In today's practical you will continue with the GIS work you started last week. You will calculate the size (area) of the formal terrestrial protected areas and vegetation types 2006 and 2009 in the shape files you created last week.
You can calculated the forest patch, FEPA wetlands and river FEPA areas as well but you must complete the PA and vegetation types areas.
We will also calculate the threat (ecosystem or conservation) status for your municipality's vegetation types which means you will need land cover to clip out the remaining natural vegetation - remember the vegetation map you clipped to produce your municipality's vegetation types contains the original extents of the vegetation types in South Africa. Lesotho and Swaziland before the large scale land use changes of the modern era.
Before we do anything more download QGIS and install it - we will use this software for projecting the shape files. You can download the installation file for QGIS from a local server below. Install it on the machine you are working on. Do this first as it takes a while to install. Don't install any of the sample data to save time.
This is freeware so you are welcome to install it on your own computer. i will only explain how use QGIS for projecting your shape files today but you are welcome to do the entire practical using it. I will explain how to do most of the practical in ArcView 3.2 however.
Once it has started to install continue reading what you will be doing today.
Projecting shape files to Albers Equal Area projection so that area can be calculated.
The shape files you created last week are not projected they are in geographic- you cannot use these to calculate area, I will explain in class. We are going to use the Alber's Equal Area projection for South Africa for calculating area as it allows comparison across the country and is not too complicated.
The zipped file below contains the SA municipalities shape in the correct Alber's projection.
- Unzip the SA municipalities albers and open it in QGIS by going to "Add vector layer" under the Layers tab
- Once it has opened right click on this layer in the layer list and set the project CRS from this layer.
- Now save a custom CRS by going to "Setting custom CRS" under the Settings tab and add a new CRS by copying it as an existing CDS and give it a name so that you can reference it from now on when projecting your other shape files.
- In the Project Properties box under the Project tab enable "on the fly" projection under CRS menu item
- Open all the shape files you must project i.e. your municipality's protected areas, vegetation types and forest patches.
- Change these to the new projection by right clicking on each layer and choosing "save as". add the new shape file name and location and choose the new custom CRS you just created. Make sure it looks like this before saving.
+proj=aea +lat_1=-33 +lat_2=-24 +lat_0=0 +lon_0=25 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs
Go to the new shape file and open the .prj file QGIS created in Notepad and check that it looks like this
Well done! you can now safely calculate the area of your municipaliy's vegetation types and protected areas. We will use ArcView 3.2 to do this.
Adding areas to the municipality's vegetation types and protected areas shape files
- Add the projected vegetation types and protected areas shape files as themes in ar4cView 3.2.
- Open the attribute table of the shape file to which you want to add an area field/column
- Choose "Start editing" under the Table tab
- "Add a field" under the Edit tab, make sure the field is of type number with 2 decimal places and name it "origarea" for the original extents area of your vegetation type
- Click on the new field column heading, then choose "Calculate" under the Field tab and enter the following
[shape].ReturnArea/10000. This will populate the new column with the areas of that collection of polygons in hectares there are 10000 square metres in a hectare.
- Add a new field and use the field calculator to calculate the % area of you municipality which was originally covered by this vegetation type. This will look something like [orinarea]/<the are of your municipality*100. Note you MUST use the field called AreaAlbers I included in the SA municipalities albers shape file downloaded today. The area on the BGIS website for municipalities is out of date and the updated summaries are being poblished this year. Some municipalities have changed in size due to municipal boundary changes by the Demarcation Board since these summaries were published.
Calculating the area of the remaining extents of the municipal vegetation types
You have to clip the vegetation types layer with a land cover layer which shows what is still natural and not transformed to other land uses e.g. forestry or agriculture. We will use the national land cover 2009 rater grid available on BGIS. Download the raster grid, unzip it and open it in ArcView. Note you will have to enable the Spatial Analyst extension in order to be able to see this layer.
open the attribute table and take note of which gridcodes are classed as natural.
In order to clip the vegetation by these two classes we must first vectorize the raster grid to a shape file. As this take a long time to compute I have done so for you.
- Add the land cover shape and the SA municipalities Albers to an Arcview. Both of these are in the Alber's projection and should be visible when opened together along which your other projected files you have made..
- First clip the land cover shape by your municipal boundary as you did last week by selecting your municipality and clipping the land cover to a new shape file of your municipality's land cover.
- Select the "natural" gridcodes in your municipality's land cover by using the query tool
- Now clip to a new shape file of your municipality vegetation types in Albers which contains the original extents area calculations by the selected gridcodes of the municipality's landcover. The result is is the remaining extents of the vegetation types of your municipality.
- You will need to dissolve the remaining extent vegetation types shape file on Bookcode as the clipping will have added numerous separate new polygons.
- You can now add a field called "remarea" to its attribute table and calculate the area of the remaining extents
- Calculate the percentage of the original extent of the vegetation type which is still remaining. [remarea]/[origarea]*100
- Use the BGIS website to find out the cut off points for classifying a vegetation type as Vulnerable, Endangered and Critically endangered. Add a field called ThreatStatus and classify each vegetation type as Least Threatened, Vulnerable, Endangered and Critically Endangered.
- Make a map showing your municipality's vegetation types threat status with the correct colours used in the legend. Check on BGIS to see what these colours are.