Manipulating a DEM and creating new layers from it
overlaying vector layers onto raster cells to gain insight for a locational model
Creating a new DEM from point data at the Census Block Level
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.
Raster and Vector Compared
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 and the “DEM shaded relief shader” from the fetch plugins method. Now load a Digital Elevation Model (DEM) as a GeoTiff called:
/home/labs/geog204/tutlab6/unbc_dem.tif. You are not loading a vector layer – what are you loading?
You can play around with the presentation of the data in the same manner as with vector data. Look at the style tab and play around with the grayscale presentation (with a stretch to min and max) and others in the colour map drop down. Scott can example.
We are going to save and create some DEM based layers. Saving the DEM layer to your home directory Saving raster layers from QGIS is different that with vector layers. You cannot right click and save as usual. Instead you go to the raster pull down and use the translate function to convert your layer to a raster file (in this case a GeoTif file) in you lab6 directory. Try it and make sure the load into canvas button is turned on. Once you have saved it, loaded into your project – remove the original DEM layer from your project.
Lets derive a graphical representation of this elevation data layer by making a shaded relief use the default setting – Scott will explain this). Scott or Carolyn will illustrate how to create a shaded relief using standard values and the shaded relief plugin.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, query them to find slopes less than 10 degrees and all values that have southerly aspects. using the map query under the analysis pulldown. Why can we not use the regular query methods we employed before? How do we query these layers? HINT: Where did we look to see how we saved or converted raster layers?
Question 2: What would the query expression 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:
- Southerly facing slopes less than 10 degrees are preferred
- The community will be located close to a lake (within 1000 metres)
- No closer than 500 metres to a swamp and
- should have a creek running through the main village. What could you do to rank polygons that favor having a creek running through it (this would include all the selected polygons, but rank them to as whether a creek ran through them).
- these particular people prefer to be located in a primarily spruce forest type.
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.