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The use of GIS, GPS and aerial imagery for mapping urban agricultural activities on open space in cities Analysis of aerial photographs This was an essential step which made it possible to map a large area in a short time, and to get very precise results. The analysis was based on the most up-to-date available set of aerial photographs of the Dar es Salaam region (black and white orthophotos, stereo pairs, scale 1:12500), taken in 1992. The photographs were analysed with a stereoscope in order to identify all open spaces used for vegetable production by the time the photographs were taken. The borders of the identified locations were drawn into the respective cadastral maps (scale 1:2500) of the Dar es Salaam region. The high resolution of the aerial photographs and the large scale of the cadastral paper maps made it possible to obtain high accuracy regarding the position of the boundary lines. For ground control, several accompanying field visits were done to avoid mistakes during this working phase. An alternative option would have been to use digital remote sensing imagery with a resolution higher than 1m. However, such digital imagery was not available in Dar es Salaam by the time of the survey, and would have been less accurate than the analogue aerial pictures. Field work (using cadastral maps and GPS) All areas identified as productive open spaces during the aerial photograph’s analysis were visited with a motorcycle as means of transport. All sites were checked. However, some sites turned out to be inaccessible (e.g. army land). The extent of the site was compared to the one of 1992. In almost all cases this was possible simply by working with the cadastral maps and observation. In case of any changes, the new situation was marked in the map. In a few cases, GPS measurements were taken to map newly emerged open spaces, and to measure significant changes in unintelligible areas. If farmers were found at the site, they were asked if they agree to participate in a short questionnaire survey regarding ownership of the site, access to water and general problems/comments. By the time of the survey in 1999, it was necessary to use Differential GPS technique in order to obtain adequate accuracy (error less than 3m). This required the simultaneous use of two GPS receivers, one for mobile use in the field, one for stationary measurements. The geographical coordinates of the corner points of agricultural open spaces were measured and saved in the receiver. Later, the coordinates were downloaded to a computer. Experience showed that it was much more efficient to work without the GPS receivers for the purpose of the study. In most cases, it was sufficient and even more accurate to observe the areas and mark the observations directly in cadastral maps. An alternative option would have been to use enlarged copies or prints of the aerial pictures to mark the results of the field observations. Unlike the use of paper maps, which made it easy to show and explain the purpose of the work to community leaders and members, the use of GPS equipment in the field was sometimes problematic and aroused suspicion, as most city residents were not familiar with this technique. Therefore, it is recommended to use paper maps or printed aerial pictures as a basis for field mapping, and to use GPS only if orientation in the field is not possible otherwise. Digitization and analysis of results (using GIS) Finally, the results from the aerial photograph’s analysis and the field work were digitised. This was done by the use of MapInfo software, as the city council in Dar es Salaam used it as standard GIS by the time of the survey. The borders of open spaces drawn in the cadastral maps were digitized as polygons by the use of a digitizing tablet. In cases where the areas were measured by GPS, the downloaded coordinates were read by the GIS software and manually connected to polygons. A digital base map of Dar es Salaam (including roads, railway lines, rivers and the coastline) was already available. Attribute data such as the questionnaires was entered and linked to the respective polygons.  Fig 2: Map of vegetable production on open spaces in Dar es Salaam, showing the spatial changes from 1992 to 1999 (source: Dongus 2001) . Outlook The created database can be used by town planners for further analysis and planning purposes, and to make overlays with other relevant spatial data. There is a high potential to use the developed methodology in other cities and to expand it to other topics of interest. This is currently done in a research project on use of waste water in urban and periurban agriculture in India and Pakistan and in the analysis of the impact of urban agriculture on health in Dar es Salaam . References
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