Digital Elevation Model (DEM) is a numerical representation of the Earth’s surface that contains actual height points representing the topography, as well as the method to calculate elevations between the height points. Typically, DEM is stored in a data system as a regular grid or a triangulated irregular network (TIN). In a discussion related to DEMs, common terms are Digital Terrain Model (DTM) and Digital Surface Model (DSM).
Digital Terrain Model (DTM) is a model that represents the Earth’s surface together with other topographic information, such as data about land cover, slopes, and aspects of the terrain. The most important element of DTM is DEM.
Digital Surface Model (DSM) is a concept that has become common due to widespread use of airborne laser scanning. It means a model that represents the highest elevation of the terrain. Thus, DSM represents the Earth’s surface only in open areas, while in other regions the model follows the forest canopy and the roofs of buildings.
Digital Elevation Model (DEM) is an invention that has liberated gathering and storing of elevation data from the principles of traditional cartography. Before the diffusion of using DEMs, country-wide elevation data was stored on the contour lines in printing plates and paper maps. While contours are still a valid method for visualising topography, from the perspective of data storage, they have two deficiencies:
- Contours are a non-continuous representation of the terrain, in which the surface forms between the selected contour interval are unknown.
- Generation of contours is conditioned by visualisation, which enforces the use of cartographic generalisation rules. In this process, some details of the topography are removed, while other forms are intentionally over-emphasised.
Nowadays, since airborne laser scanning has become general in DEM production, storage and visualisation of elevation data are truly separated for the first time in the history of cartography.
Figure 1. Glaciofluvial ice-marginal formation and littoral deposits at the First Salpausselkä, Lohja, as seen on the (a) topographic map, (b) DEM25, and (c) DEM2 by the National Land Survey of Finland.
Figure 2. Ravines on clay deposits in Siuntio, as seen on the (a) topographic map, (b) DEM25, and (c) DEM2 by the National Land Survey of Finland.
DEMs are considered as very significant geospatial datasets due to the versatile possibilities for using them. They are used for ortho-rectification of aerial photographs, cartographic representations, 3D visualisations, geomorphological, biogeographical, hydrological, and hydraulic analyses and models, water management, analysis of landscape dynamics, climate and climate impact studies, geological applications, agriculture and forestry applications, palaeogeographical mapping, visibility analyses, road and dam planning, cut-and-fill analysis, automatic drainage basin delineation, flood risk analysis, planning of telecommunication networks, and geophysical modelling, among many other uses and applications.
DEMs available in Finland are DEM2 (2 m grid) based on airborne laser scanning, and DEM10 (10 m grid), DEM25 (25 m grid), and DEM200 (200 m grid) based on contours and other elevation related data of the Topographic database all created by the National Land Survey of Finland. The new generation country-wide DEMs are typically produced by using airborne laser scanning, but on a global scale, DEMs based on map data will still be used concurrently for the foreseeable future. However, the quality and level of detail in DEMs based on airborne laser scanning are superior compared with the previous generation DEMs.
Figure 3. Terminal moraines in Siikala, Karkkila, as seen on the (a) topographic map, (b) DEM25, and (c) DEM2 by the National Land Survey of Finland.
Figure 4. The bog Keihässuo, Loppi, as seen on the (a) topographic map, (b) DEM25, and (c) DEM2 by the National Land Survey of Finland. The typical circular structure of bog hollows is visible only in the most detailed model based on airborne laser scanning, DEM2 (c).
More about DEMs
DEM error and error propagation in terrain analysis - http://urn.fi/URN:ISBN:952-10-3350-9
DEM2 by the National Land Survey of Finland
DEM10 by the National Land Survey of Finland
Global ETOPO1 (topography and bathymetry) - http://www.ngdc.noaa.gov/mgg/global/global.html
Global ASTER GDEM Version 2
Global GTOPO30 - http://www1.gsi.go.jp/geowww/globalmap-gsi/gtopo30/gtopo30.html
Near-Global SRTM (Shuttle Radar Topography Mission) DEM - http://eros.usgs.gov/#/Find_Data/Products_and_Data_Available/SRTM