July 25, 2018

A “duct tape and WD-40” approach to digital topography and geologic mapping

Posted by larryohanlon

By Philip S. Prince

Google Earth is amazing, but sometimes tree cover and land use obscure subtle but significant surface features critical to mapping and interpretation. Digital elevation and terrain models provide a way around this, but without ArcScene, SketchUp, or a similar program they lack the 3-D reality of Google Earth in oblique perspective. Addtionally, hillshade terrain models can produce an inversion effect for observers when seen in plan view, making it impossible to distinguish highs and lows. No elevation information is communicated in pure hillshade, so some spatial relationships within the clearly represented topography remain unclear. They still communicate more information than the forested Google Earth image below, but this image is less likely to produce inversion.


Not much to see here…do you see the blockslide with the really cool anticlinal toe?


The exact same perspective with hillshade overlay. The slide mass is at top center. It is a very thin “sheet;” it’s 10m thick at most, and is entirely invisible on topo maps and even Google Maps Terrain.


The folded toe is particularly nice. I haven’t been to this feature yet, but it’s on the short list.

The above images represent my approach to getting the most out of Google Earth and ArcMap. As a user of an ailing, state-supplied computer that struggles to run anything beyond the most basic ArcMap applications, I have attempted to combine the best of Google Earth and DTM in a way that appeals to my “Xennial” discomfort with most software. By producing and exporting basic hillshades in ArcMap, trimming them in Photoshop, and overlaying them as spatially-referenced .kmz’s in Google Earth, I end up with a fast-opening and easy to use platform to combine top-notch digital topography with Google Earth. The ability to fade the overlay to reveal satellite/air photo imagery below is a key aspect to this approach. I can immediately correlate features only visible in LiDAR DTM to actual land surface features that can be used as references when I’m on the ground in the field.


The Green “stripe” down the ridge at center is large landslide deposit, but this leafy imagery leaves something to be desired.


Same view with overlay. The slide is a bit more apparent here.


Individual sandstone beds as well as the failure plane are easy to see.

Panning, tilting, and fading a .kmz in Google Earth is so much more efficient than manipulating plan-view DTM that it speeds up field work preparation significantly. Surficial interpretation obviously benefits from this approach, but structural work can also be enhanced. Stratigraphic distinctions between age-correlative intervals on different thrust sheets are not always clear in areas of limited outcrop, but digital topography can provide a useful perspective.

The Cove Mountain “2” from a few weeks back is fault-separated from Catawba Mountain, which is supported by the same topographic interval. Whether the fault between them is minimally displaced or a high displacement structure cannot be determined from structure alone. The slide from above is visible at the bottom of the image, right of center.

You could count individual beds in this overlay…but just looking at overall patterns is better. The Cove Mountain stratigraphy is somewhat different from Catawba Mountain. Can you see it?


Carbonate and shale intervals distinguish Cove Mountain from Catawba Mountain. These rocks weren’t next door neighbors prior to deformation.


Screenshots from tilted Google Earth overlays are also a nice foundation for block diagram figures that meld surface imagery, geology, and cross sections. With attention to some basic drawing techniques, you can make an attractive figure from a simple JPEG screenshot.

This diagram was made in Adobe Illustrator (PowerPoint works too) from an oblique Google Earth image, an overlay of a 1:24K map I made last spring, and a cross section I produced for the same map.