LESSON ON Procedures of Photogrammetric Mapping

Procedures of Photogrammetric Mapping

The procedures in
photogrammetric mapping include project planning (recce), flight planning (or
image acquisition planning), imaging, ground control (ground truth)
establishment, image restitution (interior, relative, absolute and exterior
orientations), QA/QC analysis, collection of vector data and final map production.

 1 FLIGHT PLANNING FOR AERIAL IMAGE COVERAGE

PHOTOGRAMMETRIC PROCEDURES

As
presented in the block schematic above, photogrammetry, the science of
measurement by aerial photographs, consists
of five stages which are summarized below:

Site Identification and flights/image-acquisition
planning. Aerial surveying is
generally carried out over large
areas which involve extensive photography or image coverage. Such surveys are, therefore, made by the government,
organizations or large private
companies. In general, prior to the acquisition of the
images, the site must be identified, possibly using existing maps or mosaics. All the possible ways of
covering the area with images are considered and the most appropriate method
selected. For aerial photography, the imaging will usually be done in strips
and so the process(of image acquisition) will involve the determination of the
number of images per strip and the number of strips to cover the area. Of
course, flight planning makes use of the photographic parameters such as focal length, image frame size, flying height and desired
ground sampling distance (gsd) and the amount of overlaps needed
both between successive images and between strips. Consideration
for photo scale is also made. The photo scale is directly linked with the accuracy of the map which is the
final product. After
agreeing on the map accuracy with the client, the scale of the aerial
photography is then determined. For topographical maps
of small and medium scales, the height accuracy attainable and the image
quality are the governing factors
for the choice of the scale, whereas for large scale maps, planimetric accuracy
is more important. The largest scale
of maps that can be produced photogrammetrically
in an economic way is 1: 500. It is
therefore essential to ensure that both planimetric and height accuracy requirements, are met with the decided
photo scale. The flying height of the
camera depends on the
scale of the photography. For simple
plotting, the average scale of the
photography is generally kept the same or slightly larger than the desired scale of the final
compiled map. Hence, by knowing the average scale of the photography and the
scale of the compiled map, the flying height of the aircraft above mean ground level,
may be calculated. If planimetric
accuracy is important, the flying height depends on the photo scale chosen to meet the planimetric
accuracy. On the other hand, if height accuracy is of prime
importance, the flying height may be
first decided, depending on the required contour interval. The flying height is often related to the contour interval of the final map.

The following points may be noted :

(i) The area of ground covered by each photograph
increases with the increase of the flying height
of the aircraft and hence less number
of photographs are required for any particular area of land.

(ii) The scale of the photography
increases with the decrease in the
flying height and consequently more details for greater height accuracy are obtained.

(iii)Due to increased flying height, the haze and dust
reduce the quality of the photographs.

(iv)The cost of the flying
at greater flying height is excessive as compared to that of low flying height.

The details of this process are given
later. In the case of satellite imagery, the total number of frames to cover
the area is calculated from the available frame size, and the amount of
overlaps that exist on the images. In some cases, only one frame or two may be
sufficient for the purpose when the frame is large.

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