How Satellites Capture DEM Data

 Satellites capture Digital Elevation Model (DEM) data through sophisticated remote sensing techniques that measure the height of the Earth’s surface from space. These measurements are not direct visual images but are derived from differences in viewing geometry, signal return times, or phase shifts—depending on the sensor technology involved. The two primary methods used by satellites to capture elevation data are stereoscopic imaging (photogrammetry) and radar interferometry (InSAR). Both are highly effective, but they function based on different principles and are suited to different types of terrain and accuracy requirements.

In stereoscopic imaging, the satellite captures at least two images of the same area from slightly different angles. This is typically done either by using a pair of cameras mounted with a known baseline (in the case of satellites like SPOT or Cartosat) or by capturing the same area at different times during a satellite's orbit. This method mimics human depth perception, where the distance between our eyes allows us to perceive depth by comparing differences in the two images. Using photogrammetric principles, software algorithms calculate the displacement of features (called parallax) between the two images and convert that into elevation data. This technique is especially effective over terrain with strong texture—like mountains or valleys—where parallax is easier to detect.

Another highly advanced technique is Synthetic Aperture Radar Interferometry (InSAR), which is used by missions such as the Shuttle Radar Topography Mission (SRTM) and TanDEM-X. In this method, radar waves are emitted from the satellite to the Earth’s surface and reflected back. When two radar images of the same location are captured from slightly different positions, the phase difference between the radar signals can be calculated. These phase shifts correspond to differences in elevation and are used to construct highly accurate 3D terrain models. Unlike optical methods, radar interferometry has the advantage of penetrating through clouds and capturing elevation data at night or during adverse weather, making it ideal for consistent global elevation mapping.

Some satellites, like ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer), also use stereo imaging from optical sensors mounted on a single platform to create global DEMs, particularly in mountainous and rugged regions. While not as precise as LIDAR or radar methods, these optical systems are still widely used due to their availability and moderate accuracy.

It's important to note that while satellites provide raw data, the processing and modeling of DEMs are just as crucial as their capture. Raw elevation data must undergo geometric correction, gap filling, noise reduction, and often interpolation to become usable as a DEM in GIS platforms. These final DEMs are usually delivered in raster formats such as GeoTIFF or IMG files and are georeferenced to global coordinate systems like WGS 84.

Overall, satellite-based DEM acquisition has enabled large-scale, consistent, and repeatable elevation mapping of nearly every landmass on Earth. These datasets are critical for applications in topographic modeling, climate change studies, and hazard risk assessment. Understanding how satellites capture this data is fundamental for any GIS professional working with elevation models. Learners can get hands-on experience working with DEMs derived from SRTM and other satellite missions in the Complete Remote Sensing and GIS - ArcGIS – ERDAS course, which includes practical exercises on visualizing and analyzing elevation data captured from space.