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

S. Dongus

S. Dongus
University of Freiburg
Department of Physical Geography
Section on Applied Geography of the Tropics and Subtropics (APT)
Werderring 4
79085 Freiburg, Germany
Email: [email protected]
Web: www.geographie.uni-freiburg.de/ipg/welcome-engl.html
Tel. +49 761 203 9126
Fax: +49 761 203 3596

Prof. Dr. Axel Drescher

Prof. Dr. Axel Drescher
University of Freiburg
Department of Physical Geography
Section on Applied Geography of the Tropics and Subtropics (APT)
Werderring 4
79085 Freiburg, Germany
Email: [email protected]
Web: www.geographie.uni-freiburg.de/ipg/welcome-engl.html
Tel. +49 761 203 3513
Fax: : +49 761 203 3596

Urban agriculture is a phenomenon that can be observed world-wide, particularly in cities of developing countries. Only little is known about the actual extent of inner city areas used for agricultural purposes, and the spatial distribution of such areas. A methodology for mapping vegetable production on open spaces has been successfully implemented in the city of Dar es Salaam, Tanzania. The mapping procedure comprised analysis of aerial imagery, mapping in the field, and integration of the results into GIS. The basic functions of GIS proved to be a very useful tool, whereas the use of GPS was only necessary in few cases. Integrated in local government and planning processes, the GIS database can contribute to raise public awareness for the situation of urban farmers, help to improve extension services, and can be used by town planners for further analysis and planning purposes. The developed methodology can also be applied in other cities and expanded to other topics of interest.

Urban agriculture, i.e. agriculture in the city or in peri-urban areas, is a phenomenon that can be observed world-wide. This is particularly true for cities of developing countries, where food production for subsistence and marketing contributes to the improvement of livelihoods, food security and urban ecology (Smit et al. 1996, Mougeot 2005). In spite of all ongoing research on urban agriculture, in most of the developing countries’ cities few systematic surveys have been carried out on the actual extent of urban agriculture in terms of inner city areas used for agricultural purposes. Also little is known on the spatial distribution of urban agriculture in the cities. Many questions are still open: Where do urban agricultural activities concentrate and why, who is involved, what kinds of crops are grown and by which groups of city dwellers, which kinds of soils are occupied, how is water availability and quality, what is the distance to markets, are there potential health risks? Furthermore, only few data and knowledge is available regarding the extent, the importance, the development and the output of urban agriculture, as well as and its connection to small enterprise development.

In the world-wide context, only very limited experience with the application of Geographical Information Systems (GIS) to urban food production activities is available. For Santiago de los Caballeros (Dominican Republic) a GIS based database has been developed by the University of Santiago (Del Rosario et al. 1999). In Ouagadougou (Burkina Faso), a mapping of urban and peri-urban agricultural areas has been carried out based on IKONOS satellite imagery (Kemeling 2001). The ‘Resource Center of Urban Agriculture and Food Security'(RUAF) recently conducted mappings of the presence of urban agriculture and available open spaces in six cities: Villa Maria del Triunfo (Lima, Peru); Pikine (Dakar, Senegal); Accra (Ghana); Bulawayo (Zimbabwe); Hyderabad (India); and Beijing (China) (RUAF 2006). To date, the city with the most comprehensive experience in this respect is Dar es Salaam, Tanzania. A survey carried out in 1999 showed that one possible way to close the information gap is to map urban agricultural areas by combining analysis of aerial photographs with field work, and using GIS as a tool (Dongus 2001). Drawing on this work in Dar es Salaam, the authors suggest a procedure to map urban agricultural areas, and explain it with practical examples.

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.

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) .


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 .


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