Akm Saiful Islam. WFM 6202: Remote Sensing and GIS in Water Management. [Part-B: Geographic Information System (GIS)]. Lecture-7: Digital Terrain Model. Institute of Water and Flood Management (IWFM) Bangladesh University of Engineering and Technology (BUET). January, 2008. DEM. - PowerPoint PPT Presentation
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• A DEM (digital elevation model) is digital representation of topographic surface with the elevation or ground height above any geodetic datum. Followings are widely used DEM in GIS:
Aspect identifies the steepest downslope direction from each cell to its neighbors. It can be thought of as slope direction or the compass direction a hill faces.
It is measured clockwise in degrees from 0 (due north) to 360, (again due north, coming full circle). The value of each cell in an aspect dataset indicates the direction the cell's slope faces. Flat areas having no downslope direction are given a value of -1.
Surface Specific points • + is assigned if the height of the central point is higher than
the one of the eight neighbors and - if lower.– A peak can be detected if all the eight neighbors are lower. – A pit or sink is formed if all the eight neighbors are higher – A pass can be extracted if the + and - alternate around the central
• The altitude is the slope or angle of the illumination source above the horizon. The units are in degrees, from 0 (on the horizon) to 90 degrees (overhead). The default is 45 degrees.
• The azimuth is the angular direction of the sun, measured from north in clockwise degrees from 0 to 360. An azimuth of 90 is east. The default is 315 (NW).
• By placing an elevation raster on top of a created hillshade, then making the elevation raster transparent, you can create realistic images of the landscape.
• Contour lines are one of the terrain features which represent the relief of the terrain with the same height. There are two types of contour lines in visualizing GIS data:
• Vector Line DrawingIn case when the terrain points are given in grid, the simplest method is to divide the square cell into two triangles mechanically.
• Raster ImageContour image with painted contour terraces, belts or lines instead of vector lines will be generated in raster form.
Contours • Digital elevation model (DEM) is very often generated by
measuring terrain points along contour lines using a digitizer. DEM with contour points should be provided with an algorithm interpolate elevation at arbitrary points. There are several interpolation methods as follows.
Profile MethodA profile passing through the point to be interpolated will be generated and linear or spline curve applied.
Proportional Distance MethodAccording to distance to two adjacent contour lines, the elevation is interpolated proportionally with respect to the distance ratio.
Window MethodA circular window is set up around a point to be interpolated and adjacent terrain points are used to interpolate the value using second order or third order polynomials.
TIN MethodTINs are generated using terrain points along contour lines.
• Triangulated irregular network or TIN is a DEM with a network of triangles at randomly located terrain points.
Contouring of TINs is based on the following procedure.
step 1: find the intersect of contour and a side.step 2: assign the "reference point" with the symbol r to the vertex above the contour height and the "sub-point" with the symbols to the vertex below the contour height.step 3: shift over to the transversing to find the third vertex in the triangle by checking whether it is a reference point (r) or sub-point (s).
Automated Generation of DEM • Automated generation of DEM is achieved by photogrammetric
methods based on stereo aerial photography and satellite stereo imagery.
• Parallax is defined as difference between left and right photographs or image coordinates. The higher the elevation is, the bigger the parallax is. If the parallax is constant, equal elevation or contour lines will be produced.