Making Slope Classes Map from SRTM

SRTM is a raster data that can be downloaded in various format (HGT, ASCII, TIFF). This article describe us how to make slope classes map from a SRTM data.

1. Download the SRTM data

The first step is, off course, you have to download the data if you don’t have yet. The SRTM data can be downloaded from http://srtm.csi.cgiar.org that available in ArcInfo ASCII and GeoTiff. In this post we use ASCII format. The extracted result is file with extension ASC.

2. Open the ASCII SRTM on ArcGIS

Opening ASCII format SRTM files are not bothered. Same thing like opening a SHP file. But there may be some question whether to make a Pyramid or not. Pyramid is one of a set of raster storage facility-level differences raster data reading at different zoom levels (like the format of ECW / ERS ERmapper). Select Yes / No for the same pyramid will have no effect on this exercise.

SRTM data in ArcGIS display is just like the following:




3. Clip area of interest

Is a good practice to do an analysis (computer charge) only in areas that we examine only. To clip the area, you can use extract facility on spatial analyst (on ArcToolbox).

4. Convert the projection to UTM zona

To convert the raster projection, use this step:
ArcToolbox -> Data Management Tools -> Projections and Transformation ->Raster ->Project Raster



Initial SRTM data ASCII format can be eliminated from ArcMap. Sets of projection data frames (layers) to UTM Zone too.

5. Drive the slope in percent

We can drive the slope of the SRTM data with the 3d or Spatial Analyst. For this post we use the spatial analyst.

Reassure the spatial analyst toolbar in a position to perform,and the extensions in the active position.


Click on the menu Spatial Analyst -> Survey Analyst -> Slope. Then click OK

* Parameter unit selected percent slope grade (or degree)
* Output cell size is the resolution of raster data. Can be left as it is as long as the resolution of SRTM data
* Output can be saved on the hard drive or
* Data generated slope is as follows



5. Slope classification

Slope of the resulting data is continuous data. We can convert the continuous data (no tables) to integer data (a table). Alternatively, we can reduce the continuous data into class intervals of data through a classification

Click on the menu Spatial Analyst -> Reclasifiy
Fill with the range classes that we want. As an example of slope classes 0-8, 8-15, 15-25, 25-40, 40 up



New raster data generated in the form of slope classes. No more data slope, there is a value of 1 to 5 according to the value of the class.

If you need to make generalizations to a small class with a small area can be neglected, for example by Majority Filter (ArcToolbox -> Spatial Analyst Tools -> Generalization -> Majority Filter)

5. Export to SHP (if necessary)

Click on Spatial Analyst -> Convert -> Raster to Features

Credit: http://www.gistutorial.net
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