Stats Can Boundary Layers
Many students have been confused about the boundaries of the different datasets we have been talking about in lecture and tutorial (as well as using them in labs). Here is a link from StatsCan that illustrates these relationships for the 2006 census.
Lets take a look at a few data layers to see how they relate to each other. Open all the shape files in the home/labs/geog204/boundary_file_tutorial directory in QGIS. We are going to compare how the different boundaries of each layers compare to each other. We should be able to manipulate each data set to see relationships between these layers for Prince George.
Using the chart from the above web site. Lets start at the top and build each box of the flow chart. As we do this, we should keep looking at the attribute table to see if it can help. Once we figure how to query each layer to get the census layer, we can try to play with the dissolve function in QGIS (although we may leave that for the lab tomorrow).
Looking at the grouping of spatial layers for stats can
As a group lets create a boundary that represents the CD that Prince George is in.
We do this exercise in lab 3 using the Dissemination Areas. How many DA polygons are there in the CD?
After loading the 3 layers into Quantum, lets see if we can display the layers in a fashion that allows us to see all the layers at once (and therefor the relationship they have spatially to each other. Try to play with the transparencies and line thickness of each layer as I did below:
Figure 1: Prince George with Block, Dissemination Areas and Census Tracts
We can see as we move around the relationship amongst the borders.
In lab 3 we will be using attribute values to search and aggregate polygons of different boundary units. Lets take a look at how we use QGIS to make use of attribute data. In the end we what to have look at the Census boundary that Neil and Gail referenced in their tutorials and lectures last week (the Census Tract that UNBC is in). Below is the DA polygons selected that represent this area.
I did this by using the information (identity) tool to get the value of the CT, copied it and then used it for a query from the attribute table. Lets do this as a class – take some notes!
How may DA polygons are selected?
If we do the same for the Blocks – how many are there in the CT that UNBC is in?
Lets take a look at the Census Tracts.
Comparing DA, CD, CSD and CA
Dissolving to create an outside boundary
We have been aggregating layers into larger boundaries using attribute data. We can now remove all the interior polygons by using the dissolve tool in QGIS. See if you can make one large polygon for the CD of Fraser Fort George
Take a look in google to see if there is any help for dissolve (such as the one I found in wiki.gis.com).
Stats Can Boundary Layers to Google Earth
Lets take the boundary layer we just created using the dissolve function. In order to do this we need to same the layers as something Google Earth can understand. Do you remember the file type we used in the first lab? What do get if you type file extension google earth?
Once you have determined the file extension type, load in this layer and then create and load a block layer for Prince George into Google Earth – of course we have to wait until we are in the lab (or we can go there on the way out today if you wish).