Cadastral Maps
Cadastre is a technical term for a st of records showing the extent, value and ownership (or other basis for use or occupancy) of land. Strictly speaking, a cadastre is a record of areas and values of land and of landholders that originally was compiled for purposes of taxation. In many countries there is, however, no longer any land tax and in practice the cadastre serves two other equally important purposes. It provides a ready means of precise description and identification of particular pieces of land and it acts as a continuous record of rights in land.
A modern cadastre normally consists of a series of large-scale maps or plans, and corresponding registers. Both the plans and the registers may be stored in computers, as discussed in the chapter “computerization of maps and registers”. The present chapter deals with the essential features of cadastral maps with particular reference to the form they take when drawn on paper or displayed on a computer screen. While the survey of an individual parcel of land has in some countries resulted in a “cadastral map” for that plot of land and may have been unconnected to any adjoining land parcels, the true cadastral map covers all parcels within an area rather than isolated plots. It can act as an index for other land parcel surveys that show more detailed information or can be of sufficiently large scale for the dimensions of each plot to be obtainable from the map. In this chapter, and throughout this monograph, the term ‘cadastral map’ will be associated with any parcel of land whether defined by ownership, value or use provided that the parcel has an independent identity and is relevant to the management of land as a resource. A cadastral map will show the boundaries of such parcels but may in addition incorporate details of the resources associated with them, including the physical structures on or beneath them, their geology, soils, and vegetation and the manner in which the land is used.
The scale of cadastral maps is of great importance. Since the object of the map is to provide a precise description and identification of the land, the scale must be large enough for every separate plot of land which may be the subject of separate possession (conveniently called a “survey plot” or “land parcel”) to appear as a recognizable unit on the map. When map data are stored in a computer, they may be drawn at almost any scale and this can give an impression of greater accuracy than the quality of the survey data may warrant.
Since the map and the corresponding registers form complementary parts of the same system of description and identification, there must be some system of cross-referencing between what is shown on the map and what is recorded in the registers. This usually means that either names or numbers must be given to each separate land parcel. These references are known as property identifiers (PID) or unique parcel reference numbers (UPRN). Various reference systems have been developed including:
- The name of the grantor or grantee
- A sequential title number
- The volume and folio numbers on which the plot is registered
- The name of a farm or locality with an individual plot number
- The registration block and individual plot numbers
- A post office address
- A street index reference and parcel number
- A grid coordinate or “geocode”
The reference chosen should be easy to understand and easy to remember; easy to use for the public and by computers; permanent so that it does not change with the sale of a property, but capable of being updated when there is for example a subdivision of the land; unique; accurate; and economic to introduce.
It is essential that when these numbers or names are drawn on a map that they do not obscure the details of the map itself. The cadastral map should show the boundaries of each land parcel and in some jurisdictions may also show its area and the actual length and bearing of each boundary line. These considerations may obviously demand a scale somewhat larger than that required merely to indicate each surveyed plot.
The smallest satisfactory scale depends primarily on the area of the smallest survey plot likely to be met with, and may thus vary greatly in different circumstances. A much larger scale will be necessary for cadastral maps of towns than for those of rural areas. Similarly a closely occupied countryside consisting of small fields and holdings will require maps on a larger scale than is necessary in areas where there are large farms with open fields.
The maps with which most people are familiar are topographic maps at scales of around 1:50,000. Such maps make it possible to show accurately (though not always to scale) the position of roads, railways, footpaths, villages, rivers, streams, bridges, important buildings, administrative boundaries and other similar features as well as the relief of the land, the depth of water and variations in tide level. These maps are however quite inadequate for cadastral purposes. A simple example will make this point clear. A carefully drawn pencil line will have a width of perhaps half a millimetre. On a map on the scale of 1:50,000 this would represent a line of 25 metres wide of the ground. There are many countries, especially hilly countries, with separate fields less than 25 metres wide. Most cadastral maps need to be at scales of between 1:500 and 1:2,500 although in densely developed areas a larger scale may be needed while in open countryside much smaller scales may be acceptable.
Large-scale plans are initially much more expensive to make per unit area than small-scale maps, but it must always be remembered that, once the large- scale survey has been completed, accurate maps on any smaller scale can be derived from them. The converse is not however true for although larger-scale maps can easily be constructed by using computers, they can never be more accurate than the original data from which they were first compiled.
Usually cadastral maps need only be “planimetric” maps, that is to say, they need not show topographical relief. There may be special reasons why altitudes should be recorded on cadastral maps, but ordinarily all that is needed is a plan of what is seen, without stereoscopy, from a point vertically above the piece of land observed. Distances recorded on such plans are the horizontal distances between points and not the surface distances actually measured on the ground. Thus the area recorded for a plot of land on a steep hillside will be the horizontal equivalent which may be significantly less than the actual surface area.
A third important requirement of cadastral maps is that they should show a sufficient number of points which can be accurately identified on the ground to enable any other point on the ground to be identified on the map (or vice versa) by eye or by simple and short measurements. Professionally this requirement is satisfied by marks recording the original triangulation stations, or the stations on supplementary theodolite traverses, but this is usually inadequate or inconvenient for practical purposes. In areas where there are permanent fences or fields surrounded by embankments, the fences and banks may provide an adequate means of detailed identification, but in unfenced open fields without any embankments, some means of indicating the land parcel boundaries on the ground will be necessary.
A good mark must be durable in itself and not easy to remove either accidentally or wilfully. In many countries it is also desirable that the material of which it is made should not be of a kind that encourages theft. Since the marks must be easily recognizable they must be fairly conspicuous on the surface but for important points, such as those used as control for surveys, there are advantages in supplementing surface marks with marks that are set in concrete and buried beneath them.
One principal method of identification used in cadastral maps is the “grid”. In some countries, such as much of the public lands in the United States, a grid has been laid out on the ground creating a “rectangular system”. All parcels of land are formed by straight lines, often running north to south and east to west. The problem with such a system is that it is unsympathetic to the natural topography but its advantage is its simplicity and the relative clarity of the boundaries on the ground. More commonly however a grid is used as a referencing system so that the coordinates of all boundary turning points can be measured, calculated and recorded. The data can then be stored in a computer and used either to draw the cadastral maps or else for helping a surveyor to re- establish lost boundary marks.
Cadastral Plan showing bearings and distances of sides, areas, and plot numbers
The land parcel reference number can be used to identify the plot. It can be cross-referenced both to the files that contain more detailed survey information about the parcel such as its dimensions, and to the data on ownership, value and use. In many countries the records of survey are held in one government department (the Survey Department) while text data and details of title are held in another. The latter may, for example, be the Lands Department, the Ministry of Justice or even the Government Treasury. It is important that wherever records of land parcels are maintained, every authority adopts the same standard land parcel referencing system.
It is also essential that changes in land parcel boundaries be recorded as soon as they are agreed. All interested parties must be notified immediately of any changes that have taken place affecting land parcels, for example where there has been formal subdivision. A cadastral map must be up to date at all times.
SOURCE: FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
