Terrain Relief Models, Unabridged

How To
3

Let there be data!

Collecting tools (software) may be interesting, but the real project starts with selecting your stock (data). For terrain relief models, what you need is elevation data. Additionally, if you would like to engrave trails onto your terrain relief models (or just carve companion maps), you’ll need vector data for those as well.

Acquiring Elevation Data

  • If your area of interest spans continents, you need nothing more than low-resolution data. Download the appropriate section of 250m resolution resampled SRTM data in Esri ASCII format (or GeoTIFF, but it’s not my personal favorite).
  • If your area of interest is reasonably large, 30-meter resolution SRTM data may be adequate. Download the appropriate tiles from the 30m resolution SRTM data in Esri ASCII format (or GeoTIFF, if you insist).
  • If your area of interest in the US, you’re living the dream. Free and unfettered high-resolution elevation data is readily available. Complete coverage in 1/3 arc-second (about 10m resolution) is available, with some areas in 1/9 arc-second (about 3m resolution) and select areas in one meter resolution. See the next section, “Acquiring USGS Elevation Data”.
  • If you have access to digital elevation model data from other sources, you should be able to load it into QGIS. After all, you can load almost any GIS data into QGIS.

Note: If you’re not using USGS data, some other datasets may require a license for commercial use or may have other restrictions. That is outside the scope of this documentation.

Acquiring USGS Elevation Data

Go to The National Map’s download system: https://viewer.nationalmap.gov/basic/

Use the right side of the window, above the map, to find your area of interest. If you want precision, one way is to use Google Earth (or anything else) to get the north/south latitude limits and east/west longitude limits. Then choose the Coordinates option here and enter your four values as the pair of latitudes and longitudes. A less precise but simple alternative is to choose Box/Point and draw a box. Either way, the map will zoom to a highlight box showing the area you’ve specified.

On the left side, click Elevation Products (3DEP) under Data. Now use the Show Availability links to find the best data available. If you want the very highest resolution available, start with 1 meter DEM, and if your area is covered, it will be shaded. If you’re not covered, try 1/9 arc-second DEM next, as it’s more or less a third the resolution of 1 meter DEM. If you’re still not covered, try 1/3 arc-second DEM, which basically covers the entire US with something approximating 10m resolution. Some of the formats only provide IMG output, but if you do have an option for File Format, you will want to explicitly choose IMG (Erdas Imagine Image), as it will be the most convenient.

If you’re making a relief map of a large area and using a 1/4" ball nose mill, you probably don’t want to bother with the 1 meter DEM data. You don’t need that much detail, so why would you want to download gigs of data you’re just rounding off. On the other hand, if you’re wanting to make a model of just one smallish mountain and you have 1-meter coverage, rejoice! I usually start with the best data available, since hard drive space is cheap and download speeds aren’t bad these days.

With your search area highlighted and your preferred DEM variety chosen (as an IMG file), now click the Find Products button to do your search. On each result, you can click the Footprint and Thumbnail links in the Actions column to see what that file covers. Choose a data set that covers your selected area, or as many as it takes to cover everything. If your area is split across multiple data sets, you’ll combine the data in QGIS later. Download the data, and extract the zip file or files to a folder of your choosing.

Acquiring Trail Data

QGIS can handle trail data from many sources. OpenStreetMap data may be a particularly useful source for some things – you can use the OpenStreetMap Export page if your area of interest is reasonably sized. I have also had excellent results for some projects by simply tracing trails, roads, or waterways in Google Earth. Most of the terrain relief models with trails that I’ve made have been either National Park Service or Forest Service areas, and large trail datasets for both of those are readily available. Note that both the datasets are vast , and you’ll definitely want to clip just the portion you’re using in a project to make things more efficient. I’ve successfully used them in Google Earth Pro and QGIS 3, but it does take a noticeable few moments to load the full data.

National Park Service

National Forest System Trails

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