ERT247 GEOMATICS ENGINEERING AIRBORNE IMAGERY PERPARED BY: SAMERA BINTI SAMSUDDIN SAH SCHOOL OF BIOPROCESS ENGINEERING

Aerial Camera Systems 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. Advantage of digital camera: can store, transmit and analyze the image data. Types of camera systems: small-format and large-format.

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.

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.

Flight Lines and Photograph Overlap 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

For photographic scale of 1: 10,000 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.

Ground Control for Mapping 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.

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

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.

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

Mosaics 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.

Advantages: Disadvantages: Can be produced more rapidly. Less expensive (cheaper). Shows more terrain details. Can interpreting subtle terrain characteristics ( tone, texture , and vegetation) Disadvantages: Horizontal scale measurements are limited due to relief displacement. Not topographic maps (do not show elevations)

Aerial Surveying and Photogrammetric Mapping Advantages using AS and PM over traditional ground surveying methods: 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)

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

Aerial Photography Interpretation 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: 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.

Applications of Air Photo Interpretation for Engineer and the Surveyor. 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.

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