Digital elevation model


A digital elevation model is a 3D CG representation of a terrain's surface – commonly of a planet, moon, or asteroid – created from a terrain's elevation data. A "global DEM" refers to a discrete global grid.
DEMs are used often in geographic information systems, and are the most common basis for digitally produced relief maps. While a digital surface model may be useful for landscape modeling, city modeling and visualization applications, a digital terrain model is often required for flood or drainage modeling, land-use studies, geological applications, and other applications, and in planetary science.

Terminology

There is no universal usage of the terms digital elevation model, digital terrain model and digital surface model in scientific literature. In most cases the term digital surface model represents the earth's surface and includes all objects on it. In contrast to a DSM, the digital terrain model represents the bare ground surface without any objects like plants and buildings.
DEM is often used as a generic term for DSMs and DTMs, only representing height information without any further definition about the surface.
Other definitions equalise the terms DEM and DTM, equalise the terms DEM and DSM,
define the DEM as a subset of the DTM, which also represents other morphological elements, or define a DEM as a rectangular grid and a DTM as a three-dimensional model.
Most of the data providers. It is possible to estimate a DTM from high resolution DSM datasets with complex algorithms.
In the following, the term DEM is used as a generic term for DSMs and DTMs.

Types

A DEM can be represented as a raster or as a vector-based triangular irregular network. The TIN DEM dataset is also referred to as a primary DEM, whereas the Raster DEM is referred to as a secondary DEM. The DEM could be acquired through techniques such as photogrammetry, lidar, IfSAR, land surveying, etc..
DEMs are commonly built using data collected using remote sensing techniques, but they may also be built from land surveying.

Rendering

The digital elevation model itself consists of a matrix of numbers, but the data from a DEM is often rendered in visual form to make it understandable to humans. This visualization may be in the form of a contoured topographic map, or could use shading and false color assignment to render elevations as colors.
Visualizations are sometime also done as oblique views, reconstructing a synthetic visual image of the terrain as it would appear looking down at an angle. In these oblique visualizations, elevations are sometimes scaled using "vertical exaggeration" in order to make subtle elevation differences more noticeable. Some scientists,
however, object to vertical exaggeration as misleading the viewer about the true landscape.

Production

Mappers may prepare digital elevation models in a number of ways, but they frequently use remote sensing rather than direct survey data.
Older methods of generating DEMs often involve interpolating digital contour maps that may have been produced by direct survey of the land surface. This method is still used in mountain areas, where interferometry is not always satisfactory. Note that contour line data or any other sampled elevation datasets are not DEMs, but may be considered digital terrain models. A DEM implies that elevation is available continuously at each location in the study area.

Satellite mapping

One powerful technique for generating digital elevation models is interferometric synthetic aperture radar where two passes of a radar satellite, or a single pass if the satellite is equipped with two antennas, collect sufficient data to generate a digital elevation map tens of kilometers on a side with a resolution of around ten meters. Other kinds of stereoscopic pairs can be employed using the digital image correlation method, where two optical images are acquired with different angles taken from the same pass of an airplane or an Earth Observation Satellite.
The SPOT 1 satellite provided the first usable elevation data for a sizeable portion of the planet's landmass, using two-pass stereoscopic correlation. Later, further data were provided by the European Remote-Sensing Satellite using the same method, the Shuttle Radar Topography Mission using single-pass SAR and the Advanced Spaceborne Thermal Emission and Reflection Radiometer instrumentation on the Terra satellite using double-pass stereo pairs.
The HRS instrument on SPOT 5 has acquired over 100 million square kilometers of stereo pairs.

Planetary mapping

A tool of increasing value in planetary science has been use of orbital altimetry used to make digital elevation map of planets. A primary tool for this is laser altimetry but radar altimetry is also used. Planetary digital elevation maps made using laser altimetry include the Mars Orbiter Laser Altimeter mapping of Mars, the Lunar Orbital Laser Altimeter and Lunar Altimeter mapping of the Moon, and the Mercury Laser Altimeter mapping of Mercury. In planetary mapping, each planetary body has a unique reference surface.

Methods for obtaining elevation data used to create DEMs

The quality of a DEM is a measure of how accurate elevation is at each pixel and how accurately is the morphology presented. Several factors play an important role for quality of DEM-derived products:
Common uses of DEMs include:
The US Geological Survey produces the National Elevation Dataset, a seamless DEM for the United States, Hawaii and Puerto Rico based on 7.5' topographic mapping. As of the beginning of 2006, this replaces the earlier DEM tiled format.
OpenTopography is a U.S.-based community access source of a large amount of high-resolution topography data for the U.S.

DEM file formats