raster_chunk_processing.py Examples

This is part 3 of a three-part post. Read part 1 (Introduction) here and part 2 (Installation and Usage) here.

Ok, enough dry words. Let's see the goods!

These are all clips of the Grand Canyon of the Yellowstone River/Dunraven pass area from the USGS 1/3rd arc-second DEMs. For the smoothed DEMs, I created a hillshade with the gdaldem hilllshade -combined command. The elevation shade is the North_America_State_Utah_2 ramp from the ESRI Terrain Tools download.

Original Hillshade

Mean Radius 10 Hillshade

The simplest smoothing is averaging the value of the nearby pixels. This gets rid of noise, but also gets rid of the ridges and valleys. The sharp brown line is an error from importing the Terrain Tool's ArcMap styles into ArcGIS Pro.

Gauss Radius 10 Hillshade

A Gaussian kernel does a better job at smoothing away the high-frequency noise (ie, small sudden changes in elevation) while preserving the broader changes. Note the smoothing in the sides of the canyon, and the dampening of the contour-like artifacts in the gulleys.

mdenoise Hillshade

This is an aggressive smoothing setting; a similar effect might be possible with a higher-radius Gaussian blur.

Skymodel Hillshade

Behold the bold shades of the skymodel (using the original, unsmoothed DEM). With such tweakability available, this is but one possible output from this DEM. Much experimentation is needed to see if it's worth using for your data. Note that this particular output can be approximated by a simple slope calculation, with a white-to-black, shallow-to-steep-slope color ramp

CLAHE
This is a CLAHE stretch on the unsmoothed DEM's hillshade with a radius of 30 and a clip of 0.01. CLAHE gives a slightly different look than what's possible with the contrast stretcher tools in ArcGIS. It also permanently alters the file, which can be useful for bump-mapping imagery with a hillshade.

TPI 10
TPI gives different looks depending on the radius you use. It's usually symbolized with a green-yellow-red color ramp, but I like to use it as a layer in a terrain stack (set partially transparent, it can help highlight some of the more exciting terrain areas) either as is or hillshaded. This is the raw output from a kernel radius of 10.

TPI 10 Hillshade
Here's the raw hillshade from the TPI 10 dataset. Definitely not something useful on it's own, what it's really showing is channels and ridglines (which is what the raw TPI is often used to identify).

TPI 10 Combined
And here's the TPI 10 hillshade at 75% transprency layered on top of a smoothed skymodel hillshade. Look at how well-defined the draws and ridge lines are; this could make for a great basemap for a stream map.

TPI 30 Hillshade
We can get a slightly different look just by increasing the kernel radius to 30. We lose the sense of height in Mount Washburn, but the ridge lines really stand out.

Default Hillshade
Here's the default hillshade (without the -combined option) just to remind us of where we came from. Quite the snoozer, if terrain is your thing. Also note how the massive amounts of grey midtones muddle the colors of the elevation ramp. This is particularly abhorrent when laying it over imagery.