## •  Relief

There are many ways to describe the relief of a region. You can use verbal description (“a valley, a plateau, a steep slope, etc.”), use 3-D drawings (“perspective view”), etc., but the most efficient is usually to describe the altitude (or elevation) of all points on the map.

Elevation is the vertical distance between the surface of the Earth and a reference “zero level”, usually set at sea level. Defining precisely sea level is not an easy task, because of its variations related to waves, but mainly to tides. In Namibia , the reference level is given by the mean level measured over a long period, and elevations are measured in metre.

It is important to notice that elevations given on a map are always referring to the elevation of the ground level. The height of trees, buildings, etc. is never included in the measurement.

There are several solutions to represent the relief on a map:

Hypsometric tints , where all areas having an elevation comprised between two values (for instance 100-200 m) are represented in a given colour, part of a colour gradient. This method is very popular (and often used for school maps), but using background colours for elevations reduces the possibility of using colours for other type of information, maybe more useful for the users. The previous edition of the 1:250'000 maps of Namibia was based on this solution.

Shaded relief is used in many countries (here is an example of the topographic map of Morocco ) It gives a very “sculptural” view of the relief, but can also impede the reading of small text, especially in mountainous areas.

Contour lines is certainly the most common way to represent relief, especially on topographic maps. All points of a contour line have the same elevation. As there is an infinity of elevations on a map (comprised between the elevations of the lowest and highest points, of course), it is theoretically possible to draw an infinity of contour lines, witch would of course completely clutter the map. Instead, only contours corresponding to a limited number of fixed values are represented. These values are defined by the contour interval . On most Namibian 1:50'000 maps, the contour interval is 20 metre. This means that only points having an elevation of 0m, 20m, 40m, 60m, etc. are highlighted by a contour. On the 1:250'000 maps, the contour interval is 50 metre. In some cases, this rule is adapted: In very flat area, the amplitude of the relief may be only a few meters, and accordingly, only very few contours (if any) would be visible, and this will reduce the possibility to analyse the relief. IN this case, the author of the map may decide to add contours at intermediate elevations. For instance, on the 50'000 maps, the contour interval is 20m, but in some areas, “sub-contours” with an interval of 10m are added. These contours are represented as dashed lines of the same brown colour.

On the other hand, in mountainous are, difference of altitude of a few hundreds meters can be observed on very short distance, and the corresponding contours will be too close to be readable. The contour interval of a map sheet can be adapted to reflect this situation (using 40m contour interval for this map instead of 20m on other maps). This will of course be indicated in the legend.

In some places however, a map sheet includes both mountainous and flat area, and it is not possible to find a good compromise on the contour interval. In this case, some intermediate contours can be locally interrupted, but this solution should be avoided when possible, as it is not compliant with the principles of the representation of contour lines. This solution was used on a few 1:250'000 maps (3 rd Edition), and it is always indicated in the legend.

In order to facilitate the reading, a common practice is to enhance some contours, defined as “major contours” . These contours are displayed in the same colour, but with a slightly thicker line width.

On all Namibian maps, contours are diplayed in brown colour (this is an international convention).

Because of the continuity of the surface of the Earth, contours obey to certain rules:

•  A contour cannot be interrupted.

•  Each contour forms a closed line.

(this is of course valid for a series of map. Actually, contours are interrupted at the edge of a map sheet, and/or at international boundaries)

•  Two contours should never cross (the intersection point would have 2 different elevations, witch is of course impossible)

•  Contours are always organised in a logical sequence: between a 100m contour and a 200m one, there must be a 150m contour, if the contour interval is 50m)

It is practically impossible to indicate the elevation value on every contour. Such values are only indicated in a few places. However, thanks to the logical rules explained above, it is always possible to determine the elevation of one contour from its neighbours.

The most important is to determine the direction of the slope, to know if an unlabelled contour is higher or lower than a contour with a value. To do this, you have to analyse other elements of the map: rivers and most waterpoints are found in the lower parts of the landscape. Closed depressions are rare in most natural landscape; a closed contour usually corresponds to the top of a hill, etc. Assessing the form of the relief is not always easy, especially in relatively flat areas. To facilitate the appreciation of the relief, a special convention was used on the new Namibian maps: the elevation value placed on a contour is always oriented with the bottom of the text oriented towards the lowest part of the slope. Because of this convention, it is possible to find some values placed “bottom-up”, witch is a little bit inconvenient to read, but has the advantage to indicate also the direction of the slope, in addition to the altitude.

As seen on this figure, additional information about the relief is provided by spot height values. These are points with a known elevation. The text is usually placed on the right of the point (indicated by a small “+” sign), but this position may be adapted to enhance readability. Spot height points and their values are also displayed in brown colour.

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