Understanding Digital Elevation Models (DEMs)

1. What is a Digital Elevation Model?

In the simplest terms, a Digital Elevation Model is a digital representation of the Earth's terrain. Unlike a flat, 2D map, a DEM contains height information, allowing us to model and visualize the landscape in 3D.

Technically, a DEM is a raster data layer. As you may recall from our earlier modules, a raster is a grid of cells (or pixels). In a DEM, each cell in the grid has a single numerical value that represents the elevation of that specific spot on the ground.

The key takeaway is that a DEM simplifies the continuous, complex surface of the Earth into a regular grid of elevation values. This simple structure is incredibly powerful and is the basis for most terrain analysis in GIS.

2. The DEM Family: Distinguishing DEM, DTM, and DSM

While "DEM" is often used as a general term, it's crucial to understand the distinction between two specific types of elevation models: the Digital Surface Model (DSM) and the Digital Terrain Model (DTM). This is one of the most common points of confusion for beginners.

To clarify this, let's watch a short, clear video.

Let's break down what you just saw:

• Digital Surface Model (DSM): This model represents the elevation of the Earth's surface including all objects on it. This means it captures the tops of buildings, trees, powerlines, and any other features. Think of it as the "first-return" surface that a sensor flying overhead would see.

• Digital Terrain Model (DTM): This model represents the bare-earth surface. All natural and man-made features (buildings, vegetation, etc.) have been digitally removed. This model shows you the shape of the ground itself.

In many contexts, the term "DEM" is used interchangeably with "DTM," but it's important to know the difference and to check the documentation for any dataset you use. The table below, adapted from the GEOPERA article, provides a handy summary.

3. How Are DEMs Created?

Now that we know what a DEM is, you might be wondering where the data comes from. Creating an elevation model requires methods that can measure depth or height, not just capture a 2D image.

The following video segment explains the most common modern and traditional techniques for acquiring elevation data.

Here's a summary of the main techniques you saw:

• LiDAR (Light Detection and Ranging): An aircraft fires millions of laser pulses at the ground and measures the time it takes for them to reflect back. This creates a highly accurate "point cloud" of elevation measurements, which is then converted into a raster DEM. LiDAR is often considered the gold standard for high-resolution DTMs because some laser pulses can penetrate vegetation to hit the ground.
• Stereo Photogrammetry: This technique uses two or more aerial or satellite images of the same area taken from different angles. By analyzing the parallax (the apparent shift in object position between the images), software can calculate the elevation and generate a DEM. This is similar to how our two eyes give us depth perception.
• InSAR (Interferometric Synthetic Aperture Radar): This radar-based method uses two radar images to measure phase differences in the reflected signals, which can be translated into elevation. A major advantage is that radar can "see" through clouds. The famous global SRTM (Shuttle Radar Topography Mission) dataset was created using this method.
• Digitizing Contour Lines: A more traditional method involves scanning old topographic maps and digitally tracing the contour lines. GIS software can then interpolate a continuous elevation surface from these lines.

4. Visualizing a DEM

A DEM is fundamentally a grid of numbers, which isn't very intuitive to look at. To make sense of it, we use various visualization techniques in GIS.

The final segment of the video we've been using demonstrates the most common ways to visualize DEMs. This will also serve as a great preview for our next lesson!

As you saw, the main visualization methods are:

• Color Ramp (Hypsometric Tinting): Assigning colors to different ranges of elevation values. Typically, greens are used for low elevations, yellows for mid-elevations, and reds/browns/whites for high elevations.
• Hillshade: Simulating how the terrain would look if it were illuminated by a light source (usually from the northwest). This creates shadows and highlights that give the map a 3D, textured appearance, making it much easier to interpret landforms.
• Contour Lines: These are lines that connect points of equal elevation. They can be generated directly from a DEM and are excellent for showing the steepness and shape of the terrain.

Conclusion

In this lesson, we have defined the fundamental concept of a Digital Elevation Model. You've learned that a DEM is a raster grid representing terrain height and that it's crucial to distinguish between a Digital Surface Model (DSM), which includes all features, and a Digital Terrain Model (DTM), which represents the bare earth. We also explored the primary methods for creating DEMs—like LiDAR and photogrammetry—and the key techniques for visualizing them.

Key Takeaways:

• A Digital Elevation Model (DEM) is a raster data structure where each cell value represents elevation.
• A Digital Surface Model (DSM) captures the ground plus all objects on it (buildings, trees).
• A Digital Terrain Model (DTM) captures only the bare-earth surface. The choice between DSM and DTM depends entirely on your application.
• DEMs are created using technologies like LiDAR, photogrammetry, and radar interferometry (InSAR).
• DEMs are visualized using color ramps, hillshades, and contour lines to make the numerical data understandable.

Preview of the Next Lesson:

Now that you have a solid understanding of what a DEM is, we are ready to put it to work. In the next lesson, we will generate and interpret terrain analysis outputs (slope, aspect, and hillshade) from a DEM. We will take a raw DEM and use GIS tools to calculate these important derivatives, which are essential for countless applications in geography and environmental science.