WFM 6202: Remote Sensing and GIS in Water Management

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WFM 6202 Remote Sensing and GIS in Water
Management
Part-B Geographic Information System (GIS)
Lecture-7 Digital Terrain Model

• Akm Saiful Islam

Institute of Water and Flood Management
(IWFM) Bangladesh University of Engineering and
Technology (BUET)
January, 2008
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DEM

• 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

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DTM

• A DTM (digital terrain model) is digital
  representation of terrain features including
  elevation, slope, aspect, drainage and other
  terrain attributes.
• Usually a DTM is derived from a DEM or elevation
  data.
• several terrain features including the
  following DTMs.
• Slope and Aspect
• Drainage network
• Catchment area
• Shading
• Shadow
• Slope stability

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Examples of DTM
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1. Slope and Aspect

• (i) Slope
• The steepest slope (s) and the direction from the
  east (?) can be computed from 3 x 3 matrix.

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Slope calculation
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Slope calculation

• Slope is defined by a plane tangent to a
  topographic surface, as modelled by the DEM at a
  point (Burrough, 1986).
• Slope is classified as a vector as such it has a
  quantity (gradient) and a direction (aspect).
• Slope gradient is defined as the maximum rate of
  change in altitude (tan ?)

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Example Slope from elevation data
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• (ii) Aspect
• The aspect that is, the slope faced to azimuth is
  180 opposite to the direction of q

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Figure 1. Slope components, note that slope
gradient can be express in percent or in degrees
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Aspect calculation

• 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.

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Example aspect from the elevation data
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2. Drainage Network and Watershed

• The lowest point out of the eight neighbors is
  compared with the height of the central point to
  determine the flow direction.

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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 point with at least two
  complete cycle.

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4. Shade and 5.Shadow

• Shade is defined as reduced reflection depending
  on the angle between the terrain surface and the
  incident light such as the sun.
• Shadow is projected areas that the incident light
  cannot reach because of visual hindrance of
  objects on terrain relief

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Hill Shading

• The effect of hill shading on the assumption
  of an ideally diffused reflecting surface (called
  Lambertian surface) can be computed as follows
• Relative shading cos ? nxsx nysy nzsz
  1.0
• where ? angle between incident light vector s
  and surface normal n

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Altitude

• 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.

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Azimuth

• 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).

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Hill shading from elevation data

• The hillshade below has an azimuth of 315 and an
  altitude of 45 degrees.

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Examples A slope and hillshade maps of Glacier
National Park
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Using hill shading for display

• 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.

Hillshade elevation
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Generation of Contour Lines

• 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.

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Interpolation of Elevation from 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.

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Interpolation Methods
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Examples A Digital Elevation Model and
associated contour map of Glacier Nat'l Park
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Triangulated Irregular Network (TIN)

• 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).
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Example TIN Creation
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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.