Africover cartographic standards

Cartographic standards are all the rules used to represent complexity of the reality in form of a map (bi-dimensional way).

They can be summarized in three main groups:

• Scale of interpretation: Representing the level of detail given (or a level of detail compared to a given scale). In AFRICOVER a scale of 1:200,000 or 1:100,000 are used respectively for large or small countries (or specific areas of interest in a country –Nile river delta). However, this is general information, for a more specific inside of the information content see the variable minimum mappable area (V.M.M.A) concept.
• Geodetic Framework: All AFRICOVER thematic layers (land cover, rivers, roads, toponomy) are referenced to a unified geodetic and cartographic framework for the whole African continent, following the recommendations by the AFRICOVER Working Group for Geodesy and Mapping. It has been an important requirement, considering that at least seven reference ellipsoids and over twenty horizontal geodetic datums are used in Africa.

The following geodetic and cartographic framework was selected for Africa-wide land cover mapping by the AFRICOVER project:

- Reference ellipsoid: Geodetic Reference System 1980 (GRS 80);
- Geodetic datum: World Geodetic System 1984 (WGS 84);
- Cartographic projection: Universal Transverse Mercator (UTM);
- Planimetric accuracy: 50m (dominant base map features) to 100m (land cover polygons);
- Map sheet layout: based on the International Chart of the World (ICW).

• Cartographic representation of thematic information.

It is a large set of rules to represent the thematic content in a cartographic context that AFRICOVER has fully standardized. Only the most important of them are listed below:

1. The Minimal Mapping Area (M.M.A.) is a concept applied by cartographers when addressing the smallest area that can be shown on a map. Historically, the cartographers determined one particular minimum size of area to be mapped; this was applied to all classes contained in the legend. The disadvantage of the method applied in this rigid way is that classes with a difference in importance would follow the same rules. AFRICOVER has introduced the concept of Variable Minimal Mapping Area (V.M.M.A.) where the user can relate the size of the minimal mapping area to the eight major land cover groups of LCCS and to all the classes belonging to them. This gives to the map producer the possibility to better calibrate the level of information given. In Africover more emphasis is given to agriculture, natural vegetation classes instead of natural bare areas, etc. In the same national database, therefore, the use can have some classes calibrated to the level of information similar to 1:200,000 scale and others similar to 1:100,000 or larger scale.

2. The mixed unit is a concept related to point 1. When two features A and B are both too small to be mapped alone according to the V.M.M.A. concept, a polygon comprising the two can be traced and coded as mixed unit (A/B or B/A). In all this complex matter AFRICOVER has produced detailed rules to help both the photo-interpreters and the final users of the database. A specific syntax for example exist:

A/B: the use of a single slash between two classes represents a cartographic generalization. It means that in the polygon having a similar code both features A and B exist. The feature A is predominant (larger area extent in the polygon) as compared to B (B covers at least 20 % of the polygon area). The only exception to this rule is when a class of "isolated fields" is present. In this case the area of the second class is between 10-20% of the polygon area. According to the Africover rules, a cartographic mixed unit can be done up to three classes: A/B/C. The "cartographic mixed unit" should not be seen as a limitation of the database. In other mapping exercises, generally this information, being smaller than the M.M.A, will disappear. In Africover they will remain even if they lose the full geographic location. In other exercises mixed units are used sometimes to show "cartographic representation problems" sometimes "thematic uncertainty" of the interpretation. Africover has fully standardized the difference between the two with a different syntax.

A//B: is a thematic generalization, it means the polygon can have or the feature A or the feature B.

A///B: is a time-related information. It is used only for agricultural classes, it means the polygon has one year the class A and the other year the class B.