OpenStreetMap

Hadjer Lamis, Chad - Tracing guide (villages and buildings)

Posted by IvanGayton on 14 May 2016 in English (English)

Overall goals

The Hadjer Lamis area is very poor, and there is an unusual burden of disease and malnutrition amongst the population which contributes to high mortality in children under five years old. In order to better understand, assess, and respond to this, we need to know more about the population.

We are mapping villages and taking their names on the ground, but identifying all of the inhabited areas and counting the structures is much more efficient from aerial photos. Knowing where all of the villages are scattered through the savanna helps us to map them, and counting the buildings within each village gives us a quick and fairly accurate method to estimate population (important to understand the spread of disease and identify areas of highest need). Perhaps surprisingly, structure counts are often more accurate than asking how many people live in the villages.

The tasks at hand are:

  • Find all of the villages in the area, and draw an area around them, tagging each as Land use, Residential.

  • Map the structures in each village. This is done by tracing all buildings as polygons and tagging them as Building Features=Building (in iD editor) or building=yes (in JOSM) , or by simply counting them and adding a tag Structures with the appropriate number.

Village structure

Settlements in this part of the world are generally organised into extended family compounds, each containing a number of small shelters and often a few crops. You can see the outlines of the compounds as dark lines dividing the village in to smaller sections. The villages are generally more or less circular.

Overview of Absoufa village

This village (Absoufa) is quite typical of the villages in the area. Here's what a little part of it looks like on the ground.

A small part of Absoufa

Villages much like this are scattered all over the landscape in Hadjer Lamis. It is difficult to find them all on the ground, as even the local people do not necessarily know all of the locations of all of the villages! If they are already traced, it is much easier for our field teams to find the villages to tag them with the appropriate local names and other information.

Try to trace around all of the compounds, not just the houses; the compounds often extend well past the inner circle of houses but are definitely part of the village.

Houses

There are two basic types of houses here: round huts, often called Tukuls, and rectangular buildings. All are made of some combination of mud and straw (in some cases the straw has been pre-processed by donkeys or cows, making it stickier).

Tukul and rectangular house

The tukuls are easy to spot from the aerial imagery. They are round, with a distinctive central point where the roof peaks. They look a little bit like, well, um, a rounded mound with a darker-coloured point at the top centre... you know, like a mushroom cap.

Peaked roof

Please simply tag all structures as "building". There is a distinction between "house" and other types of building, but it's incredibly difficult to tell the difference from an aerial photo, so best to simply leave them as "building" for now.

Peaked roof

Peaked roof

Peaked roof

The rectangular houses may have a thatched roof or a corrugated metal roof. A metal roof is usually quite obvious from the aerial photo as it reflects a lot of light. In general, the metal-roofed buildings are more likely to be important buildings such as a mosque, chief's home/office, or health clinic. If you see an obvious metal roof, you can add a tag for roof:material - metal.

Some of the buildings may be for storage of grains or animals (i.e. chicken coops). You can't tell from the aerial photo, so don't worry about it! Just trace all of the standing buildings you can see. The primary goal of the building tracing is to estimate population, and we can do this simply by multiplying the number of structures or roof area by a constant determined by surveys we do on the ground.

annotated village

It is fairly common to see a faint ring or rectangle where a building once existed (mud and thatch homes do not last forever). It is best to count only currently standing structure, as this is most likely to correspond to the actual population. However, in the event that you can't tell whether the building is still there or not, please trace it! When in doubt, add the feature. It's easier to remove it than to find a missed one.

Compounds

Unlike areas further south in Africa, where villages usually consist of a collection of homes without much internal separation, villages in Hadjer Lamis are basically organised by family compounds. Each compound is surrounded by a fence of mud, wood, or brush, and contains several buildings (sometimes including grain storage buildings as well as houses). Many compounds also include some cropland, in what is effectively a small farm or large garden.

compound wall

These fences really define the internal structure of the villages.

compound wall

Sometimes buildings are integrated into the compound wall; often you'll see that a rectangular building shares an outside wall with the compound perimeter.

There is no existing default tag for "compound" in OpenStreetMap. Therefore, when they are mapped as an Area and simply tagged "compound", they do not appear in the OSM map (though they are present in the data). For this reason, until we are able to resolve this, we are not tracing the compounds in the current task. However, please learn to recognise the compounds as they are part of the village; if you only trace around the houses the village perimeter will appear smaller than it actually is.

Location: 12.713, 16.408

Comment from Glassman on 14 May 2016 at 14:57

Thanks for posting the images. It should be helpful to Missing Maps Mapathons when instructing new mappers.

Have you considered proposing a landuse=compound on the tagging list? The number of compounds would probably be too small for the standard layer. You might be able to convince HOT to add the landuse=compound to the Humanitarian layer.

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Comment from dcp on 14 May 2016 at 16:35

user=carlitos gomez has already added nodes tagged with place=hamlet. Would it be sufficient to add a second tag with structures=n. It would be very quick to do, but is it sufficient for your purpose? Can you provide a boundary box. We don't want to do more than required!

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Comment from PlaneMad on 16 May 2016 at 10:14

This is very similar to the structure of villages in India near the deserts in the northwest. The compound is essentially a fenced land that includes multiple landuses.

My suggestions for tags is to use barrier=fence for the perimeter and landuse='residential' for the enclosed property. The gardens and crops within a very much residential in nature and are completely for domestic consumption than to sell to a market.

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Comment from PlaneMad on 16 May 2016 at 10:14

Btw, great post. its interesting how humans so far away geographically evolve very similiar habitations.

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