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GoalObjectivesComputer and Data RequirementsPart 1. Slope calculationsPart 2. New Orleans Flood Volume Calculation.Summary of Items to turn in:CEG 795 – Water Resources Modeling and GISExercise (HW) #3: Spatial Analysis DUE: Monday, February 13th thModified from:Prepared by David G. Tarboton, Utah State UniversityDavid Maidment and Oscar Robayo, University of Texas at AustinGoalThe goal of this exercise is to serve as an introduction to Spatial Analysis with ArcGIS.Objectives- Calculate slope from a grid digital elevation model- Apply model builder geoprocessing capability to program a sequence of ArcGIS functions- Use raster data and raster calculator functionality to perform calculations of flooded area and flood volume due to the Hurricane Katrina flooding of New Orleans. Computer and Data RequirementsTo carry out this exercise, you need to have a computer, which runs the ArcInfo version of ArcGIS. The necessary data are provided in the accompanying zip file, exercise3.zip. The first thing you should do is set your current workspace to a directory where files (layers) that are created will be stored. To do this, select Tools, Options, Geoprocessing, Environments, General Settings. Put in a folder of your choice. 1Part 1. Slope calculations1.1 Hand CalculationsGiven the following grid of elevations. Calculate by hand the slope and aspect (slope direction) at the gridcell labeled A using(i) The standard slope function(ii) The 8 direction pour point modelRefer to the powerpoint slides from lecture 4 to obtain the necessary formulas for each of these methodsGrid cell size 100m57 55 47 48 4853 67 56 A 49 5253 52 45 B 42 4351 58 40 41 40Comment on the differences and indicate which you think is a most reasonable approximation of the direction of water flow over the surface.Question #1 (To turn in): Hand calculations of slope at point A using each of the three methods and comments on the differences.1.2 Verifying calculations using ArcGISVerify the calculations in (1.1) using ArcGIS Hydro and Geoprocessing functions. Save the following to a text file 'elev.txt' (This file is also included in exercise3.zip)ncols 5nrows 4xllcorner 0yllcorner 0cellsize 100NODATA_value -999957 55 47 48 4853 67 56 49 5253 52 45 42 4351 58 40 41 40This shows how raw grid data can be represented in a format that ArcGIS can import.Open ArcMap and ArcToolbox. Use the tool Conversion Tools  To Raster  ASCII to Raster to import this grid file into ArcMap. Specify a name and location for the Output raster (elev) and specify the Output data type as FLOAT (It is more consistent to think of elevation data as including floating point data, rather than integer, even though this specific case is integer data).2You can use the identify button on the grid created to verify that the numbers correspond to the values in the table above.Open Tools  Extensions and verify that the Spatial Analyst function is available and checked. This is where the spatial analyst license is accessed, so if Spatial Analyst does not appear you need to acquire the appropriate license ($$$). 3Select View  Toolbars  Spatial Analyst. This displays the Spatial Analyst toolbar in the ArcMap interface. You may dock the toolbar somewhere convenient.Select Spatial Analyst  Surface Analysis  Slope on the Spatial Analyst toolbar.4At the dialog that appears select elev as the input surface and select whether you want the output in Degreeor Percent (to be consistent with your hand calculations above). You can also save this grid file into your directory. Use the identify button on the slope grid that is created to verify that the numbers correspond to the values you calculated by hand and resolve or reconcile any differences. Record in a table the ArcGIS calculated slope at grid cell A. Select Spatial Analyst  Surface Analysis  Aspect on the Spatial Analyst toolbar. At the dialog that appears select elev as the input surface. You can also save this grid file into your directory.Use the identify button on the aspect grid that is created to verify that the numbers correspond to the valuesyou calculated by hand and resolve or reconcile any differences. Record in a table the ArcGIS calculated aspect at grid cell A. Question #2 (To turn in): Table giving a comparison of the slope and aspect using the three different methods (Standard Slope, 8 direction pour point model, and ArcGIS) for grid cell A The table should have two rows (slope and aspect) and three columns corresponding to each method. Finally, compare your results and determine which method ArcGIS is using to compute the slope and aspect.51.3 Automating procedures using Modelbuilder.Modelbuilder provides a convenient way to automate and combine together geoprocessing tools in ArcToolbox. Here we will develop a Modelbuilder tool to automate the importing of the ASCII grid and calculation of Slope and Aspect. First you need to set the location where the Toolbox you develop will be stored. Select Tools, Options, Geoprocessing and find the space where you are asked to specify a location to store My Toolboxes. Put ina directory of your choice. Right click on the whitespace within the ArcToolbox window and select New Toolbox. Name the toolbox "slope_aspect" (or something else you might like).6Right-click on the new toolbox and select new model.The model window should open. This is a window where you can drag, drop and link tools in a visual waymuch like constructing a flow chart.In the Toolbox window browse to Conversion Tools  To Raster  ASCII to Raster. Drag this tool onto the model window.Double click on the ASCII to Raster rectangle to set this tool's properties.7Set the Input ASCII raster file to elev.txt and Output raster to some name (I used elevm so as not to conflictwith elev that already exists). Set the output data type to be FLOAT. Click OK to dismiss this dialog. Themodel elements on the diagram are now colored indicating that their inputs are complete.Locate the tool Spatial Analyst Tools  Surface  Slope and drag it on to your window. Also, locate thetool Spatial Analyst Tools  Surface  Aspect and drag it on to your window. The output from the ASCII to raster function needs to be taken as input to the slope function. To do this use the connection tool and draw a line from elev_a, to Slope. Double click on the Slope and Aspect rectangles to set this tool's properties such as the output


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UNLV CEG 795 - Spatial Analysis

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