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3D Analyst - Watershed from SRTM


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Creating Watershed From SRTM DEMs
by Muhammad Arief Arianto

Published in: Business, Technology
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3D Analyst - Watershed from SRTM

  1. 1. <ul><li>Creating Watershed </li></ul><ul><li>From SRTM DEMs </li></ul><ul><li>Advance Geographics Information System </li></ul><ul><li>Muhammad Arief Arianto </li></ul><ul><li>G051060071 </li></ul><ul><li>MIT-NRM </li></ul><ul><li>Bogor Agricultural University </li></ul>
  2. 2. <ul><li>Watershed is the upslope area contributing flow to a given location. Such an area may also referred to as a basin, catchments, subwatershed, or contributing area. </li></ul><ul><li>Watersheds can be delineated from a DEM by computing the flow direction and using it in the Watershed function. </li></ul>
  3. 3. STUDY AREA
  4. 4. <ul><li>GENERAL PROCEDUR </li></ul><ul><li>This process will conducted by using arcgis92 software. There are seven main steps to create a watershed, they are : </li></ul><ul><ul><li>Create DEM from SRTM data; </li></ul></ul><ul><ul><li>Remove Error Sink in the DEM; </li></ul></ul><ul><ul><li>Generate Flow Direction; </li></ul></ul><ul><ul><li>Generate Flow Accumulation; </li></ul></ul><ul><ul><li>Generating Channel; </li></ul></ul><ul><ul><li>Generating Stream Link; and </li></ul></ul><ul><ul><li>Generating Watershed </li></ul></ul>
  5. 5. Creating Dem using 3DEM <ul><li>Open 3Dem SRTM software </li></ul><ul><li>Click file and chose load terrain model and a new windows will appears </li></ul><ul><li>Chose SRTM Data (hgt) on file type and than press OK button </li></ul>
  6. 6. <ul><li>Load the filled HGT tiles. </li></ul><ul><li>Chose the smaller area from hgt files by clicking on operation menu and click select smaller area or by pressing F8 button on keyboard. </li></ul>
  7. 7. <ul><li>Reproject the DEM to UTM by clicking on operation menu and choose change projection </li></ul><ul><li>Write the data out as GeoTiff DEM by clicking file menu. </li></ul><ul><li>The data is now ready to be added to ArcMap, but still there are negative values even though SRTMFill was used, and that Should be cleaned. It is necessary to replace these negative values. To remove negative values use Spatial Analyst raster calculator. </li></ul>Use the conditional statement : “ con([Palu.tif] < 0, 0, [Palu.tif]) ” This is a Visual Basic- like command and is interpreted as if the raster value of Pamong.tif is less than zero then replace the raster value with zero otherwise replace it with the original value.
  8. 8. Remove Error Sink in the DEM <ul><li>Before removing error sink, we should convert DEM type from floating point to integer Because elevation more dealing with integer number. Procedural steps for converting are </li></ul><ul><ul><li>Activate the spatial analyst extent ion, click on spatial analyst menu bar and chose raster calculator. Apply formula below “Int ([palu.tif])” and press evaluate button. The result will be automatically named by arcmap as Calculation, and store the information on folder Calc </li></ul></ul>
  9. 9. <ul><li>The output is a temporary GRID. To make it permanent right click the file named calculation and select Data and choose make permanent </li></ul><ul><li>If a cell is surrounded by higher elevation cells, the water is trapped in that cell and cannot flow. The Fill Sinks function modifies the elevation value to eliminate these problems. To remove an error sink, use the Fill tool in Hydrological Modeling Tools. Input surface raster by choosing calculation file name, choose the file name want to used, and let the Z limit because this is optional </li></ul>
  10. 10. Result Before Applying After Applying
  11. 11. Generate Flow Direction <ul><li>The Flow Direction function takes a grid as input, and computes the corresponding flow direction grid. The values in the cells of the flow direction grid indicate the direction of the steepest descent from that cell. To showing the direction of flow, Click Spatial analyst tools, choose hydrology and choose flow direction by clicking the tool. To generate a flow directions use the previous result (result from removing sink error). Let the force all edge cells box and output drop raster as a default </li></ul>
  12. 12. Generate Flow Accumulation <ul><li>The Flow Accumulation function takes as input a flow direction grid. It computes the associated flow accumulation grid that contains the accumulated number of cells upstream of a cell, for each cell in the input grid. To create a flow accumulation, select the output flow direction file, and goto hydrologi toolbox which in on the spatial analyst tools, choose flow accumulation by clicking it, input the file name want to used, and let others as default </li></ul>
  13. 13. Generating Channel <ul><li>Based on the flow accumulation, we can map stream channels where the water flows. This channel is based on the number of cells which flow into a specific cell. If we want large channels we use a larger number. For this Assignment will mappings all cells that has at least 400 cells that flow into it. Because the cell size is 90 m, these means we are mapping all cells that have a drainage area of at least 90 x 90 x 500 = 3.24 km2. This operation will done by choosing raster calculator and applying formula : ([FlowAccumulation]) >= 400. </li></ul>
  14. 14. Generating Stream Link <ul><li>After mapped stream channels we have to assign each individual stream segments a unique number. This is done with the tool Stream Link tool </li></ul>
  15. 15. Generating Watershed <ul><li>After conducting those operation, now ready to do surface hydrology analysis to generate watersheds. Use the tool watershed from hydrology toolbox. Input flowdirection file on the first input box, input streamlink file on second input box and let others as default </li></ul>
  16. 16. <ul><li>The last step After applying watershed operation are converting the result to vector format and doing some cliping, because some part of watershed crossing the coastline </li></ul>
  17. 17. <ul><li>THIS IS END OF SLIDE… </li></ul>