Digital Ortho-Photography and other Techniques in the Creation of a Cadastral Register

1. Introduction

Furthermore, it must be considered that a correct integration of the map within a geographic reference system is required to avoid overlapping or gaps in the subdivision of the lands. This integration can be more or less strict, dependent on the degree of approximation of the boundaries that can be tolerated, but is essential in that, otherwise, it would not be possible to perform any subsequent updating operation.

2. Cadastral mapping

2.1 Direct topographic survey

We should consider 100 as the cost value allocated to cartography produced with this technique.

2.2 Photo-restitution

The operations which are at the base of this technique are those of the photo-restitution proper, with the important difference that the preparation of the photo-restitution draft is directed at the photointerpretation of only the details that have a "cadastral sense", leaving aside others that would encumber and render confused the representation. Moreover, the photo-restitution draft does not constitute in itself the fundamental basis for preparing the final maps (as occurs, instead, for technical maps of a general use); in fact, in the case of Cadastre, the ground after-survey, which always follows photo-restitution, is oriented towards ascertaining the correct delimitation of property boundaries, eventually by associating to the maps an administrative information database. Therefore, the photo-restitution is a method representing a good compromise between the high precision required for cadastral purposes and the productivity required to keep the cartography constantly updated. It is a more economical medium than direct topographic survey, but is still relatively costly, in absolute terms.

The cost is about 70% with respect to the cartography produced with direct survey.

2.3 Digital ortho-photography:

The objective of the digital ortho-photography technique is not to ensure the production of a complete and perfect cadastral map; in fact such scope is by definition unachievable through any type of territorial photogrammetry alone. However, this represents an adequate tool to accelerate and economize on cartographic procedures, exactly like photo-restitution compared with ground survey, but with an even higher productivity and value added.

The idea of overlaying cadastral maps on photo-mosaics is not new, but it has always been faced with the problem of photo deformation, or with the very high cost of producing orthophotos through the use of ortho-photoscopes. Through digital aerial photogrammetry the first of these two problems is solved, obtaining a perfectly framed map projection image, whereas the second problem becomes redimensioned. Furthermore, the fact that digital ortho-photography already represents by itself a digital file and that such file can subsequently be updated through a simple repetition of the aerial photogrammetry represents an essential and exclusive advantage of this technique.

In practice, the digital ortho-photographic image is a file already qualified to accept the overlaying of other vectorial files and the associated alphanumeric data. These two options are very important when any type of old property mapping is already existent, and are not possible with a traditional ortho-photography, unless a rastering of this successive to production is performed, with a further cost increase.

the degree of precision offered by this technique is dependent on the scale and the scanning density of the aerial photographs. Contrary to the classical aerial photogrammetry, this technique does not require a photo-restitution operation and it is possible to apply it even with the use of rather simple equipment (PC and scanner). Consequently, it is by far the most rapid and most economic medium; thanks to these characteristics it is very adapt for use in the initial stage of work concerning creation of cadastral mapping. It is also adapt for the updating of existing cadastres, when there is the need for a rapid identification and finding of easily visible elements (e.g. buildings not yet represented in the old maps).

The cost is about 40% that of the direct survey method; it is to be considered that this estimate takes into account the fact that the aerotriangulation operations (see 3.1 and 3.2) are performed through ground survey of the control points. If instead the control points are derived from existing cartography the cost diminishes further, down to 30% with respect to the traditional method.

3. Digital ortho-photography: work phases

3.1 Topographic operations

3.2 Creation of a terrain model

3.3 Rastering of air photos

3.4 Production of ortho-photographic imagery

3.5 Overlaying of cadastral maps

4. Use of the ortho-photographs

The raster image definition values to be used can be varied within wide limits; they can even arrive at scanning densities similar to the grain of photographic emulsion. It shall be noted that the level of quality that can be achieved is nearly identical to that of a photographic print; conversely, it should be borne in mind that as the degree of definition increases, the dimension of the raster files increases (to the square), and this may generate problems in management with PC-level equipment (also, the cost of scanning machines sky-rockets, for increasing definition).

5. Comparison among cartographic products

• High to Very High scale topographic maps (1:500-1:1.000-1:2.000), made from direct topographic survey. (very high resolution, very high accuracy, very high cost, very low productivity)

• High scale photo-restitution (1:1.000-1:2.000-1:5.000), made from aerial photogrammetry of low-altitude air shots (1:10.000-1:15.000). (high resolution, very high accuracy, high cost, low productivity)

• High scale ortho-photos (1:1.000-1:2.000-1:5.000), made from low-altitude air shots (1:10.000-1:15.000) rectified by ground surveyed control points. (high resolution, high accuracy, medium to high cost, medium to high productivity)

• Medium scale ortho-photos (1:10.000-1:20.000), made from high-altitude air shots (1:30.000-1:40.000) rectified by ground surveyed control points. (medium resolution, high accuracy, low cost, high productivity)

• Medium scale ortho-photos (1:10.000-1:20.000), made from high-altitude air shots (1:30.000-1:40.000) rectified by cartography derived control points. (medium resolution, medium accuracy, very low cost, very high productivity)

• Medium scale ortho-photos (1:10.000-1:20.000), made from enlarged satellite high resolution images. These images are native ortho-rectified (as SPIN-2). (medium to low resolution, medium to low accuracy, very low cost, ultra high productivity)

The final choice among these methods is not as simple as it could appear. Once indentified the needed scale/accuracy, the choice depends, particularly, on the initial conditions: for example, on the availability or lack of a good existing cartography, or on the availability of rather good old cadastral maps (and on their original scale). Also, the comparison between high-altitude air shots and satellite ortho-rectified images, strictly depends on the cost of a photogrammetric flight, which can wide vary in different countries and regions, while the cost of the satellite imagery do not depends on local conditions. Thus, some of these factors can make the difference for the final choice, and all the cartographic tools we have got nowadays are good for the task of rebuilding or updating a cadastral cartography.

Anyway, we are convinced that the digital ortho-photo technique will increasingly be used to update cadastral maps. But, we also believe that this new technique will find its major application in the initial phase of establishment of a cartographic base for the Countries which do not yet have a cadastre (as in the case of many Third World Countries) or which must remake the old cadastral maps (former-Socialist countries). Undoubtedly the technique represents the most advanced and economic means for obtaining an accurate and easily updated representation of an area.

Of course, we repeat, the digital ortho-photo doesn't exhaust the task of the creation (or full remake) of a real and complete cadastral registry.

Cartography is the background only for the geometric reference of the whole cadastral informations. Either in the case of a new cadastre creation or of a full remake, many function about the administrative management have to be implemented. For example, the data-base related to the cartography may include:

• Land estimation data
• Real estate estimation data
• Routines to manage the division or the sharing of the properties
• Routines to introduce new buildings
• Fiscal data
• Mortgage data
• Inheritance data

This list is incomplete, and all data and routines can vary according to the different legislations in different countries. Anyway, is a fact that the cartography is the core of a cadastral system. A cartography which allow an easy relation to any kind of alphanumeric data: the features of such a cartography are nowadays well defined.

6. Costs

1. The extension of the area to be updated: the prices, of course, will decrease according to the extension as well as according to the scale of the maps.

2. The scale of the new cartography: please note that in our example the scale is high. Prices, for lower scales, will drop in a quasi-geometrical way.

3. The average area of the parcels: in this example (and perhaps anywheres in Europe), the properties are intensively divided.

4. The number of administrative data to be entered. In this case there was a huge quantity of any kind of variations into the property deeds: that the reason why the figure and the relative weight of the item 7 is slightly higher than normal.