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March 2003 |
Step-by-Step approach to build a municipal GIS
Mapping
Maps are indispensable if a GIS is used. In most cases, if not all, there will be no accurate map of the towns are available. There are two ways to handle this problem:
- A detailed topographic map can be prepared. If it is decided to prepare a full-fledged map then professionals should preferably carry out the mapping survey. This can be a municipal survey department or a private company. The mapping survey should include all visible objects that are under the responsibility of the municipality.
- A so-called 'sketch-map' can actually be used as basis for an urban management GIS. This sketch-map should be reasonably true to scale. The sketch map will then be used to do the field survey. This will typically be measuring the size of the houses and plots fronting the road and the width of the road. In combination with information on the depth of the plot or house (obtained from the owner or tenant) a good sketch map of the road and the plots and properties can be made. This can be done on paper for easier adjustment, unless a skilled operator is available who can enter the survey results directly into a computerized map. The municipality staff, supported by temporarily assigned staff experienced in applying this method, should prepare the sketch map.
The typical scale of the base map for municipal management is 1:1,000 or 1:1,250 (e.g. 1 cm on the map represents 10 or 12.5 meter). The typical accuracy of such a map is usually taken as 0.5 mm on the map itself, e.g. 0.5 to 0.6 meter in the real world. Regular surveying techniques will easily yield that accuracy but a sketch map created using the GPS method will be less accurate by a factor 10, e.g. the equivalent of a 1:10,000 or 1:15,000 map accuracy. This is important to realize when using the maps to calculate areas, lengths or distances, and when the map is to be overlaid with maps from other sources, which may have a different level of accuracy. Once the base map is prepared it can be digitized; the plots can then be assigned unique numbers.
As mentioned above it may be possible to enter plot / property information directly on a map if a sketch road network map was prepared using the GPS method. Otherwise digitizing is generally done by, first scanning maps and then digitizing on the screen. The best way to do this is to have it done by a professional company. A GIS operator (or preferably two) to work in the municipality should be trained and should help with the digitizing. GIS operators may be staff members of the municipality or may have to be recruited. It may be wise to use a numbering scheme that allows for subdivision of one plot; this can be achieved by adding a 0 to the number (130, 140, 150 and not 13, 14, 15). Alternatively it is possible (and easier to implement) to assign completely new numbers to any new plot that results from division of an old plot or merging of old plots. This option is the preferred option.
It is very important that the relevant authorities in the municipality accept the finished map as correct - this has to be formally in writing.
| GPS - sketch map |
A simple method, which can be used to create such a map, is to use the 'track' function in a simple hand-held GPS. A basically unskilled individual moving through the town streets, either by car, motorcycle or bicycle or even walking will generate a relatively detailed line map. Averaging several repeats will lead to a higher accuracy. The line map can be converted to a street map by adding the street width, which can be simply measured by a tape measure. The error of such a map will be in the order of 5-10 meters, while the cost will be minimal. An additional advantage is that the map is already in digital format and does not have to be scanned; there will be no geo-referencing problems or errors if the right coordinate system is selected.
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