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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.