Digital Elevation Models & their Applications in
Geological Studies

 Digital Elevation Models

A Digital Elevation Model (DEM), also referred to as the Digital Terrain Model (DTM) is a digital representation of earth's topography, i.e. an elevation map. DEMs can be used to derive topographic attributes, geomorphometric parameters, morphometric variables or terrain information in general. In combination with other spatial data, digital elevation models are an important database for topography-related analyses or 3D video animations (e.g. fly-throughs). Different georeferenced 3D products can be derived and complemented by a coordinate system and presented in a 2D-map projection or as a 3D perspective view.

The process of quantitatively describing terrain is known as Digital Terrain Analysis (DTA. Common synonyms arc geomorphological analysis, landform parameterization and land surface analysis. A distinction must be made between DTA and the term Digital Terrain Modelling, which also refers to the generation of terrain data.

In order to avoid confusion of terminology, we use the term DTM to describe a set of interpolation/filtering techniques used to derive the topographic surface, and the term DTA for a set of techniques used to derive terrain parameters. Note that these are arbitrary definitions, which might differ from the other literature.  The following terminology is commonly used:

  • DEM – Digital elevation map, i.e. representation of the Earth's surface topography.

  • DTM – a set of techniques used to derive or present a DEM.

  • DEM filtering – a set of techniques used to improve the geomorphic resemblance of a DEM.

  • Terrain analysis or parameterization – Terrain parameterization is a set of techniques used to derive terrain parameters from a DEM, i.e. a process of quantifying the morphology of a terrain. Terrain analysis (DTA) is used as a general term used for derivation of terrain parameters and their application.

  • Terrain parameter – Parameter (maps or images) derived from a DEM using DTA, e.g. slope.

  • Topography or relief – The shape or configuration of the land, represented on a map by contour lines, hypsometric tints, and relief shading.

DEM data sources

At present, there are five main sources of the elevation data:

  1. Ground surveys;

  2. Airborne photogrammetric data capture;

  3. Existing cartographic surveys (e.g. topographic maps);

  4. Airborne laser scanning and

  5. Stereoscopic or radar-based satellite imagery.

These DEM collection methods can be compared considering four aspects:

(a)    price

(b)   accuracy

(c)    sampling density and

(d)   pre-processing requirements

Traditionally, elevation data has been collected by land-surveyors from ground surveys or by semi-automated digitizing using stereoplotters. This is the most accurate but also the most expensive data collection method. The most recent developments consider automated stereo-image matching, use of laserscanning and remote sensing imagery, either with stereoscopic overlap (SPOT, ASTER) or interferometric imagery. Note that in the case of elevation data derived from the remote sensing sources, the sampling density is closely related to the ground resolution.

From the above-mentioned techniques, laserscanning seems to be the most accurate method with the highest sampling density. Moreover in the case of laserscanning, both object surface and ground surface can be recorded, so that the elevation data is better defined as the Digital Surface Models (DSM). A comparison of several elevation surfaces can then be used to map three heights or estimate volume of objects. Laserscanning has already been applied for mapping buildings, power lines, open pits, surface textures and even waves in the water. The second highly cost-effective new technique is the airborne and spaceborne interferometric radar system, which can be used to accurately derive both the land cover and terrain data. Typical elevation Root Mean Square Error RMSE(z), achieved with the use spaceborne interferometric images ranges from few to ten meters.

DEMs are increasingly becoming available in the market today. Many countries already provide elevation grids at course resolutions (> 250 m) and at a commercial price. Free source of elevation data with the global coverage is the global digital elevation map with a horizontal grid spacing of 30 arc seconds, which is approximately 1 x 1 km. It is derived from several raster and vector sources of national topographic information and is available via the website of the US Geological Survey.

In February 2000, the Shuttle Radar Topography Mission (SRTM) radar system gathered topographic data over approximately 80% of the land surfaces of the Earth, creating the first-ever near-global data set of land elevations at 1 arc second (about 30 meters) and 3 arc seconds (about 90 meters) ground resolutions. In the USA, this data have been released to the public and are available at the US Geological Survey's EROS (Earth Resources Observation Systems) Data Center for download via FTP (File Transfer Protocol). Data for areas outside the USA is available at a resolution of 90 m and can be downloaded via FTP from the Global Land Cover Facility at www.landcover.org.

DEM data structures

In a GIS environment, a DEM is commonly modeled and visualized using two main data structures:

1.      Rectangular grid or elevation matrix (GRID) and

2.      Triangulated Irregular Network (TIN).

The GRID DEM is typically stored as a raster map (or image), where each pixel carries the information on elevation or terrain parameter. The TIN DEM is based on the triangular elements with their vertices at the sample points. The advantage of TIN DEM compared to the GRID DEM is that it can incorporate structural features such as peaks, slope breaks and conic pits, and by some is considered a more accurate structure for terrain parameterisation especially when contour data is used. Although the gridded DEM-data model is non-adaptive and commonly over-samples in low-relief areas and undersamples in high-relief areas, it is somewhat more attractive than the TIN DEM due to its simple data structure and high possibilities of GIS operations. It is easier to manipulate, process and integrate it with other GIS data, especially in the DTA applications and has been used as the primary structure in ILWI5 and other similar GIS packages.

Availability of Digital Elevation Data:

One of the most practical and valuable returns from the United States space programme is the SRTM digital elevation model. Until the production of the SRTM DEM, good-quality measurements the Earth's surface at practical levels of detail did not exist or were not generally available for much of the planet. SRTM was developed at NASA's Jet Propulsion Laboratory (JPL) as a joint venture of NASA, the United States National Geospatial Intelligence Agency (NGA), and the German and Italian Space Agencies. The mission collected 12 terabytes (1012 bytes) of data over nearly all of Earth's landmass between 60°N and 56°S in just 11 days in February 2000. Elevation measurements were derived from interferometric analysis of the C-band radar signal and were processed at JPL. The resultant DEM has 1 arcsecond (c. 30 m) postings, with an absolute vertical resolution significantly better than the mission specification of 16 m. The SRTM DEM is now freely available (at a somewhat reduced effective resolution for non-US areas). However, the DEM is not spatially comprehensive.  It did not cover areas within 30° latitude of the poles and, more troublesome for most users, it has substantial gaps (voids) where the radar interferometric system failed to provide a signal adequate for DEM generation.

Meanwhile, generally coincident with the SRTM Project, but continuing to 2010 and beyond, ASTER has been acquiring imagery across all areas of the planet up to within 8o latitude of the poles.  ASTER is one of the sensors operating on Terra, a satellite launched in December 1999 as part of NASA' Earth Observing System (EOS).  The ASTER Project is a co-operative effort between NASA, Japan's Ministry of Economy, Trade and industry, and Japan's Earth Remote Sensing Data Analysis Center. ASTER covers wide spectral region with 14 bands from visible to the thermal infrared, with high spatial, spectral and radiometric resolution. The spatial resolution varies with wavelength: 15 m in the visible and near infrared (VNIR 0.55-0.80 ΅m); 30 m in the short wave infrared (SWIR 1.65-2.4 ΅m); and 90 m in the thermal infrared (TIR 8.3- 11.32 ΅m). An additional band is the key to producing digital elevation models. This band (named 3B) is the same as nadir band 3 (NIR), except that it observe at a backward angle of c. 28°, producing a store pair for each daytime ASTER image. Each ASTER scene covers an area of 60 x 60 km, and the sensor has up to 8.55° of pointing capabilities. Standard DEMs produced by the United States Geological Survey Eros Data Center (USGS-EDC) have 30 m postings, similar to SRTM's 1 arcsecond posting.  However, users can also produce their own DEMs from the band 3 stereo pair using any chosen software. ASTER DEMs are comparable in resolution to those from SRTM. However, potential improve ments are still possible since the DEMs do not capture all of the topographic detail that is visually apparent in the stereo imagery.
 

Geological Applications of DEMs 

Exact information about the Earth’s surface is of fundamental importance in all geosciences. Topography exerts control over range of Earth surface processes (evaporation, water flow, mass movement, forest fires) which are important for energy exchange between the physical climate system in the atmosphere and the biogeochemical cycles at the Earth surface. Ecology investigates the dependencies between all life forms and their environment such as soil, water, climate and landscape. Hydrology draws upon knowledge about the relief of ground surface to model the movement of water, glaciers and ice. Geomorphology describes the relief, recognizing form-building processes.  Climatology investigates fluxes of temperature, moisture and air particles – all influenced by topography.

Another area of application of DEMs is the global land cover classification. Precise mapping and classification of the Earth's surface at a global scale is the most important prerequisite for large-scale modeling of geo-processes. It has been demonstrated through numerous studies that radar images are suitable for documentation and classification of natural vegetation and agricultural areas. In remote sensing DEMs are used to correct images or retrieve thematic information with respect to sensor geometry and local relief to produce geocoded products. Thus, for the synergic use of different sensor systems (and GIS), digital elevation model are a prerequisite for geocoding satellite images and correcting terrain effects in radar scenes.  This is detailed in the following section.

Applications in Topographic Studies:

Topography is a graphic representation of natural features of the earth's surface including hills, valleys, rivers, lakes and other similar features. Typically, topography is drawn on maps and charts or as shaded relief. These methods of displaying relief are, however inadequate in that they do not give any information on the elevation above sea level of all points on the map or how steep the slopes are. The knowledge of surface topography is of major importance to Earth sciences. It is essential in any discipline concerned with process modeling like hydrology, climatology, geomorphology and ecology. It is also a prerequisite for many applications in civil and military agencies and in industrial areas like telecommunications (specifically, radio wave propagation), navigation, hydrology, disaster management (prevention, relief, assessment), transportation and infrastructure planning.

Today, the techniques of radar interferometry with Synthetic Aperture Radar systems (SAR) and laser interferometry (LIDAR) are currently the most advanced technology and the most effective way of acquiring topographic information. It is independent of cloud covers, sun illumination and the contrast of the Earth's surface.

Applications in Remote Sensing and Topographic Mapping:

DEMs have many applications in remote sensing and mapping, such as topographic mapping (contours), thematic mapping, orthoimage generation and image analysis, map revision, and so on. To make images useful as backdrops for other thematic information and base maps, it is desirable that the images have characteristics similar to those of maps. This means that the same scaling, orientation, and projection into a geo-referencing system should be adopted.

Remote sensing images, either satellite or aerial images, do not have such good characteristics due to the distortions caused by the imperfections of camera or scanner systems, the instability of platforms (tilts and flying height variations), atmospheric refraction, the earth’s curvature, and terrain height variations. The two most serious factors are the instability of the platform and terrain height variations. Therefore, geometric rectification is required.

Use of DEMs in Geological Mapping:

DEMs are important in providing valuable geological information that can be used as a guide in defining the geology of a given area. Geological structures and rock unit boundaries showing a strong correlation with relief can be mapped with detailed topographic analysis. Digital Elevation Models (DEMs) are the most suitable tools for such kind of analysis because they yield an accurate representation of relief and can be processed with computers. Using DEMs, topographic attributes (elevation, slope, etc.) are easily quantified and can be displayed as output images called DEM derived surfaces. Through these images, DEMs display the relationships between topography and geology.

Although DEMs are currently being used for describing geological features related to geomorphology, hydrology and tectonics, they still have not become a common tool in geological mapping projects.

Of particular importance to geological mapping are DEM derived surfaces.  The following functions have been found to be most useful in depicting geological information:

1.  Slope – Displays the grade of steepness expressed in degrees or as percent slope. This image can reveal structural lineaments, fault scarps, fluvial terrace scarps, etc.

2.  Aspect – Identifies the down-slope direction. Aspect images may enhance landforms such as fluvial networks, alluvial fans, faceted fault related scarps, etc.

3.  Shaded topographic relief or hill-shading – This image depicts relief by simulating the effect of the sun's illumination on the terrain. The direction and the altitude of the illumination can be changed in order to emphasize faults, lineaments, etc. This image is probably the most useful to display geological data related to landforms in terrains that show a close correlation between geology and topography.

4.  Flow direction – Shows the direction of flow by finding the direction of the steepest descent or maximum drop. This DEM derived surface depicts the drainage.

5.  Basin – Function that uses a grid of flow direction (output of flow direction) to determine the contributing area.

To summarize, the science community, for example, employs DEMs for research on

•         Climate impact studies

•         Water and wildlife management

•         Geological and hydrological modeling

•         Geographic information technology

•         Geomorphology and landscape analysis

•         Mapping purposes and

•         Educational programs

Applications in Mineral Exploration & Water Resources:

Digital elevation models deliver basic information on geologic structures. These information sources are especially important in remote areas where coverage by topographic maps is limited. Exploration geologists are possibly the most experienced users of digital elevation models and multipectral remote sensing data. By analyzing digital elevation models they determine promising regions of potential mineral deposits which find an expression as a topographic prominence or depresion, or placer deposits found along stream channel. More and more, a combination of remote sensing data, especially DEMs, with gravity maps the identification of oil spills on satellite imagery and other phenomena and combinations leads the prospecting companies to successful explorations. In addition to exploration activities digital elevation models are also used for monitoring the exploration consequences. The problems of subsidence in mining regions, for example, can be studied and evaluated by the use of DEMs.

DEMs are used to extract information relevant to ground water studies such as potential areas or groundwater recharge or contamination. Information on hydrological patterns, lineaments, morphological structures, tectonic and sedimentological anomalies can also be extracted and merged with optical satellite data and available topographic and geological maps. DEMs can be used to search for promising groundwater bearing areas and to designate locations of wells. High resolution digital elevation models can deliver much needed additional information for the interpretation of the ground water relevant structures and catchment areas.

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