How To Calculate Slope From Digital Elevation Model

Many times you need to see how a terrain is changing in terms of slope over certain area, in GIS terms a slope is defined as rise/run is defined as Maximum rate of change in elevation over distance between cell and its eight neighbors. 

Image by Elias Sch. from Pixabay

Data you need: A DEM of your area easily available over internet 

Start Arccatalog

ArcCatalog Window 

Make Connection to Folder containing Data Set by right click on Folder Connections at top left tab “Folder Connections”.

   Launch Arc Map

Tool Interface

   Right Click Layer and select Add Data -> Add “gridfile.txt” or DEM 

Click on the search icon (Ctrl+F)

Ascii To Raster Tool 

Using local search. Type “ASCII to raster” in the dialog box. 

Click on function ASCII to Raster (Conversion) (Tool) as shown in figure 

A window as shown below appears

ASCII to Raster Tool Interface 

  Leave output name as it is by default

   Specify the Output data type as FLOAT because the given elevation data includes decimal values.

   You can use the identify button  on the grid created to verify that the numbers correspond to the       values in the table above.

   We are going to use the Spatial Analyst Tool of ArcGIS to calculate slope

Spatial Analyst Tool 

Open Customize-> 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.

Extension Enabling in ArcMap 

If the spatial analyst tool does not show up, then you will need to speak to the software administrator to get it installed.
Open the tool Spatial Analyst Tools under the Arc Toolbox.
Then double click on Surface, double click on Slope (As shown in figure below)

Slope Tool in ArcMap 

 You should see the “Slope” Menu as shown in figure.

Slope Tool Interface in ArcMap 

Select ASCIITo_txt1 as the input raster and leave names by default (Slope_ASCIIT1) for the output raster. Set the Output measurement to PERCENT_RISE and leave the Z factor at 1. Click OK. Note that name may change based upon your data set name 

Note: you can click on Tool Help button to read details on this tool.  Note that when you click on each field in the dialog box, the help part of the dialog to the right explains the content of the file or option.

The resulting Slope grid should now be added to the display.  

Slope Calculation Process