Use of Geographic Information Systems (GIS) and Global
Positioning Systems (GPS) for mapping urban agricultural activities
and open space in cities
*)Dongus, S. & Drescher, A.W.
In spite of all ongoing research on urban agriculture, in most of the world’s cities only little is
known about 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 open questions arise: 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?
Therefore we face a lack of data and knowledge concerning the extent, the importance, the development and the output of urban agriculture. Geographical Information Systems (GIS) are widely used for urban planning purposes since decades.
Some limited experience with the application of GIS to urban food production activities is already existent. For Santiago de los Caballeros (Dominican Republic) a GIS based database has been developed by the Center for Urban and Regional Studies (CEUR) of the University of Santiago (Del Rosario et al. 1999). Experiences from a survey in Dar es Salaam (Tanzania), which was carried out end of 1999 show that one possible way to close the information gap is to map urban agriculture by using GIS. This survey concentrated on open space production within the urban area of Dar es Salaam (Dongus 2000).
Objectives of the Dar es Salaam survey
The main aim of the research project was to elaborate an inventory of the currently existing open spaces used for vegetable production in Dar es Salaam, but also of the open spaces used for vegetable production in 1992, including their different sizes and locations. The results will close the information gap regarding the actual area under vegetable production, the dynamic development in the last decade and thus allow to draw conclusions about the importance of this type of urban food production and its role in regard to urban food security. Furthermore, the results will contribute to a knowledge base, which can be used by town planners, city officials and policy makers to make future decisions concerning the place of urban agriculture – especially vegetable production – in the city’s development. The inventory will also make it
easier for support organizations to get in contact with urban farmers.
Another main aim of the research project was to raise public awareness in respect of vegetable production within the urban area of Dar es Salaam. It was intended to incorporate already existing material on Dar es Salaam’s agricultural land-use (Dar es Salaam City Commission,
University of Dar es Salaam, Mapping Department).
*) AGT – Applied Geography of the Tropics and Subtropics, University of Freiburg, Germany. Emails: firstname.lastname@example.org, email@example.com
The thought behind the project was to make it possible for urban farmers to get more support from various stakeholders concerning the question of land tenure, water supply, infrastructure, extension services and training.
The medium to present the results of the survey will be a map in digital and paper form that comes with additional tabular data showing further information, created with GIS Software.
Why using Geographical Information Systems (GIS)?
; GIS allow to visualize and analyze geographical data, for example the distribution of
agricultural open spaces in a city
; GIS automatically calculate the sizes of digitized areas
; the maps in digital form can be overlaid with already existing digital data, e.g. water
quality or designated land use
; digital maps can be updated in the future and extended to a greater range of topics
; GIS make it possible to print hardcopies of maps showing any selection of topics and
areas in any desired scale
; It is possible to link the spatial data in maps created by GIS with further attributes like
roads with their state, water with its quality and agricultural open spaces with the type
of crops grown on them or the number of farmers working on them.
; GIS enables linkages of socio-economic data with urban production (extent, location,
kind of e.g. aquaculture, cash crop, vegetable, animal husbandry etc…)
; Satellite images offer great opportunities in combination with GIS, especially if they are
available in digital form. Currently, satellite imagery does not reach the same resolution
as aerial photographs do, but considered that satellite imagery is becoming more and
more accurate, it might soon be useful for mapping urban agriculture even on a small
Methodology used for Dar es Salaam survey
This chapter describes the methodology used in Dar es Salaam. However, given that GIS and either aerial photographs or GPS receivers are accessible, it can be used in a similar, maybe slightly adapted way in most parts of the world.
In Dar es Salaam, a total area of 165 km? was surveyed in a period of four months, and different working techniques were used:
; Analysis of aerial photographs
; Field work
; Digitization and analysis of results using GIS
Analysis of aerial photographs
This was an essential step which made it possible to cover 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; scale 1:12500), taken in 1992. The photographs were used in combination 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 transferred to the set of cadastral maps (scale 1:2500) of the Dar es Salaam region. The high resolution of the aerial photographs and the large scale of the 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.
Field work (using cadastral maps and GPS technique)
All areas identified as productive open spaces during the aerial photograph’s analysis were visited with a motorcycle as means of transport. In almost all cases the sites were checked, only few sites turned out to be inaccessible for checking (e.g. army land). The extent of the site was compared to the one of 1992. In a lot of cases this was possible only by working with the cadastral maps. In case of changes, the new situation was marked in the map. In some cases, Differential GPS technique was used to measure newly emerged open spaces and to measure significant changes in unintelligible areas. If farmers were found at the site, they were asked a few questions concerning ownership of the site, access to water and general problems/comments.
The reason to use Differential GPS technique (which requires 2 GPS receivers – one in the
field, one stationary) was the high accuracy (<3m). The geographical coordinates of the corner points of agricultural open spaces were measured by GPS and saved in the receiver. Later on, the coordinates were downloaded to the computer.
Without the aerial photographs, the amount of work would have been considerably higher. In this case, more work in the field would have been necessary, also more measurements by GPS.
Digitization and analysis of results (using GIS)
Finally, the results from the aerial photograph’s analysis and the field work were digitized. This
was done by the use of MapInfo, which is a common Geographical Information System (GIS) Software that allows visualization and analysis of geographical data, for example to calculate the sizes of all digitized areas. Other common GIS which could have been used for this purpose are e.g. ArcView and AtlasGIS.
In cases where the borders of the open spaces were marked in the cadastral maps, they were digitized 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 connected to areas manually. A digital base map of Dar es Salaam (including roads, railway lines, rivers and the coastline) was already available. Otherwise it would have been no problem to digitize a paper map which is accurate enough for this purpose.
Additional data like the information gained in short interviews with farmers was also added to the digital map (in form of attributes to objects), visible in tables.
This database can be used by town planners for further analysis and planning purposes. There was no compatible data available yet to create map overlays, but the Dar es Salaam City Commission (DCC) has already started to adapt its existing GIS database to MapInfo/Arc View format.
For the purpose of mapping agricultural activities an open space in cities, the use of GIS has some obvious advantages:
GIS will allow planners to monitor changing urban food production trends more easily as cities continue to undergo rapid changes.
Hall (1998) explains major advantages of GIS in a few words: “Geographic Information Systems are capable of integrating geographical data with other data from various sources to provide the information necessary for effective decision making in planning sustainable development. Typically a GIS serves both as a tool box and a database. As a tool box, a GIS allows planners to perform spatial analysis using its geo-processing or cartographic modeling functions such as data retrieval, topological map overlay and network analysis. Of all the geo-processing functions, map overlay is probably the most useful tool for planning and decision making - there is a long tradition of using map overlays in land suitability analysis. Decision makers can also extract data from the database of GIS and input it into different modeling and analysis programs together with data from other database or specially conducted surveys”.
The use of GPS also proved to be quite useful for measuring big agricultural open spaces. Even more effective is the use of remote sensing data such as aerial photographs, and in case it is accurate enough, satellite imagery.
Del Rosario, P. J. , Y. Cornelio, L.Y. Polanco, A. Russell, H. López & P. Escarramán (1999): Manejo de Residuos Sólidos y Agricultura Urbana en La Ciudad de Santiago de los Caballeros. CEUR, Santiago.
Dongus, S. (2000): Vegetable Production on Open Spaces in Dar es Salaam – Spatial
Changes from 1992 to 1999. Urban Vegetable Promotion Project, Dar es Salaam, Tanzania.
Hall, P.A.V. (1998) [EDITOR]: Software for Land Management. Decision Support Systems for Sustainable Development: Experience and Potential - a Position Paper. The Open University, UK, IDRC Ottawa.