GIS for the Development of a Cebu Geographic Planning Atlas
Unfortunately, the data situation in the Philippines and for the Province of Cebu particular, with regard to age is generally very poor.
The official topographic map series which is widely used in the country is based on 50 years old aerial photographs (1947) as is available at full coverage only at a maximum scale of 1:50,000. larger scales, such as 1:25,000 or 1:5,000 are only available for a few square kilometers .
However, since the scale 1:50,000 is also widely used for thematic map series, we decided to use his as our base map for the atlas , in order to have full coverage of the island of Cebu.
Relevant data ( map and statistical data ) was then collected from the different sector departments and agencies and evaluated for their usefulness. After this, data input started
3.2 Data Input
The analog spatial data ( such as geological map series, soil maps, etc .) had to be manually digitized using a digitizing tablet . for each theme approximately 27 sheets at scale 1:50,000 had to be digitized as either point, line or polygon information .
For digitizing the 20-m contour lines of the entire island of Cebu over one year was consumed . ( the often quoted statement hat more than 80% of the time of a GIS project is consumed for data input cab certainly be confirmed ). Digitizing of spatial data was carried out with ILWIS 1.4, after which the data was exported ( in DXF format or BNA) to Arc/Info for further processing.
Attribute or statistical data were taken form official record and entered through the keyboard into tables. Then the records were assigned individual record numbers (ID or object identifiers ) to allow linkage with spatial data in Arc/Info ( DBF format ). Some data had been entered directly into the Arc/Info databases.
Digital attribute data which were obtained form outside sources also needed to be preprocessed to match them with our own database structures.
3.3 Data management
All digitized spatial data were further processed in Arc/Info, where the topology was built-up. Topology defines the absolute location in a coordinates system and the neighborhood relations. All spatial data were registered in the same coordinate system (UTM), unlike in CAD system, where the locations are always relative, not absolute.
There are three basic forms of spatial data processed: the points, lines, and polygons Points and lines are the basic structures, polygons are closed lines . Examples are:
Point : Location of miner deposits , rain gauges , etc .
Line : Rivers and streams, contour lines, road network
Polygon : Mapping units like geology, soils, administrative units .
Under this step, all points , lines and polygons received a unique code ( object identifier ). For instance, over 500 polygons ( mapping units ) of the geological map needed to be labeled individually. The mapping units of another, far more complicated map ( Land Management Units ), had to be code individually with 1983 labels.
Through this coding, it was eventually possible to link the attribute ( or non-spatial ) data with the spatial data ( e.g. political boundaries such as municipalities ) . this combination of spatial with the non-spatial data is the distinguishing characteristics of geographic information system .
For the Atlas production, who kinds of data were distinguished. These are the spatial base data and thematic data. The base data , once generated, served s basis for all other maps to ensure the spatial consistency of e maps. Base data were for instance .
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Shoreline
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Administrative Boundaries
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Rivers and streams
Some other spatial data like the contour lines were also used several times for the generation of several maps , for instance contour lines for the development of an elevation map, a slope map and a DEM.
In this way, over 45 themes were generated and organized as layers. In each layer spatial data are organized , according to their them. While in one layer contour information about river are stored ( as line ), in another layer contour information ( as line ) may be stored, and in a third layer land use information (polygons ) maybe available. All information layers are equal and to hierarchy exists. All layers are referenced through a common coordinate system .
3.4 Data Analysis and Generation of New Information with GIS
New information were generated through spatial operations ( geometric, logical combination, buffers, etc.) or statistical analysis.
3.4.1 Natural Resources Data Analysis
One example of f data analysis done was through digital overlay and intersection of administrative, boundaries with the soil erosion map to calculate eroded arras for each of the 53 municipalities and cities of the island province.
The contour lines were also used to generate a slope map, which could be overlaid with the administrative boundaries. Hence, it was possible to quantify areas above 18% slope inclination should be taken out agricultural or other use and made available for reforestation according to law ).
Interpolated contour lines were also used ot generate a suede-three-dimensional model and to delineate automatically all major watersheds for the island. Hence, it was possible for the first time to calculate the areas of all major watersheds more all less accurately.
Such area calculation can be done quickly with GIS, one the necessary database are already available, and can be used as decision-support for spatial planning purpose
