorges in places where sub-glacial rivers subdivided the terrain.
The TPI attempt I did for the unglaciated portion roughly defines the ridge tops in red, stream valleys and slope bases in blue, and slopes as yellow/yellow green. The map that follows is the overlay of TPI performed in gdaldem at multiple scales, but we are somewhat limited in what we can do with gdaldem, because we can’t control our window size.
Then I performed a proper TPI analysis at 80 feet and 300 feet for a glaciated portion of the plateau in Northern Ohio, following Jensen’s guidlines. What follows is a photo of a printed map, cause I forgot to bring an jpeg of pdf home… . Here, upland flat portions are dark green. They are adjacent to light blue areas that are also upland, but exceed 5% slopes. We can see upland areas adjacent to cliff areas as well as peaks in red, lower areas of high relief in orange, gorge valleys in dark blue, U-shaped valleys in a lighter blue (stream polygons and lines are also overlayed, adding to color confusion– hence the warning in the title).
What intrigues me about this map are the areas around the streams just northwest of the center of the map. Capturing the gorges and upland areas successfully was not a surprise. What is surprising is that we capture and map the areas that seem to me likely contribute directly to surface flow into the streams as distinct areas from the upland plains. Next, I think I’ll compare this analysis with the soil map for the area, as well as the detailed vegetation cover we have for the area.