<br>Dear Simon,<br><br>Thank you for your assistance, going to try it now. Thanks.<br><br>Adi<br><br><div class="gmail_quote">On 11 July 2011 11:14, Simon Cropper (The fosGIS Workflow Guides) <span dir="ltr"><<a href="mailto:scropper@botanicusaustralia.com.au">scropper@botanicusaustralia.com.au</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><div><div></div><div class="h5">On 11/07/11 13:33, Adi Fajar RAMLY wrote:<br>
> Hi All,<br>
><br>
> Greetings from Indonesia. I want to ask if possible to create slope class<br>
> from contour lines? I got a lot of questions from colleagues from<br>
> environment office in Provincial/districts, where we try to socialize the<br>
> use of opensource GIS for environmental officer in<br>
> Provincial/Districts/Municipal Environmental Office rather than using<br>
> proprietary software such as ESRI GIS Products.<br>
><br>
> By the way, is it also possible to generate contour lines from the data dem<br>
> / SRTM (HGT file format) using gvSIG such as in Global Mapper?<br>
><br>
> thanks for the assistance...<br>
><br>
> Regards,<br>
><br>
> Adi<br>
><br>
<br>
</div></div>Adi,<br>
<br>
Here is a draft tutorial I have in the mill to publish on my website.<br>
<br>
I am willing to release the text here under a Creative<br>
Commons-Attribution-ShareAlike (CC-BY-SA Australia 3.0) licence, as the<br>
process was thrashed out with the help of all the great people on this list.<br>
<br>
<a href="http://creativecommons.org/licenses/by-sa/3.0/au/deed.en" target="_blank">http://creativecommons.org/licenses/by-sa/3.0/au/deed.en</a><br>
<br>
Eventually I will be releasing the tutorial using actual data and images<br>
showing each step, so if you are interested you can put yourself on the<br>
email notification list.<br>
<br>
If you have any problem following the steps in the following tutorial<br>
let me know and I will see if I can help you through the issues.<br>
<br>
**Terrain analysis using contour data**<br>
<br>
*Step 1. Preparation*<br>
<br>
Open a project file, create a new view and add your contour data file to<br>
the view. This shapefile should contain a numeric field indicating the<br>
elevation or altitude.<br>
<br>
*Step 2. Create a DEM*<br>
<br>
Open Sextante and choose “Rasterize Vector Layer” under the<br>
“Rasterization and interpolation” category.<br>
<br>
You should see the 'Rasterize Vector Layer” Dialog.<br>
<br>
Choose the vector layer and elevation/altitude field, and input a<br>
legitimate output file name.<br>
<br>
In the Raster Output Tab, select 'Use Extent from layer” and under cell<br>
size input 20, 50 or 100m for the size of the cell. You will need to<br>
experiment with this value to obtain the best output.<br>
<br>
You should end up with a black box with white cells overlapping the<br>
contour positions. The black areas are voids in the grid file and do not<br>
contain data. This file is referred to as a Digital Terrain Model or DEM.<br>
<br>
*Step 3. Filling in the blanks*<br>
<br>
The next step is to fill in the values between the grid cells that<br>
appear in the DEM.<br>
<br>
Open Sextante and choose “Void Filling” under the “Basic Tools for<br>
raster layers” category. The “Void Filling” dialog should appear.<br>
<br>
Choose the raster layer, and input a legitimate output file name. Leave<br>
the Tension threshold at 0.1.<br>
<br>
You should end up with a graduated black-and-white raster file.<br>
<br>
If you move your contour file above the raster file in your Table of<br>
contents, you can visually confirm that the elevation matches the raster<br>
file created.<br>
<br>
Right click the raster file and select “Raster Properties” then open the<br>
“Bands” tab. Check and uncheck R, G, B radio buttons to see how the map<br>
changes. Although not part of the analysis it helps if your choose a<br>
colour you find appealing and contrasts well with your contours.<br>
<br>
Right click the raster file and select “rename” and allocate a name that<br>
makes sense (e.g. rast_void_filling) to distinguish the raster from<br>
others versions.<br>
Step 4. Clean up the DEM<br>
<br>
Open Sextante and choose “Crop grid with polygon layer” under the “basic<br>
tools for raster layers” category.<br>
<br>
*Step 4. Establishing the Slope*<br>
<br>
Open Sextante and choose “Slope” under the “Geomorphometry and terrain<br>
analysis” category. The “Slope” dialog should appear.<br>
<br>
Select complete raster file (e.g. rast_void_filling), the type of slope<br>
and units required (in my case I use 'Maximum slope Travis et al.1975'),<br>
and a legitimate name for the output file.<br>
<br>
In the “raster output” tab select “Use extent from layer” and select the<br>
raster file being used (e.g. rast_void_filling) and leave the cell size<br>
the same (i.e. 20m).<br>
<br>
You should have a black screen with faint lighter areas where slope are<br>
greater. Depending on the variation across the site then this raster<br>
file can be difficult to visualize. Move your contour file above the<br>
raster in the ToC and use the information tools to query individual<br>
cells to see if the results are as expected. You can play with the<br>
values in the enhance tab to try and visualize the changes in the cell<br>
values.<br>
<br>
*Step 5. Create classification groups for slope*<br>
<br>
Open Sextante and choose “Unsupervised classification (clustering)”<br>
under the “Raster layer analysis” category. The “Unsupervised<br>
classification (clustering)” dialog should appear.<br>
<br>
Select Raster Layer, set the number of classes to 5 and define a<br>
legitimate raster file name and database name. The database will contain<br>
the boundary of the classes.<br>
<br>
Open Sextante and choose “Vectorize raster layer (polygons)” under the<br>
“Vectorization” category. The “Unsupervised classification (clustering)”<br>
dialog should appear. Select the raster and a legitimate name.<br>
<br>
<br>
I hope this helps.<br>
<br>
--<br>
Cheers Simon<br>
<br>
Simon Cropper<br>
Website Administrator<br>
<a href="http://gis.fossworkflowguides.com" target="_blank">http://gis.fossworkflowguides.com</a><br>
The fosGIS Workflow Guide<br>
<br>
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</blockquote></div><br><br clear="all"><br>-- <br>Adi Fajar Ramly<br>GIS Division<br>Ministry of Environment<br>