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A GIS-based modeling of environmental health risks in populated areas of Port-au-prince, Haiti

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par Myrtho Joseph
University of Arizona - Master in Natural Resources Information System 1987
  

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3. METHODS

3.1 Study Area

Port-au-Prince (Figure 1) is the administrative, commercial, and political capital of Haiti, but regarding the size it is the second smallest commune of the country. It measures 36 km2 (IHSI 2003). The study area, which is the populated areas within Port-au-Prince, is about 28 km2. Elevation in the study area ranges between the sea level and 600 meters. The last census realized in 2003 indicated that more than 730,000 inhabitants (9% of the country's population) populated this place, which represents about 20,500 people per km2. This pressure of dense population on this narrow strip of land is not without negative impacts on the environment. The Atlantic Ocean forms the northwesternmost boundary of the study area, which in turn verges on several slums. The study area was obtained by cutting off the south section of the base map approximately at latitude UTM 2049000 meters as indicated on Figure 1(left). This cut was done for several reasons. First, the topographic map sheet used for digitization missed a portion of the south section of port-au-Prince. Efforts to find the missing part at the same scale (1:12,500) were fruitless. The largest scale found, 1:50,000 would difficultly allow digitizing the contours 10 meters apart. Another fundamental reason was the fact that the missing area was poorly inhabited with the density of housing close to zero. Since we wanted to assess health risks in populated places, we felt that the exclusion of this area in the study would not substantially affect the study. The last reason concerned time-efficiency. The elevation at these excluded areas was the highest (about 600 meters). Consequently, a lot of contours needed to be digitized, which would add to the burden of digitizing tasks without contributing to the improvement of the study. Therefore, the most convenient choice was to take this section off of the study area.

Figure 1: left: Base map of Port-au-Prince and the study area; right: Port-au-Prince's view from the southeast hills.

3.2 Data collection

The features included in the dataset derive from two main sources: a) data readily available from the Remote Sensing and GIS Unit of Haiti's Planning Ministry (formerly UTSIG, currently CNIGS) and IHSI; b) digitization of multiple layers from topographic map, scale 1:12,500 prepared in 1994 by the Defense Mapping Agency, Hydrographic/Topographic Center, Bethesda, MD. The first source category includes the administrative boundary of Port-au-Prince, the habitat density, and the land use. The IHSI's SDE delimitation contributed to the reattribution of the habitat density layer. Features digitized within an ArcMap interface included: contours, rivulets and other waterways, high voltage power lines relay-centers and power energy centrals, the main roads and other high-traffic-density streets, the waste collection network, formal and informal marketplaces, the main hospitals, the cemetery, the seashore, and the very important points (VIPs), which are landmarks found on the topographic map. All the layers were standardized to UTM projection, NAD83, Zone 18N, unit in meters. As can be seen most of the features were obtained by the laborious digitization process.

After digitization the features where edited in accordance to pre-established set of topologic rules in order to ensure the integrity of the database. Overlaps, dangles, unwanted intersections, wrong attributions, and any other topologic errors revealed by the topology validation tool were corrected with the editing tools provided in ArcMap until all the errors were adjusted.

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