Friday, July 10, 2015

Digital Elevation Models (DEM)

01 Digital Elevation Models (DEM)
Digital Elevation Models (DEM)


INTRODUCTION
A DEM is a raster representation of the Altitude provide basic, quantitative information about the Earth’s surface. The accuracy of this data is determined primarily by the resolution (the distance between sample points). Other factors affecting accuracy are data type (integer or floating point) and the actual sampling of the surface when creating the original DEM.

Most data providers and professional users use the term DEM for both the digital terrain model (DTM) and digital surface model (DSM). A DTM usually refers to the physical surface of the Earth (elevations of the bare ground surface) without objects such as vegetation or buildings, while a DSM describes the upper surface of the landscape, includes the height of vegetation, man-made structures and other surface features, and only gives elevations of the terrain in areas where there is little or no ground cover (Maune, 2007). 

Elevation data sets, from which DEMs are generated, are obtained by a broad range of measurement techniques, such as ground survey (GPS, total station, terrestrial, and laser scanner), airborne photogrammetric imagery, airborne laser scanning (LiDAR), radar altimetry and interferometric synthetic aperture radar (InSAR).

TERMINOLOGY
Digital Elevation Model (DEM): generic term for altitude grid.
Digital Terrain Model (DTM): ground elevation model.
Digital Surface Model (DSM): ground + cover elevation model.
Digital Height Model (DHM): cover elevation model.
DEM Types

The digital elevation model corresponds to a regular grid of elevation. Each node of the grid shows an altitude value.
The resolution of the grid corresponds to the distance between to neighbor nodes.

DEM Scales Vs Sources
DEM Scales Vs Sources


Global Scale
The GTOPO30 DEM was created based on heterogeneous topographical maps. The quality of the elevation data varies consequently over space.
The SRTM30 DEM was acquired through space shuttle radar interferometry. This new source of elevation data overcome the major quality problems of the GTOPO30.
They both present a resolution of 30 arc seconds (~900 m) and are freely available for the Earth surface.

Regional Scale
SRTM 90 Vs 30 DEM.The SRTM 90 & 30 m DEM were acquired through space shuttle radar interferometry.
They present a resolution of 3 arc seconds (~90 m) respectively 1 arc seconds (~30 m) and are available for the Earth surface.
The SRTM 90 m is freely available. The SRTM 30 m costs being of 0.5 $ per square kilometer. 

Local Scale (LASER DEM)
LASER DEMThis new acquisition technology allows the capture of very high resolution DEM (~1 m). Both terrain (ground) and surface (objects) are captured in the same time. Such detailed digital elevation model offers good potential for local relief analysis in applications such as hydrology, hazard mapping. The cost of acquisition are relatively high (150-300$ per square kilometer).

(LASER DEM)

The ASTER GDEM is provided at a one arc-second resolution (approximately 30m). The absolute vertical accuracy of ASTER GDEM is 20 m at 95% confidence level.
(ASTER GDEM2) was introduced to improve the spatial resolution, and increase the accuracy of water body coverage.
The Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) was generated at three separate resolutions of 30 arc-seconds, 15 arc-seconds, and 7.5 arc-seconds (approximately 1 km, 500 m and 250 m, respectively).

Examples of the topography detail displayed by the selected digital elevation models.

Usage of DEM
(1) Hydrological modelling including flood simulation, delineation and analysis of watersheds and drainage networks,
(2) Soil erosion and sediment transport modelling,
(3) Delineation and study of physiographic units,
(4) Soil and ecological studies,
(5) Geomorphological evaluation of landforms,
(6) Civil engineering and military applications such as site and route selection, landslide hazard assessment, visibility analysis (viewshed analysis), and
(7) Remotely sensed image enhancement for 3D analysis. Groundwater and climatic models also use digital topographic data as essential components. Digital elevation models provide an opportunity to characterize quantitatively land surface in terms of slope gradient and curvature and yield digital terrain information not blurred by land cover features which is often a problem in stereo-aerial photograph interpretation and remotely sensed image analysis.


Displaying Digital Elevation Model (DEM)/ (DTM)

Displaying Digital Elevation Model (DEM)-(DTM).

Analyzing Surfaces Terrain / DEM analysis tools
Some of these tools are primarily designed for the analysis of raster terrain surfaces. These include Slope, Aspect, Hillshade, and Curvature tools.

1.      Calculating Slope
It affects where structures or trails can be built, crops can be planted or harvested, the speed of flowing water and consequent erosion, landslide potential, and the list just goes on and on.
The Slope tool calculates the maximum rate of change from a cell to its neighbors, which is typically used to indicate the steepness of terrain. (0-90) degree.

Slope Calculation

  2.  Calculating Aspect
Aspect identifies the slope direction in compass degrees from 0 (due north) to 360.
The aspect of a surface typically affects the amount of sunlight it receives (as does the slope); in northern latitudes places with a southerly aspect tends to be warmer and drier than places that have a northerly aspect. Aspect is an important contributor to vegetation and habitat type, as north-facing slopes often have very different conditions and temperatures than south-facing slopes.

Aspect Calculation

     3.  Hillshade
Hillshade allows us to determine the illumination of a surface (the DEM in the case) given a direction and angle of a light source (i.e. the sun). The resultant grid contains values ranging from 0-255 with 0 representing complete darkness.
Hillshading is an extremely useful way to depict the topographic relief of a landscape. Few methods are as intuitive and easy to understand as a hillshade. A good hillshade lets you understand immediately what areas are ravines, ridges, peaks or valleys.

Hillshade Calculation.

     4.  Curvature
Calculates the slope of the slope (the second derivative of the surface), that is, whether a given part of a surface is convex or concave. Convex parts of surfaces, like ridges, are generally exposed and drain to other areas. Concave parts of surfaces, like channels, are generally more sheltered and accept drainage from other areas. The Curvature tool has a couple of optional variants, Plan and Profile Curvature. These are used primarily to interpret the effect of terrain on water flow and erosion. The profile curvature affects the acceleration and deceleration of flow, which influence erosion and deposition. The planiform curvature influences convergence and divergence of flow.

Curvature Calculation.

Note
From an applied viewpoint, the output of the Curvature tool can be used to describe the physical characteristics of a drainage basin in an effort to understand erosion and runoff processes. The slope affects the overall rate of movement down-slope. Aspect defines the direction of flow. The profile curvature affects the acceleration and deceleration of flow and, therefore, influences erosion and deposition. The plan form curvature influences convergence and divergence of flow.
Displaying contours over a raster may help with understanding and interpreting the data resulting from the execution of the Curvature tool. An example of the process follows >>
1.       Use Contour to create contours of the raster.
2.      Create a slope raster.
3.      Contours of the slope.
4.      Add the curvature raster as a layer in ArcMap. Overlay the two contour coverage just created, and apply different color symbology for each.

Sources 
A book of "DTM Principles and Methodology"
Arc GIS online Courses 
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