Have your Hillshade and Elevate it too

For years and years we’ve been using Shaded Relief a.k.a Hillshade images in order to visualize and assess the qualities and deficiences of elevation models. It’s hands down the quickest and most comprehensive method for humans to get a feel for the shape of the land and the relative accuracy of a given model, but it’s not good at interrogating elevation itself.

TL;DR: use “return raw value” on ‘Properties >> Processing Templates’ tab to have Identity report Elevation instead of colour.

Context

Take this example of the Yukon River near Carmacks, looking at the elevation layer with Standard Deviations stretch applied. There are flaws in this elevation model, but a person without signifcant experience in interpreting models combined with local knowledge is unlikely to note them. The white splotchy areas look questionable and what is probably a seam line down the right side.

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With a hillshade applied it quickly becomes clear that the ghostly white area is actually ok, the right vertical is confirmed a seam line, and that there is an additional invisble seam line in the center, plus hitherto invisible problems with the water surface (the triangles).

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Hillshades suck at data interrogation though. The Identify tool reports the rendered pixel value (the colour) at the point of interest, when what we most likely want is the elevation:

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Solution

There is a cure. When the source is an Elevation Service or Raster Mosaic Dataset (as distinct from a single raster file on disk) there is an additional properties tab called “Processing Templates”. At bottom of that tab see “Return raw pixel values when using the identify tool” checkbox.

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[Later-2] I didn’t know raster functions could be stored as templates. That’s very cool! And solves one of the itches I’ve had for years: to have Hillshades for viewing, but have Identify return the Elevation value instead of the rendered colour value. Yay. Smile

Closing

Thanks to Ryan Gould (Geomatics Yukon) for tipping me off to this new-to-me feature this morning!

There is also the “Use hillshade effect” of the Symbology tab, which also allows human-accessible visualization and have Identify return elevation values. It’s effects are heavy handed relative to a true hillshade and can make things appear worse than they are. It does have the benefit of being available for any raster source though.

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Further Exploration

The elevation layer portrayed is “DTM_LiDAR_YukonAlbers”, available from Geomatics Yukon.


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