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.