G. DE WISPELAERE *
(*) G. de Wispelaere, Cartographic Section, I.E.M.V.T.
The reconnaissance plan (or map) of pasturelands
The synthesis map
The inventory map
The selection of scales for maps of natural pasturelands is determined by numerous factors: the area of the study, the aims, the duration of the work on the field and the cost.
By reviewing different kinds of cartographic documents on pasturelands, the author analyses the factors of selection concerning the scales for each category.
There are many factors involved in the selection of scales for pastureland maps. The more important include: the area of the territory to be mapped, the duration and precision of the field work, and the intended purpose of the document. The scale of existing topographical documents as well as the type, scale, and date of available aerial photographic coverage play a fundamental role.
Due regard should also be paid to other cartographic documents such as pedological and geological maps, maps for physical planning, etc., which in the case of integrated studies require a common scale so as to ensure uniformity.
Finally, the cost, more than any other factor, often determines the choice of scale.
From the analysis of these various factors different types of cartographic documents can be envisaged.
The choice of this type of document depends mainly on the duration of the field work for the area of study, since the duration of work is itself often determined by a fixed budget.
Such maps generally cover vast areas. They are drawn on 1:100,000 or 1:500,000 scale, and might cover one or several regions or even an entire country.
However, if the duration of work is too short for the area under study, this document can be of larger scale, such as 1:200,000 or 1:100,000, up to a limit of 1:50,000.
These plans, which are often presented in a simplified manner, only give the general appearance of pasturelands. They cannot be considered real maps, especially as regards the location of vegetation, since they are too general.
Nevertheless, such reconnaissance maps are valuable for detailed studies. Furthermore, excellent topographical maps at these scales can facilitate the elaboration of such detailed studies.
The 1:100,000 and 1:500,000 scales seem to be the best suited for this type of man.
A very detailed document as regards both contents and presentation, this map can only be prepared, in our opinion, from abundant data already transcribed on the inventory type map and should cover, if not a whole country, at least a substantial part of it.
As a synthesis, this document should incorporate in addition to the conventional survey information the ecological data reflected by the vegetation. It should further indicate land use and existing or future facilities for the use and improvement of range areas (wells and watering points, veterinary infrastructure, and the like).
The most commonly used scale is 1:200,000. This is also the scale of the topographical maps in French-speaking Africa; it is of considerable advantage because the nature of these maps makes it possible to superimpose thematic information on an accurate topographical survey.
It should be noted, however, that the use of such topographical surveys has some drawbacks, especially in the Sudano-Sahelian zone, because most were done about ten years ago. Since then lines of communication as well as villages have shifted, while areas under cultivation have expanded considerably.
This scale allows mapping of areas varying from about ten to one hundred thousand square kilometers.
Maps on this scale are designed carefully and many users find them satisfactory.
Because of the surfaces they cover and the precision they require, these maps are time-consuming and costly; the mapping unit appears on the document, a 5-mm square representing 100 ha, or occasionally less for vegetation formations such as bourgou.
This scale requires thorough field work and a good knowledge of the interpretation of aerial photographs.
These maps, which are prepared most often by means of photo-interpretation, preferably utilize recent aerial photographs of good quality; if not, additional field work is needed to update the information. The scale of these photographs should be such that the number of prints to be used is as low as possible, taking into account the requisite precision.
So far the aerial photographs utilized for the preparation of inventory maps on a 1:200,000 scale are at 1:50,000.
Photographing on a smaller scale (1:100,000 for example), by using emulsions that are better adapted to the interpretation of vegetation than the panchromatic emulsion, reduces the cost of the map by diminishing the number of prints. Although they have been tested in France, before their systematic use is suggested they should be tested in tropical Africa.
Here again the area covered determines the selection of the scale.
The main thing is to draw a map on the smallest scale possible, taking into account the objectives established and the precision required.
More than in the inventory maps the scale, the type, and the period and quality of the print are decisive factors. It is therefore appropriate to examine carefully the various factors pertaining to each particular case before proposing a specific scale.
A choice can be made between 1:100,000 (for areas ranging from one to ten thousand km²), 1:50,000 (for an area varying from three hundred to one thousand km²) and 1:25,000 or 1:20,000 for smaller areas, taking into account that a map at 1:25,000 needs an air coverage of a similar or bigger scale.
The 1:10,000 scale should be reserved for very detailed studies covered by aerial photographs, which should also be blown up.
These two scales are used mostly for studies of ranches. They should be able to indicate existing facilities exactly (division of plots, fencing off, etc.) and facilitate pastureland management (rotation, enclosure, improvement through the introduction of fodder crops, and so on).
For this type of mapping the basic topographical documents available are often insufficient and sometimes non-existent.
Therefore, for a fixed scale one should consider the establishment of a topographical plan from aerial photographs either according to a photo plan, which is usually an expensive document, or a controlled mosaic or, lastly, from a simple assembling of negatives (free mosaic). In this case, however, the scale is approximate and there is a lack of accuracy.
The choice of a mapping scale is important for the publication of thematic information compiled during the field survey.
A bad choice in a study of this type can discredit the work done and fail to supply the user with the document he should rightfully expect.