[Gvsig_english] Create Slope from Contour Line

[Simon Cropper (The fosGIS Workflow Guides)]

On 11/07/11 13:33, Adi Fajar RAMLY wrote:
> Hi All,
>
> Greetings from Indonesia. I want to ask if possible to create slope class
> from contour lines? I got a lot of questions from colleagues from
> environment office in Provincial/districts, where we try to socialize the
> use of opensource GIS for environmental officer in
> Provincial/Districts/Municipal Environmental Office rather than using
> proprietary software such as ESRI GIS Products.
>
> By the way, is it also possible to generate contour lines from the data dem
> / SRTM (HGT file format) using gvSIG such as in Global Mapper?
>
> thanks for the assistance...
>
> Regards,
>
> Adi
>

Adi,

Here is a draft tutorial I have in the mill to publish on my website.

I am willing to release the text here under a Creative 
Commons-Attribution-ShareAlike (CC-BY-SA Australia 3.0) licence, as the 
process was thrashed out with the help of all the great people on this list.

http://creativecommons.org/licenses/by-sa/3.0/au/deed.en

Eventually I will be releasing the tutorial using actual data and images 
showing each step, so if you are interested you can put yourself on the 
email notification list.

If you have any problem following the steps in the following tutorial 
let me know and I will see if I can help you through the issues.

**Terrain analysis using contour data**

*Step 1. Preparation*

Open a project file, create a new view and add your contour data file to 
the view. This shapefile should contain a numeric field indicating the 
elevation or altitude.

*Step 2. Create a DEM*

Open Sextante and choose “Rasterize Vector Layer” under the 
“Rasterization and interpolation” category.

You should see the 'Rasterize Vector Layer” Dialog.

Choose the vector layer and elevation/altitude field, and input a 
legitimate output file name.

In the Raster Output Tab, select 'Use Extent from layer” and under cell 
size input 20, 50 or 100m for the size of the cell. You will need to 
experiment with this value to obtain the best output.

You should end up with a black box with white cells overlapping the 
contour positions. The black areas are voids in the grid file and do not 
contain data. This file is referred to as a Digital Terrain Model or DEM.

*Step 3. Filling in the blanks*

The next step is to fill in the values between the grid cells that 
appear in the DEM.

Open Sextante and choose “Void Filling” under the “Basic Tools for 
raster layers” category. The “Void Filling” dialog should appear.

Choose the raster layer, and input a legitimate output file name. Leave 
the Tension threshold at 0.1.

You should end up with a graduated black-and-white raster file.

If you move your contour file above the raster file in your Table of 
contents, you can visually confirm that the elevation matches the raster 
file created.

Right click the raster file and select “Raster Properties” then open the 
“Bands” tab. Check and uncheck R, G, B radio buttons to see how the map 
changes. Although not part of the analysis it helps if your choose a 
colour you find appealing and contrasts well with your contours.

Right click the raster file and select “rename” and allocate a name that 
makes sense (e.g. rast_void_filling) to distinguish the raster from 
others versions.
Step 4. Clean up the DEM

Open Sextante and choose “Crop grid with polygon layer” under the “basic 
tools for raster layers” category.

*Step 4. Establishing the Slope*

Open Sextante and choose “Slope” under the “Geomorphometry and terrain 
analysis” category. The “Slope” dialog should appear.

Select complete raster file (e.g. rast_void_filling), the type of slope 
and units required (in my case I use 'Maximum slope Travis et al.1975'), 
  and a legitimate name for the output file.

In the “raster output” tab select “Use extent from layer” and select the 
raster file being used (e.g. rast_void_filling) and leave the cell size 
the same (i.e. 20m).

You should have a black screen with faint lighter areas where slope are 
greater. Depending on the variation across the site then this raster 
file can be difficult to visualize. Move your contour file above the 
raster in the ToC and use the information tools to query individual 
cells to see if the results are as expected. You can play with the 
values in the enhance tab to try and visualize the changes in the cell 
values.

*Step 5. Create classification groups for slope*

Open Sextante and choose “Unsupervised classification (clustering)” 
under the “Raster layer analysis” category. The “Unsupervised 
classification (clustering)” dialog should appear.

Select Raster Layer, set the number of classes to 5 and define a 
legitimate raster file name and database name. The database will contain 
the boundary of the classes.

Open Sextante and choose “Vectorize raster layer (polygons)” under the 
“Vectorization” category. The “Unsupervised classification (clustering)” 
dialog should appear. Select the raster and a legitimate name.


I hope this helps.

-- 
Cheers Simon

    Simon Cropper
    Website Administrator
    http://gis.fossworkflowguides.com
    The fosGIS Workflow Guide

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