Digital Elevation Models: Building your Base

By Diana S. Sinton

One of the most widely used and valuable data sets that a GIS project can involve is a digital representation of the earth’s terrain, also known as a digital elevation model. Several major programs over the last almost-two decades have not only generated prodigious amounts of elevation data, but significant efforts have gone into post-collection processing to share high resolution data for great amounts of the earth. Two of these projects include data from the Shuttle Radar Topography Mission (SRTM) and the current 3DEP.

The SRTM involved modifying the Space Shuttle Endeavor for an 11-day trip in February 2000. By adding a second radar antenna at the end of a 60-meter-long mast attached to one of the shuttle’s wings, multiple different measurements were collected from returning waves as it passed over the earth’s surface. SRTM data does not exist for about 20 percent of the globe that is at high latitudes (below 56 degrees south or above 60 degrees north). More frustrating were the data gaps and inaccuracies at high elevations and in deep valleys. In the time since it was originally collected, however, additional and improved processing of the data continues to address many of these issues. Initially, release of the higher resolution data (1-arc second, equivalent at the equator to 30-meter grid cells) was limited to the United States, but there is now intent to make this level available wherever possible.  CGIAR’s Consortium for Spatial Information has become stewards of the data and distributes the global data set at 90 meters and a resampled 250-meter version, very useful for research and models covering large extents. 

3DEP, an abbreviation for the nation’s 3D Elevation Program, was launched in 2012 following an assessment that determined how enhanced elevation data could contribute billions of dollars in cost saving benefits each year. 3DEP relies mostly on LiDAR data as its source where possible, allowing for the creation of both highly accurate terrain surfaces as well as point clouds for 3D modeling. At this point, 3DEP has taken over where the USGS’s former National Elevation Dataset left off. The NED effort represented the traditional top-down government approach to producing data for the public, a model whose production and maintenance costs have become unsustainable. Instead, the extensive network of partners with which 3DEP connects helps to fund and set priorities for the data production program.  Current 3DEP data are available through the National Map Viewer.

USGS manages the land-based portion of 3DEP.  NOAA’s domain is the marine environments for the bathymetric counterpart to terrain data. From its Digital Coast platform, NOAA enables access to a range of elevation and bathymetric information. Each agency also includes the transition zones between coast and ocean in its domain, such as the USGS’s CoNED program.  In turn, NOAA supports the US Interagency Elevation Inventory, a program whose data viewer includes a particularly user-friendly interface for search and discovery of available bathymetric data. Though the website belongs to NOAA’s Office for Coastal Management, I was also able to find LiDAR data for the state of Iowa within a matter of seconds, which points out one of the issues within the world of digital geospatial data, for elevation or otherwise: There are almost always multiple places from which one can access and download the information. 

Redundancy allows people to use their preferred interface and to go to one agency’s site when another one is down, but it also engenders uncertainty: is this the most current and complete data? These data sets seem to be the same thing, but should I trust that? Are they the same format? Will one site require fewer hoops and button-clicks, making it faster? From which site can I most quickly go from the first browse to having the data added to my desktop software, ready to map and analyze?  Which sites are the easier ones for a novice to use, and for an expert to have the widest range of options?  

On a recent afternoon, I challenged a friend to a data download duel. This person, let’s call him C., is not a regular GIS user, at least not a full-blown desktop version. He does frequently use browser-based mapping programs and has familiarity with geospatial information in general. (He’s a marine geologist by training.) I was curious to see how user-friendly some of these sites were for an informed novice. We sat down at neighboring computers with a realistic-and-straightforward task to accomplish: to find, select, and download a DEM of Block Island, Rhode Island, from the National Map. He asked me what DEM stood for; I told him. I deliberately did not give any more clarification or specification than that, as I also wanted to know how he would go about teaching himself, or asking, about the possible resolutions available (1-arc second, 1/3 arc second, etc.). 

I was feeling rather confident in my own abilities, and decided that I’d even go one step further and add the extra step for myself to load the downloaded DEM into my mapping program — so I was pretty surprised when he hit the “done” timer first.  “Finished,” he said. “I have the 1/3 arc-second shapefile for Block Island.” Shapefile? A DEM in shapefile format?  Hadn’t I said DEM? Well, yes. C. had successfully found, selected, and downloaded the file with contour lines for Block Island, derived from the 1/3 arc-second DEM. Meanwhile, my own download was by then complete. We both had some format of elevation data for Block Island within about six minutes of when we had started.

Next step, how about bathymetric data for around Block Island?  I reset the timer and we both launched NOAA’s Digital Coast website on our respective machines. Ready, go.  By about the five-minute mark, I had found multiple possible data sets, though none of them seemed to be exactly what I wanted. I choose to err in favor of more being better, so like someone rushing around a grocery store with a free pass at the checkout, I loaded up my cart with a bunch of files and hit the Submit button. Then I needed to give them my email address. Then I needed to wait for an email. Then I needed to wait while the server processed my data sets — one at a time, for each of those data layers that I had impulsively added to my cart. It was another five minutes before I had my first email come through offering me a data download, and at some point one of the server messages suggested I might have to wait for “up to a day” for my request to be filled! 

Meanwhile, at the nine-minute mark, C. had completely given up on the NOAA website. He had moved onto GeoMapApp, his trusty go-to site for simple mapping tasks, and zoomed to Block Island. Three minutes later, he had bathymetric data from the General Bathymetric Chart of the Oceans displayed around Block Island, at 30 arc-second resolution.  The data he was displaying could be exported as an image or in GIS-ready grids. Yes, it was at a much coarser resolution than the one-meter geotiff of the area immediately surrounding Block Island that I eventually received, or the 800+MB of various other versions of digital topobathymetric imagery that I had stuffed into my NOAA cart. In my rush to win, I never managed to actually find what I would consider regular bathymetric data for the Atlantic near Block Island, or, clearly, I got what I asked for even though I’m still not sure what I should have found. Welcome to the world of publically available digital elevation data!  

Other digital elevation sites to keep in mind:

Published Wednesday, March 8th, 2017

Written by Diana S. Sinton

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