Define Sources of digital elevation models.

Digital Elevation Models (DEMs) represent the topography of the Earth's surface in digital form, providing information about elevation variations across landscapes. Various sources contribute to the creation of DEMs, utilizing diverse technologies and data collection methods. Here are key sources of digital elevation models:

1. Satellite and Airborne Remote Sensing:
   Remote sensing platforms, such as satellites and aircraft equipped with LiDAR (Light Detection and Ranging) or synthetic aperture radar (SAR) sensors, capture elevation data. LiDAR, in particular, is highly accurate for terrain mapping, as it measures the time taken for laser pulses to return after hitting the Earth's surface.

2. Spaceborne Stereo Imagery:
   Satellite systems with stereo capabilities capture overlapping images of the Earth's surface. By analyzing the parallax between these images, scientists can derive elevation information and generate DEMs. Examples include the WorldView and Pleiades satellite constellations.

3. Global Navigation Satellite Systems (GNSS):
   GNSS, such as the Global Positioning System (GPS), can contribute to elevation data through ground-based measurements. Survey-grade GNSS receivers provide accurate elevation information, and when combined with other techniques, contribute to high-precision DEM creation.

4. Topographic Maps and Cartographic Data:
   Traditional topographic maps, which represent elevation using contour lines, can be digitized to create DEMs. Additionally, cartographic datasets containing elevation information, like those produced by national mapping agencies, contribute to DEM creation.

5. Stereophotogrammetry:
   Stereophotogrammetry involves interpreting overlapping pairs of aerial photographs to create three-dimensional terrain models. By analyzing the displacement between corresponding points in these photos, elevation data is derived and used to generate DEMs.

6. In Situ Surveys and Ground-Based LiDAR:
   Ground-based LiDAR systems mounted on tripods or vehicles capture detailed elevation data for specific areas. In situ surveys, including ground-based measurements, provide accurate elevation information for validation and calibration purposes.

7. Open Data Platforms and Crowdsourcing:
   Open data initiatives and crowdsourcing platforms allow for the collection and sharing of elevation data. Projects like OpenStreetMap incorporate volunteered geographic information (VGI), including elevation data, to contribute to global DEM datasets.

8. Ice and Snow Mapping:
   Specialized satellite missions, like those dedicated to monitoring ice and snow, provide elevation information for polar regions. These datasets contribute to understanding climate change impacts on ice sheets and glaciers.

9. Digital Terrain Models (DTMs):
   DTMs focus specifically on the bare Earth terrain, excluding vegetation and other above-ground features. These models contribute to applications such as hydrological modeling, landform analysis, and infrastructure planning.

10. Interferometric Synthetic Aperture Radar (InSAR):
    InSAR utilizes radar signals from satellite or airborne platforms to measure the surface deformation over time. This technique is particularly useful for monitoring subsidence, tectonic movements, and other elevation changes.

The integration of data from these diverse sources allows for the creation of comprehensive and accurate DEMs, supporting applications in various fields such as geology, hydrology, environmental modeling, and infrastructure planning.