The use of GIS, GPS and aerial imagery for mapping urban agricultural activities on open space in cities

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The use of GIS, GPS and aerial imagery for mapping urban agricultural activities on open space in cities

Survey of open space agriculture in Dar es Salaam, Tanzania

This research project was carried out in 1999, as a co-operation between the German Technical Co-operation (GTZ), the Dar es Salaam City Council and the Ministry of Agriculture and Co-operatives. The main aim was to elaborate an inventory of all open spaces used for vegetable production in Dar es Salaam (see Fig 1) at that time, but also seven years before in 1992. Open spaces play an important role in cities regarding microclimate and can be a buffer for food security in times of crisis (Jacobi et al. 2000). The exact locations and sizes were mapped and integrated into the GIS database of the Dar es Salaam City Council. Areas smaller than 1000m² were not considered. The actual area used for vegetable production on open spaces in the city was assessed, adding up to 4% of the whole surveyed area in 1999. Furthermore, conclusions could be drawn regarding the dynamic development over a seven year’s time span (see Fig 2). This gave indications about the importance of this type of urban land use, mainly in terms of its viability in view of competing land uses such as new constructions. This knowledge base can be used by town planners, city officials and policy makers for decision-making concerning the place of urban agriculture – especially vegetable production – in the city’s development. The urban agriculture map (Fig 2) offers an entry point for support organizations to get in contact with urban farmers. The evidence created by this map can help raising public awareness in respect of vegetable production within the urban area of Dar es Salaam. The main motivation for the survey was to create an opportunity for urban farmers to get more support from various stakeholders concerning questions of land tenure, water supply, infrastructure, extension services and training.

Fig 1: Open space used for vegetable production in the city of Dar es Salaam/Tanzania (picture: Dongus 1999)

Why using Geographical Information Systems (GIS)?

For the purpose of this study, the following advantages of using a GIS proved to be most useful:

Visualisation of spatial data, particularly the distribution of agricultural open spaces in a city
Simple analytical functions such as calculation of the sizes of agricultural areas
Possibility for data overlay in order to investigate relations with various relevant factors, e.g. designated land use, irrigation water quality, socioeconomic variables etc.
Potential for updating digital maps in the future, and extension to a greater range of topics and layers
Possibility to print hardcopies of maps showing any desired selection of topics and areas in any scale, for discussions with stakeholders
Linkage of vector data in maps with attribute data such as type of crops grown or number of farmers
High flexibility: According to the respective local contexts and available data sources, a wide variety of spatial data can be integrated and combined for optimal outcome: Satellite imagery, aerial photography (digital or analogue), topographic or thematic maps of all scales, cadastral maps, GPS measurements etc.

Methodology used for Dar es Salaam survey

This chapter describes the methodology which was successfully applied in Dar es Salaam. Given that GIS and either aerial photography, high resolution satellite imagery or Global Positioning Systems (GPS) receivers are available, it can be used in a similar way in cities anywhere in the world.

In Dar es Salaam, a total area of 165 km² was surveyed in a period of four months. Different working techniques were used: (1) analysis of analogue aerial photographs (stereo pairs), (2) field work and (3) digitisation, visualisation and analysis of results by using GIS.

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