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Air surveys and cadastral maps

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 (f) and the flying height of the aircraft (H) above any ground point (h); mathematically:

the scale at any point = f/(H-h).

Since the height of the terrain (h) 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.

While photogrammetric techniques have been extensively used for topographic mapping, they have contributed less to cadastral surveying. In favourable circumstances, for example where there are rice lands with embankments around their edge, terraced hill lands, or land enclosed with well- marked hedges or walls, the photographs may be able to supply all the field detail needed for the cadastre. In other cases this may not be so - for instance in some open field systems where cultivation of the same crops is carried out by different land holders more or less continuously over a wide area, or in some areas under tree cover. The aerial photographs may show temporary detail, for example the divisions of a single field between crops, or details within the survey plot such as minor internal drainage ditches. These should not or need not appear on the cadastral map but may be difficult to distinguish from permanent field boundaries on the photographs.

Air Photo Scale = 1 / (H-h)

In many circumstances the legal boundaries of holdings cannot be determined from the photographs without extensive checking on the ground. Generally more work will be required to supplement the photographs in the case of cadastral maps than in the case of most topographical mapping. 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.

Survey work of all kinds tends to be expensive and the precise comparative cost of air and ground methods can usually only be determined with regard to particular cases. Air survey methods benefit from economies of scale where a large number of boundaries or points of detail have to be recorded. The capital and running costs of aeroplanes and the equipment used in aerial surveys are high, and the personnel required are highly skilled and relatively well paid in comparison with some field surveyors. The number of days in the year on which flying can be done is normally quite small but then in many countries the number of days on which field survey work can be carried out is restricted for climatic reasons though not so much as for flying.

There may be difficulty in getting approval for air survey from the military authorities who in many countries have traditionally opposed public access to aerial photographs for supposed security reasons. Given the ready availability and quality of modern satellite imagery, this restriction makes little sense. Would-be terrorists can easily obtain from elsewhere the sort of information that opponents of public access to aerial photography seek to hide. Opposition to the use of aerial photography has increased the cost of many surveys and reduced their effectiveness with consequent but unmeasured costs that have significant impact on the national economy.

Broadly speaking, aerial photographs have been used in cadastral mapping in five 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.

More accurate mapping may be achieved by using rectified photographs that remove any distortions occurring when the photography is not done with the camera pointing exactly vertically downwards. The Land Titling Programme in Thailand made extensive use of enlarged rectified photographs, with supplementary information provided by ground survey. Cadastral photomaps at scales of 1:4,000 and larger were produced. A similar technique was used in Botswana where the photo-maps were suitably annotated to identify particular parcels. Given a vast area of savannah country with no nearby ground survey control, it is possible to identify patterns in the vegetation and to trace on to the photographs the outlines of areas of land allocated for purposes such as veterinary stations, air strips and police posts. The dimensions of the plots can then be added from measurements made by taping on the ground. A disadvantage in using rectifiers is the relatively high cost of this equipment.

For areas that are not flat, an improvement on rectification is to produce orthophotographs in which each point on the original photographic image is transformed to its correct position on a topographic map. The resulting product looks like a photograph but has the metric qualities of an accurate map since it removes the changes in scale across the original photographs that are due to variations in ground height. Although this requires more sophisticated technology, the advantage is improved planimetric accuracy and hence the dimensions of land parcels can be measured by scaling off the plan. In parts of Australia and Canada, orthophotographs have been used successfully in the compilation of cadastral index plans. For many years in the evolution of their landscape, both countries lacked any integrated survey control network and most cadastral surveys were connected only to local features. The result was that although the size and shape of each parcel had been carefully surveyed, there was often uncertainty as to where the parcel was actually located. The orthophotomaps were used as the basis for joining together the mosaic of disparate parcel surveys, rather like fitting together the pieces of a jigsaw.

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. The resulting product would then be to graphical standards of accuracy, that is to the equivalent of the thickness of a line drawn on the map.

Many countries, however, insist on numerical information for their cadastral surveys and this has resulted in separate topographic and cadastral mapping, leading to duplication of effort even within a national survey department. In urban areas, it is not uncommon to find that large scale topographic mapping has been produced photogrammetrically for the purposes of planning and general land management while cadastral plans of the same area have been compiled separately from independent ground surveys. This separation of function between cadastral and topographic surveying is symptomatic of the polarization that has taken place between advocates of ground and air surveys with a resulting lack of homogeneity in the data.

Photogrammetric techniques can be used to produce numerical coordinate data through the use of analytical stereo plotters or comparators. These produce coordinates for points on the air photographs from which the ground coordinates can be calculated. The accuracy of such coordinates depends upon the equipment used but can be similar to that achieved by ground survey - in Germany and Switzerland, for example, such techniques have been successfully used to revise cadastral maps. The great advantage of air survey over ground survey is the speed and hence the enormous amount of time that can be saved.

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 emplaced of sufficient size that they 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.

The general conclusions drawn from the above observations may be stated as follows:

  1. 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.

  2. 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.

  3. Unless reasonably full-time employment can be found for the staff and equipment of a national mapping agency's air survey department, it will usually be better to contract the work out to a commercial agency.

  4. 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.

  5. Air survey will not supersede ground survey for cadastral purposes, but it should complement it. Where there is no great hurry to cover a large area, and especially where local ground surveyors are available or can be made available in sufficient numbers, it may be better to rely entirely on ground survey for cadastral maps, even though air survey methods may be used for some other purposes. In any event there is great merit in making the cadastral staff fully responsible for the maintenance and updating of the maps that they produce.

  6. There are many purposes for which large-scale aerial photographs, as opposed to maps based on photographs, are required. If these photographs are being taken, it is possible that with some variations in method and greater care in adjustment they can be used to make maps as well as for their primary purposes. Opportunities of this kind should not be neglected. In fact when air photographs of an area are being taken for any departmental purpose, it is sound practice to examine the possible needs of all departments for photographs or maps of the area and to consider whether it is cost effective to take the photographs, once and for all, on the largest scale required for any of these purposes.

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