[Gvsig_english] Create Slope from Contour Line

[Adi Fajar RAMLY]

Dear Simon,

Thank you for your assistance, going to try it now. Thanks.

Adi

On 11 July 2011 11:14, Simon Cropper (The fosGIS Workflow Guides) <
scropper at botanicusaustralia.com.au> wrote:

> 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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>
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-- 
Adi Fajar Ramly
GIS Division
Ministry of Environment
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