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.
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.
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.
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.