Reproject the DEM to UTM by clicking on operation menu and choose change projection
Write the data out as GeoTiff DEM by clicking file menu.
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.
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.
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
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
The output is a temporary GRID. To make it permanent right click the file named calculation and select Data and choose make permanent
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
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
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
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.
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