4.08.03 Environmental Management: Slope

Wed, 11/21/2007 - 14:06
Note: This is a first attempt to provide guidance in preparing the information product needed for the CLUP and is intended to be used hand-in-hand with Volumes 1 and 2. As more knowledge is gathered, the IP will be updated. Likewise, revisions may be required due to new or changing land-use policies. Furthermore, data will continuously be prepared by the custodians, which may require updates. For the latest update, please check HLURB Homepage: http://www.hlurb.gov.ph/ or contact HLURB, telephone +632 927 2698.       Step 1: Provide a Background and Identify the Objectives of the GIS Analysis     The slope of the land is only one of the several conditions considered in determining the suitability of land for future urban development as well as for crops cultivation and production. Low levels of terrain to moderately sloping areas with good soil characteristics are favorable for agricultural cultivation and urban development. Steeply sloping to mountainous conditions makes the land highly prone to soil erosion and is mostly suitable for forest.     The objective of the analysis is to show the slope conditions in the municipality/city and to identify the areas that are below 18% slope. The result will only be one of the inputs when analyzing the land suitable for future urban development.     Step 2: Identify the Indicators to Evaluate Objective Fulfillment     There are many examples of how slopes are indicated today: from sophisticated division of more than 10 classes to a more simplistic approach. However, the key threshold indicator is ‘more than 18% slope’ which are not suitable for urban development in the Risk and Suitability Analysis made in Step 5 of the CLUP process and consolidates it to be ‘forestland’.     Step 3: Create the Database     Attribute     The attribute table below was used for this example. EM02 Slope     Spatial     The spatial feature will be a polygon.     Not Suitable   There are slope maps available at the BSWM for some areas to start from (see below). If no slope map is available from BSWM, contour maps and spot elevation can be found in NAMRIA topographic maps. For the municipal planner with or with little experience in GIS technology it will still be cumbersome to produce a slope map in a digital format. In case there is no available digital slope map at the BSWM it is recommended that the CLUP slope map is prepared by a professional expert (in tandem with the CLUP Base Map preparation).         The following describes the steps to be taken: The image below is an extract from a digitized contour map with 20 M interval (0,5 dpi grey lines): The map shows the contour lines indicating the steepness or flatness of the land. Contour lines which are close to each other mean slope is steep while those farther apart indicates gentler slope of land. It is, therefore, used to interpret the slope % classes. There is however no need to acquire a contour map if a slope map is available.             The slope map, above in this context, shows seven classes of slopes. For simplification, the slope classes have been assigned a color coding where shades of ‘green’ indicates land that are level to moderately sloping while the shades of red indicates land which are strongly sloping to very steep(above 18%).         The contour map overlaid with the slope map. (slope map is 50% transparent )     Step 4: Analyze the Data     The next step will be to generalize the slope classes into two : areas not suitable for urban development and areas to be considered as one of the conditions of suitable areas. Below is a simplified raster layer showing not suitable areas for urban development.         Step 5: Present the Data     The data together with other Baseline maps (fault, flooding, etc) will be used in the Risk and Suitability Analysis.  
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