9. Spatial Analysis: Vector Data

Duration: 8 minutes and 34 seconds

Country: South Africa

Language: English

License: CC - Attribution Non-commercial No Derivatives

Genre: Instructional

Producer: Chief Directorate: Spatial Planning & Information, Department of Land Affairs, Eastern Cape, South Africa

Views: 343 (22 embedded)

Posted by: giacomo on Mar 30, 2010

Understand the use of buffering in vector spatial analysis. Spatial analysis uses spatial information to extract new and additional meaning. Usually spatial analysis is carried out using a GIS Application. A GIS provides normally has spatial analysis tools for feature statistics (e.g. how many vertices make up this polyline?) or geoprocessing such as feature buffering (e.g. which area falls within a certain distance of this feature?). In this worksheet we look more closely at vector analysis, using buffering as an example.

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Hi my name is Sibongile.
Welcome to the 9th topic in our Gentle introduction to GIS ...
... with QGIS tutorials series.
In this screencast, we will give you a basic introduction to the concept ...
... of spatial analysis and show you a sample analysis tool.
Spatial analysis tools are an important part of Geographical Information Systems.
Spatial Analysis is the process of extracting or creating ...
... new information about geographic features.
Spatial analysis is useful for understanding spatial data.
If you need to evaluate your data and interpret it, then spatial analysis is very helpful.
Spatial analysis can also be used for estimating ...
... and predicting things about the data you have available.
In this screencast example we will show you how a typical spatial analysis, ...
... called buffering, works in the GIS application - QGIS.
Let's take a closer look!
Buffering in QGIS is provided by an option in the 'Tools' menu.
Let's load a 'rivers layer' that we will use as the basis for our buffer analysis.
To do this, click the 'Add vector layer' icon in the toolbar.
Look for the '50k_rivers.shp file' in the 'Local directory'...
... where your sample data is installed.
Highlight the layer in the 'layer selection dialog', and then click open.
The 'rivers layer' should now appear in the map view.
Now, let's create a buffer around our rivers.
A buffer is an area surrounding the feature of interest, ...
... in this case, an area around the rivers.
Creating a buffer around rivers can be useful for conservation planning.
For example, you can use a buffer to identify areas ...
...close to rivers in which no development may take place.
To start the buffer analysis, open the 'Tools menu'.
Next, click on 'Geoprocessing Tools'.
Now click on the 'Buffers item' to open the 'buffer dialog'.
In the 'Buffers dialog', select '50k_rivers' ...
... as the 'Input vector layer'.
Next, activate: 'Buffer distance'.
Now, enter 0.0005 as a decimal degree - this is about 50 meters.
Activate the option called: 'Dissolve buffer results'
Finally, select a name: 'river_buffer.shp' ....
... for the 'output shapefile' and click 'Save'.
Now click OK to start the analysis.
Depending on how many features you have in your layer, ...
... it may take a little bit of time to complete the analysis.
After the buffer analysis is finished, you will be asked ...
... if you would like to add the new vector layer to the legend.
Choose YES and the layer will be added to the map.
To see both the 'original rivers layer' and the 'new buffer layer' ...
... you will need to do the following: ...
Drag the layer 'river_buffer' with the left mouse button in the legend ...
... so that it lands up below the '50k_rivers' layer.
Then, if you like, you can also change the colours of the buffer layer ...
... in the 'properties dialog'.
Finally, you can zoom closer into an interesting area of ...
... the 'map view' and check the results of your 'buffer analysis'.
You will see as you visualise the data that all areas ...
... within fifty meters of the river have been outlined.
Now you could use this buffer layer to check if ...
... a development application is too close to a river.
In the next screencast we will look at 'spatial analysis tools' ...
... for raster data and describe 'interpolation methods'.
See you next time!