Transcript GIS and Remote Sensing in Water Resources Management

WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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:
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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.
1. Slope and Aspect
2. Drainage network
3. Catchment area
4. Shading
5. Shadow
6. Slope stability
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
Examples of DTM
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
1. Slope and Aspect
(i) Slope
• The steepest slope (s) and the direction from the
east () can be computed from 3 x 3 matrix.
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
Slope calculation
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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 )
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
Example: Slope from elevation data
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
(ii) Aspect
• The aspect that is, the slope faced to azimuth is
180° opposite to the direction of q
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
Figure 1. Slope components, note that slope gradient can be
express in percent or in degrees
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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.
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
Example: aspect from the elevation
data
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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.
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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.
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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.
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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).
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
Hill shading from elevation data
• The hillshade
below has an
azimuth of 315
and an altitude
of 45 degrees.
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
Examples: A slope and hillshade
maps of Glacier National Park
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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 Drawing
In case when the terrain points are given in grid, the
simplest method is to divide the square cell into two
triangles mechanically.
• Raster Image
Contour image with painted contour terraces, belts or
lines instead of vector lines will be generated in raster
form.
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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 Method
A profile passing through the point to be interpolated will be
generated and linear or spline curve applied.
 Proportional Distance Method
According to distance to two adjacent contour lines, the
elevation is interpolated proportionally with respect to the
distance ratio.
 Window Method
A 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 Method
TINs are generated using terrain points along contour lines.
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
Interpolation Methods
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
Examples: A Digital Elevation Model and
associated contour map of Glacier Nat'l
Park
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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 symbol
s 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 subpoint (s).
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
Example: TIN Creation
WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful Islam
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.