There are two ways in which maps may be produced from primary sources. One is by ground survey and the other by air survey, that is by taking measurements from aerial photographs. Points identified on overlapping aerial photographs may be transformed into positions on maps either by mechanical analogue means or through the use of mathematical techniques. The processes, known as photogrammetry, require some ground measurements to be taken in order to establish the precise scale and orientation of any map in relation to the ground data.
The quality of modern instrumentation is now so good that in conjunction with computerized techniques and aerial triangulation, the ground control points that must be provided are relatively few. The actual number depends on the size and shape of the area to be mapped, the accuracy required, and the scale of the photography. The latter varies across a photograph since it is equal to the ratio between the focal length of the air survey camera and the flying height of the aircraft above any ground point. Since the height of the terrain varies across an area, then so does the scale of the photography.
Photogrammetry can be used to establish a greater density of control points from which it is then possible to compile detailed topographic maps. The outline of features as seen from the air, together with contour lines, can be identified and traced with great ease and economy. Under suitable conditions, photogrammetry can produce maps and measurements that are as accurate or even more accurate than those obtainable by standard ground methods.
In many circumstances the legal boundaries of holdings cannot be determined from the ph otographs without extensive checking on the ground. Nevertheless the techniques can be as accurate as, and significantly cheaper than, undertaking a survey entirely by ground survey methods. One of the great disadvantages of air survey is that, except for the ground control marks that are part of the basic geodetic framework, it does not leave permanently demarcated points on the ground. A large number of these are often required for an efficient cadastral survey in order to facilitate subdivision work and the future re-establishment of missing boundary features. In ground surveys most theodolite or other survey stations are, or can be, simply and permanently marked by buried pipes or concrete beacons as the work progresses. This system makes it possible to reconstruct any part of a minor triangulation or traverse at any time, a fact of obvious advantage when identifying by measurement any unmarked points or during revision of the survey. In the case of air surveys, the provision of additional ground marks will need to be done independently, often at considerable cost. In all cadastral surveys undertaken by photogrammetry there is a need for follow-up ground surveys to check the actual location of legal boundaries that may not be visible on the photography or may have been wrongly identified.
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Broadly speaking, aerial photographs have been used in cadastral mapping in different ways. In the simplest case, contact scale or enlarged photographs may be used as a base on which to outline the parcels. In some countries it is sufficient to produce a plan which acts as a signpost to the parcels, the boundaries of which can be determined by inspection on the ground. In such cases there is no need to record the accurate dimensions of any land parcel so long as its boundaries are clearly visible on the photographs. Thus in Kenya, photographs enlarged to 1: 2,500 scale have been used to form the base from which the boundaries of the parcels could be traced. Such an approach is expedient where large numbers of parcels need to be recorded over a short period of time. Where time and money allow, the areas can be resurveyed to higher accuracy at a later date.
A different approach to producing cadastral maps is through the use of stereo-plotting machines. These may be used either to show the physical features which coincide or coexist with the legal boundaries or to locate points of detail that can be used as control for simple ground surveys. Cadastral maps can then be compiled from a combination of photogrammetric plotting of physical details that are visible from the air with simple graphical methods of survey to locate specific land parcel boundaries.
In setting up a cadastral system there are obvious advantages in using photogrammetric techniques where the boundaries of parcels are visible from the air or where relatively cheap and stable marks can be seen clearly on the photographs. In the case of small areas and when adding to or revising an existing survey, the advantage of speed is much less, and must in any event be weighed against increased costs. The latter are due not only to the higher proportion of time spent in the air in relation to the amount of photography done, but also to the expense of setting up the necessary ground controls for a small job.
There are many purposes for which large-scale air photographs are required, other than for the making of maps. Such cases are erosion surveys, forest inventory surveys, land-use surveys and many others. It may often prove economical to use aerial photography that meets the needs of the cadastral survey and to make maps on this basis, even though the cost of such photography would not be justified solely for the purposes of cadastral mapping.
Where air surveys are part of the ordinary survey practice of a country, they may also prove suitable for large cadastral jobs, provided that the amount of supplementary ground work required is not so large as to make it more economical to use ground staff alone. In such cases it may be economical to use photography of sufficiently large scale to meet the needs of cadastral surveying even though this exceeds the requirements of other mapping projects. To carry out a dual or multiple purpose survey through one set of photographs may make economic sense. Often however the needs of different applications of the photography are incompatible and multiple sets of photography may be required.
Where a new cadastral survey of a large area is required to be carried out quickly, air survey is often the best practical method, unless a great deal of supplementary ground work proves necessary. Cadastral surveys of small areas, extensions to existing surveys, and revision work will usually best be done by the ground staff of the national cadastral survey organization, according to a continuous programme.
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10. Study the following words and word combinations. Compose the sentences of your own with some of them.
primary sources ;
in conjunction with ;
varies across , ;
density ;
n some circumstances ;
efficient 䳺, ;
missing ;
to undertak , ;
signpost ;
approach ;
solely ;
incompatible ;
supplementary .
11. In the text Air Surveys and Cadastral Maps find words to complete the following groups of synonyms. Compose the sentences of your own with some of them.
1. Characteristic, property, feature, trait, kind, sort,_________________
2. Common, everyday, familiar, conventional, normal,______________
3. Conflicting, contrary, antagonistic, unsuited,____________________
4. Find, discover, come across, detect, pinpoint,____________________
5. Thickness, solidity, mass, compactness,________________________
6. Assist, aid, help, make easy, simplify,__________________________
7. Border, perimeter, limit, frontier, margin, edge,__________________
8. Situation, condition, position, background,______________________
