Topics In Today’s Lab
We have an understanding of vector spatial features such as points lines and polygons. Along with this we have come to understand the importance of clean layers that are topologically correct. These spatial entities represent geographic features and are by nature discrete units that cover the entire extent of an area. For instance we have looked at EA/DA units. Each one of these shapes is unique and is linked to a database by way of a unique descriptor.
The previous illustration reveals how vector and raster data can be used to express the same geographical areas (vector on the left and raster on the right). Pixels can be gathered by their cell value and be grouped into “raster polygons”. This system can be used in a raster GIS but there would be no topology rules as in principle each cell is to be considered unique. This translates to raster data as being continuous not discrete.
To further understand continuous data lets manipulate a DEM (Digital Elevation Model).
In QGIS load the plugin “Raster Based Terrain Analysis” from the plugins manager.
Now load a Digital Elevation Model (DEM) as a GeoTiff called:
This is not a problem for us, but if you wanted to save it as another type you can do so by going to the raster menu and use the translate function to convert your layer to a different formated raster file.
Shaded Relief – HillShade
Question 1:What do the brighter and darker values of a shaded relief represent?
Slope and Aspect
Once you have perused the hillshade for awhile you can create a slope layer and a aspect layer from the DEM using the “Raster based terrain analysis” plugin.
This is quite intuitive, but do you understand what you are creating?
Querying raster data. Once you have the slope and aspect layers loaded, use the Raster Calculator to find slopes less than 10 degrees and all values that have southerly aspects. You access the Raster Calculator from the raster menu.
Why can we not use the regular query methods we employed before with shape files?
What are we actually querying from these raster layers?
Question 2: What would the expression in the raster calculator be for providing all southerly aspects?
Question 3: Can you query between layers (as opposed to attributes of one layer)? If so why can you do so?
Using raster data to locate lost communities
We can use these tools to query the grid layers to select areas of interest that may be used to solve a problem. Lets use a rudimentary model such as trying to locate a lost community to illustrate the usefulness of combining raster and vector data.
Question 4: – Class led – but make sure you write down the steps to hand in (2 marks):
We are led to believe that there was an ancient community in this area of Cranbrook Hill that would most likely have these spatial qualities:
We will need to load the vector layers: lakes, creeks, swamps and forest cover layers
In this example we modeled data in both forms (vector and raster) and then combined them by clipping raster with vector. We could have fulfilled this query is several other ways. The tools we have loaded allow us to perform many raster operations such as filtering, aggregating, mosaicing, classify, mathematical and statistical operations as well as converting the raster queries to polygons. We can also use the lack of topology to perform some direct raster sampling by directed buffered polygons. If you are interested in these topics talk to either Scott\Michael\Aseem. There are a great number a analysis methods that can be applied using raster data layers.