QGIS 2.0 - Spatial Join

Spatial Join

A common GIS task is to join the attributes from one spatial data layer to another. In this example we will join attributes from a polygon layer to a points layer, based on which polygon contains the points.

Prepare Target Points Layer

For this exercise, we'll use a set of points showing power plant sites in Taiwan from the CARMA database
Add Vector Data, set UTF-8 encoding, and browse to part_three/Taiwan_CARMA_Xian80.shp
Right click on the Layer in the Layer List and the General tab, to check the Coordinate Reference System (projection) for this layer
Note that it is in Projected CRS, EPSG:2333. Cancel the Properties Dialog window.
Target Points layer should be ready for the join

Check Join Layer

Now we want to Add Vector Layer and browse to the part_three/TW_POP.shp layer, encoding = big5
Click on the TW_POP layer in the layer list and drag it below the Taiwan_CARMA layer. Now the points should be on top of the polygons in the Map View.
Right click on the TW_POP Layer in the Layer List and the General tab, to check the Coordinate Reference System (projection) for this layer
Note that it is in Geographic CRS, EPSG 4326 WGS 84
This is NOT the same projection as the target layer, and we need to reproject the join layer before we attempt the spatial join.

Reproject Join Layer

Click OK or Cancel to close the Properties window.
Right click on the TW_POP layer and choose SAVE AS, then fill out the information for the layer export as follows:
The Format should be Esri Shapefile. Under SAVE AS, BROWSE to your desktop and give the file a name: TW_POP_NEW and hit OK. The Encoding should be set at BIG5.
Now click on the CRS drop-down list to choose SELECTED CRS, then BROWSE.
Under "Recently Used" scroll down to click on the Xian 1980 Gauss Kruger, EPSG:2333 projection, accept this CRS with OK. Now the export process should run and you will get a small warning on your Map View when the layer has been saved to disk.
Add Vector Data and browse to your TW_POP_NEW Layer to add it to your project. You should now have three layers in your layer list.
Now we can right click on the original TW_POP layer to remove it from the project. You will be asked if you want to Remove One Object, click OK.

Run the Join

Your project should now have two layers in the EPSG:2333 projection -- Taiwan_CARMA and TW_POP_NEW.
We can now proceed to use the JOIN ON LOCATION function (aka SPATIAL JOIN)
Go to menu VECTOR | DATA MANAGEMENT TOOLS | JOIN ATTRIBUTES BY LOCATION
Set the TARGET LAYER = Taiwan_CARMA_Xian80.shp, set the JOIN LAYER = TW_POP_NEW
Click on the OUTPUT SHAPEFILE BROWSE button to set output location and filename, like: "test_join", and set encoding = big5, then click SAVE
Click OK and you will be prompted to add the new join shapefile to the TOC (table of contents), click YES
Open the Attribute Table of the new join layer, "test_join." Scroll to the right, and you should now see that the all the columns from the TW_POP_NEW layer have been added to the table, and each point feature now includes the attributes for the polygon that the point was located in.