2. Digital Terrain Models
Dr. SaMeH Saadeldin Ahmed
Assistant professor of Mining and Environmental Engineering
Email: dr_sameh_mining04@yahoo.com
A short course for
Egyptian Cement Company (ECC)
April, 2005
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Outlines
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Definition
General Outlines of DTM
Sources of DTM
Principles of DTM
Interpolation of DTM
Accuracy of DTM
Applications
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1. Definition
A Digital Terrain Model (DTM) may be
defined as “ a numerical description of
irregular as well as regular surface, based
on the measurement of discrete points of
this
surface
together
with
the
corresponding interpolation rules”.
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2. General Outlines of DTM
Two different aspects
properties of DTM:
can
describe
the
1) Method of Sampling
2) Method of Reconstruction
In general, DTM is composed of sampling points
data together with the interpolation rules.
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Terrain Classification
The basic data sources of DTM are terrain
itself, its photogrammetric model as
existing topographic maps for sampling.
The classification can be approached in
three different ways, namely qualitative,
quantitative and mixed.
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a. Qualitative Classification
Where visual inspection and interpretation
of the terrain in question is the dominant
factor, the approach necessitates that
persons who carry out such classification
should be highly skilled and well
experienced.
Disadvantage its outcome differs from
person to person.
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b. Quantitative Classification
As the title suggests, it is based on
computing some parameters by which the
class of the terrain is determine.
So far, a fully quantitative method is not
available.
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c. Qualitative- Quantitative
Classification
this is the most feasible method where the
two previous methods are combined.
The parameters of classification are:
1.
2.
3.
4.
5.
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Shape of profiles
Amount of local relief
Slope gradient
Number and length of break lines
Degree of roughness
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Sources of DTM
• Direct measurements on photogrammetric
stereo-models.
• Ground surveying.
• Existing topographic maps.
• Existing orthophoto maps superimposed with
contours.
• Profiles as a by-product through the
production process of orthophoto.
• Stereo orthophoto.
• Radar and leaser altimeters carry in aircrafts &
spacecrafts.
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Principles of DTM
• Measurement of elevation data should be
taken in an efficient manner .
• The digital elevation model should be
constructed by as small number of points as
possible.
• The
digital
elevation
model
should
approximate the terrain topography in a
sufficiently high accuracy.
• The computation of interpolating point
elevation should not demand too much time.
• The DTM should be suitable for elevation
computation.
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Measuring Patterns
• Recording along closely spaced profiles.
• Scanning along contours and/or terrain
break lines.
• Recording by regular point grids.
• Recording by irregular points grid or
random points.
• Recording by regular point grids
supplemented with break points.
• Recording by cross sectioning.
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Interpolation of DTM
• Interpolation is the method of the
reconstruction of the surface from the
sampling points of terrain.
• Figure (2.2-a) shows the concept for
definition of interpolation
• Figure (2.2-b) shows interpolation and
filtering.
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Accuracy of DTM
The following are the main factors influencing
the accuracy of digital terrain models:
• Density of reference points
• Distribution of reference points
• Type of terrain
• Accuracy of editing
• Accuracy of the measure:
– The source of the reference points.
– The experience and the skills of the
operator.
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Applications of DTM
the major applications of DTM data have
covered the fields of design and planning of
civil engineering projects
highway projects,
automated cartography,
land use,
soil classification,
simulation studies,
navigation and
geographic studies.
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Thanks for your attention
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