Download ert247 geomatics engineering

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The recent introduction of digital cameras has
revolutionized photography.
Digital and film-based cameras both use optical
lenses.
 Film-based cameras: use photographic film to record
an image.
 Digital cameras: record image data with electronic
sensors.
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Advantage of digital camera: can store, transmit
and analyze the image data.
Types of camera systems: small-format and
large-format.
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Small-format system
 Consists one or more cameras
 Using smaller photographic format (negative size-
35mm)
 Do not have high-quality lens to meet the normal
measurement accuracies.
 Very useful and inexpensive for updating land-use
changes.
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Large-format system
 Consists only single camera.
 Uses fixed focal length, large-format negative
(230mm by 230mm).
 Strictly used for aerial photography.
 Equipped with a highly corrected lens and vacuum
pressure to minimize distortion.
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Factors need to consider:
 Using suitable aircraft and technical staff.
 The study area must be outlined carefully – using
GPS to maintain flight line alignment.
 Photographs must be taken under cloudless skies.
Figure 1: Flight line and photograph overlap
Figure 2: Photograph overlap along flight line
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The number of air photos required to cover
study area is very important.
For photographic scale of 1: 10,000
 1 photograph cover 1 km2
 if area need to cover 500 km2 , the number of air
photos is 500/1 = 500 nos .
 But, if the scale decreased to 1:5000, the number
of air photos is 500 x (10000/5000)2 = 2000 nos.
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Aerial photography is not perfect if it involves
exposure to sudden and a flat surface.
Ground control points is the best way to
overcome this problem.
To establish the control point;
 Existing photography used for mapping.
 Prior to the acquisition of the air photos.
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Ground control is required for each data point
positioning.
The accuracy depending on following
requirements:
 Measurements of distances and elevations
 Preparation of topographic maps
 Construction of controlled mosaics
 Construction of ortho-photos and rectified
photographs
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Selection of ground control points for existing
photograph based on following criteria:
 Must be separated in the overlap area – model more
stable and result more accurate.
 Must be easily identifiable on both air photos – useless
(if not)
 Should be selected on the assumption that there are
no changes since using the existing photographs.
 Surveyor should consider ease of access to all points
to minimize open-ended traverse lines.
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For new photography, there are criteria need
to follow;
 The areas containing few identifiable ground
control points.
 Legal surveys of densely developed areas.
 Municipal survey of roads and services
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An assembly of two or more air photos to form
one continuous picture of the terrain.
Extremely useful for the following application;
 Plotting of ground control points at the optimum
locations to ensure the required distribution and
strength of figure.
 A map substitute for field checkpoint locations and
approximate locations of natural and cultural
features.
 A medium for presenting ground data.
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Advantages:
 Can be produced more rapidly.
 Less expensive (cheaper).
 Shows more terrain details.
 Can interpreting subtle terrain characteristics (
tone, texture , and vegetation)
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Disadvantages:
 Horizontal scale measurements are limited due to
relief displacement.
 Not topographic maps (do not show elevations)
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Advantages using AS and PM over traditional
ground surveying methods:
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Low cost.
Reduced field work.
Faster in compilation (time saving).
Easy to record inaccessible terrain conditions.
Provide an accurate record of the terrain features.
Flexibility in term of scale.
More relevant (new technology)
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Disadvantages:
 Cannot get the real picture at dense vegetation
area.
 Cannot show the contour line
 Need to do site visit - type of roads, surfacing etc
Image interpretation is achieved by a
combination of direct human analysis and by
automated soft-copy processes.
 Image interpretation techniques are based on 3
fundamental assumptions:
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 The remotely sensed image are records of the results
of long- and short-term natural and human processes.
 The surface features can be grouped together to form
patterns that are characteristic of particular
environmental conditions.
 The environmental conditions and reflected image
patterns are repeated within major climatic zones.
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Can identify the land forms and site
conditions (type of soil, soil depth, average
topographic slopes, etc)
Can examine the topographic slopes, areas of
unstable ground and density, and type of
vegetation cover.
Air photo provide an excellent overview of the
site and surrounding area.
Soil test holes should be used to verify the
results of the air photo interpretation.