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Creation of a candidate areas
map for possible occurrence
of a rare vegetation type.
Quantum GIS Practical
Tim Sutton http://linfiniti.com tim@linfiniti.com

Problem Statement
You are tasked with finding areas in and around the Cape Peninsula that are a suitable habitat
for a rare fynbos plant species. The extent of your area of investigation in the Cape Peninsula
is : south of Melkbosstrand, west of Strand.
Botanists have provided you with the following preferences exhibited by the species in question:
- It grows on east facing slopes.
- It grows on slopes with a gradient between 15% and 60%.
- It grows in areas that have a total annual rainfall of > 1200 mm.
- It will only be fund at least 250 m away from any human settlement.
- The area of vegetation in which it occurs should be at least 6000m2
As a volunteer for Cape Nature, you have agreed to search for the plant on the closest suitable
piece of land to your house. Use your GIS skills to determine where you should go to look.

Due Date: Friday 13 May 2011 at 17h00
Your Task
What to hand in:
- A work log (document) that describes sequentially the process you followed where each
step is annotated to show the specific QGIS tool and tool options you used.
- a shapefile containing the areas you identified to be suitable habitat
- a shapefile containing the site from which you computed distances (e.g. your house)
- a comma separated values (CSV) text file containing the distance matrix between your
origin site and the suitable habitat areas.
- an image file containing the map layout which should include the following elements:
- a hill shaded terrain model
- a false colour semi-transparent overlay of your resampled rainfall layer
- streets presented with suitable symbology for the scale of your map
- the site from which you computed distances annotated with a suitable icon and label
(e.g. your house)
- the areas you computed to be suitable habitat, with the closest one to you highlighted and
indicated on the map using an arrow (as per example at the end of this document).
Note: The entire exercise must be completed using QGIS (version 1.7 or better).
Note: This is a practical assignment to be completed by each student individually.

Some Hints and Tips
Set your project projection to EPSG:32733 (UTM Zone 33 South), and save / convert all of your
datasets into this projection.
The DEM is provided in LO19 - which is not a commonly used projection outside of South Africa
and does not have an EPSG authority code. When converting that layer to UTM33S, make sure
to specify the source CRS explicitly by selecting it from the user defined CRS list.
Use % for slope steepness.
When dealing with large datasets, first remove any features you don't need and clip the dataset
to your AOI.
The rasterise tool needs at least one numerical attribute before it will run - use the calculator tool
on your table to create a new column and populate it appropriately.
When dissolving features, look for the 'dissolve all' option in the drop down list.
When working with rasters, e.g. your rainfall dataset, resample them to a common area and
pixel dimensions for optimal results.
Create a mask layer (1 inside the mask, nodata outside the mask) and multiply your rasters by
the mask to remove any irrelevant data from the dataset.
If you get stuck, try to approach the problem from a different angle - there is usually more than
one way to do it.
Sometimes things take a little while to compute, just be patient.

DEM
Clipped DEM
Yearly Avg
Rainfall Layer
Generalised Zoning
Warp to
EPSG:32733
Compute Slope
Steepness
Compute Slope
Aspect
Compute
Hillshade for
Visualisation
Warp to
EPSG:37733
Digitise Bounding
Box Layer (AOI)
Extract Rural
into new layer
Workflow Overview Part 1
Clip to Strand/
Melkbos
No
East Facing?
Yes
Clip to Stand/
Melkbos
Dissolve
Assign 0
>45 && < 135
Assign 1
Resample
Clip to AOI
Initial Data Preparation
No
Slope > 15' and
less then 60'?
250m
Yes
Negative Buffer
No
Rainfall >
1200mm?
Yes
Assign 0
Assign 1
Assign 0
Not 200 as
written in
assignment!
Assign 1
Rasterize

Slope Steepness
Candidate Areas
Discard
Compute Final Candidate Areas
Rainfall Candidate
Areas
No
Cross Product
Map
Vectorise
Area size >
6000m2?
Final Candidate
Areas
Workflow Overview Part 2
Slope Direction
Candidate Areas
Yes
Rural Area
Candidates
Keep
Calculate Closest Candidate to your House
My House
Make sure to
create unique id
columns for
both!!
Distance
Matrix
Analysis
matrix.csv
Final Candidate
Areas
Compute
Centroids

Used Input Layers:
DEM - CMA Dem (raster)
GMAP - Yearly Rainfall (raster)
Generailse Zoning Dissolve (vector)
Street Names (vector)
Example Results
Created Output Layers:
shade.tif - shaded relief model (raster)
candidate-rainfall (raster)
candidate-slopes (raster)
east-slopes (raster)
aoi (poly)
rural-buffer (raster)
final candidates (vector)
candidate-centroids (vector)
myhouse (vector)
matrix (csv)
InputID,TargetID,Distance
1,1,7357.27169753
2,1,7834.86228204
3,1,7639.67826118
4,1,6907.82017995
5,1,7672.92174477
6,1,10107.8244511
7,1,10223.9673274
8,1,9975.87899658
9,1,6595.79008429
10,1,7220.24749253
11,1,6688.61990682
12,1,6875.5651428
13,1,6178.38582957
14,1,6286.65277004
15,1,8530.91339767
16,1,8686.69407627
17,1,8353.71172243
18,1,8371.36534275
19,1,9003.24871874
20,1,8410.83156015
21,1,9236.64811674
22,1,8784.03231925
23,1,8427.89226257
24,1,9140.29032564
25,1,8759.32539398
26,1,8488.55454044
27,1,8513.56695278
28,1,8507.80933615
29,1,8535.50191814
30,1,8496.38863229

Workbook Example
Step 1: Preparation
- create a new blank QGIS project
- click the icon in bottom right of the status bar
- set the project coordinate reference system to EPSG:32733 (UTM33S)
- save the project : File -> Save Project
Step 2: Reproject DEM from LO19 to UTM33S
- load the DEM : Layer -> Add Raster Layer -> CT/DEM/cct_10m/prj.adf
- enable gdal tools plugin : Plugins -> Manage plugins -> Filter : gdal -> tick & press OK
- warp the layer : Ensure Prj layer is selected then, Raster -> Warp -> Use the following options:
- Input file : prf
- output file : select and give the filename as dem.tif in a personal working directory
- source srs : override the default 4326 : press select, select from user defined
coordinate reference systems the one created for the dem (it will have +lon_0=19) in its text
- target srs : set the output SRS : EPSG:32733(UTM33S) by selecting it from the select dialog
- tick the 'load into canvas when finished' option and click OK
- Remove the Prj layer : Layer -> Remove Layer
- Zoom to the extent of the dem layer : View -> Zoom to Layer
Step 3: ......and so on..........