North Africa Tagging

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Summary

These tags grew out of the 2012-3 Mali Crisis, but are appropriate to the larger Sahara and Sahel region across north Africa.

Highways

See Highway Tag Africa. TODO move/considate onto this page.

See also FR:WikiProject Madagascar#Hiérarchisation government classification used in Madagascar. Presumably some north african countries have government classifications too (These should be documented on the respective territory based wiki pages, then linked from here)

Water

Most waterways (except for the Niger River) are not full of water year-round and may be dry for many months of the year. For these waterways, add the tag intermittent=yes along with the appropriate waterway tag. You can determine if the waterway is there at least of portion of the year by zooming out of the specific location and comparing the location in question with the imagery at a low zoom level.

There are also lakes that contain no water during the dry seasons. These should be mapped as normal lakes, even if they happen to be dry at the time the imagery was taken. It is still useful to know where they are on a map as after a rain they will likely contain water. They are also good places to dig wells to recover the water that seeps into the ground after a rain. Check the area around them, usually ~50 m or so from the "high water mark" and you will often find existing wells.


Tag Description Picture/example Notes
waterway=river Niger River
waterway=stream
waterway=dam [example]

[example 2 - Niger Delta]

waterway=canal [example]

Infrastructure

Tag Description Picture/example Notes
man_made=water_well A simple dug out hole in the ground that collects water at the bottom. Well with "fan" - Well without "fan" - "Walled" well Water wells are obviously of great importance in the Sahara. The ones we can map from imagery are mostly for watering animals, and are a ~1m round black dot in the imagery. A real well should be very black and uniformly colored across the whole thing; sometimes small bushes look like wells but you can tell they are not from the different coloring due to the sun lighting/shadowing them. Wells are usually located along streams (but not right in the stream) and in other low lying areas where water collects and seeps into the ground. Also, most small villages will have one or more wells just outside the perimeter of the village, and/or a well in a "town square" at the center of the village. Finally most wells will have at least a small footpath leading to them, whereas a random bush is not likely to have such repeat traffic to it. For heavily used wells they will often develop a "fan" shape around them from the convergence of many tracks leading up to the well and animals standing around the well. Lastly, herds of cows spend a lot of time standing around wells so if you see a herd of cows there is a decent chance (but not guaranteed) that there is a well in their midst or nearby.
man_made=water_tower A large tank high up on 3 or 4 legs. [example] Water towers are also important to map, but are much less common than wells. They are located in towns, typically around the edge of town but not always. The key to determining something is a water tower is that it should be fairly tall which means it will cast a long shadow, and usually you can see multiple "legs" in the shadow leading to the tank at the top. Also fairly common are water tanks that just sit at ground level. These should not be marked as water towers, but should instead be marked as man_made=storage_tank.
man_made=tower A communications or other tall tower. Towers are usually very tall and therefore cast a very long shadow. Also the tower itself will appear to "lean" in one direction since satelite imagery is usually not taken looking exactly straight down. Because of this you want to position the node for the tower at the base of the tower where the tower itself and the shadow meet in a "V" shape. Also, when mapping a tower it is also good practice to map the barrier areound it and any buildings inside or right next to the barrier as these can give hints as to the tower's purpose.
man_made=cattle_chute A small pen with a long neck for examining animals one at a time. [example] Although normal round or square cattle pens are all over this area, the pens with a long neck forming a cattle chute are much less common. These pens are tend to be located around the more important towns where cattle are brought to market (not sure about this market statement, just a theory at this point). Because the necked pens are relatively rare and seem to indicate towns that are more important to the local economy we mark these specially. The pen itself gets mapped like any normal cattle pen with barrier=wall or barrier=fence as appropriate, and if the pen also features a long neck just wide enough for cows to line up single file it also should have a node placed in the canter of the larger holding area tagged man_made=cattle_chute

Other

For purposes of getting a good base map we have been putting landuse=residential ways around small towns and villages. For the few major towns in this area sometimes there will be obvious industrial or military areas and these should not be included as residential but should instead be mapped appropriately. For the small villages though it is safe to assume (as a first approximation) that the whole village is residential. Because this area is so vast and so sparsely populated if you see a village, mark it before moving on. It is very easy for someone to download the OSM data and go back to fill in details on marked villages, but if you don't mark it it may not ever be seen by a mapper again. Try to keep the residential area "tight" around the village as this makes it possible to estimate population more accurately, and if a village is a "cluster" of small groups of houses with fairly large gaps in between then it is preferable to make different residential areas for each small group of houses. This makes the map a bit more useful in positioning things recorded by feildworkers, and again improves the population estimate by not including empty space in the residential area. Small gaps between buildings from roads and town squares and such do not need to be split into multiple areas, only fairly substantial ones. One exception to this rule is the sparsely populated villages made of small clusters of round huts spread out over a fairly large area. These can be left as a single area or split up a bit based on the relative size of the gaps between clusters, but it is not necessary to make every single cluster of huts its own residential area. In the future we may try to add some extra tag to these areas to mark them as "low density" clusters for population estimates, but the exact scheme for this has not yet been worked out.