QGIS 2.0 - Point in Polygon Query

To find out how many points in a particular layer occur in polygons of a second layer, use this method.

Points Layer

For this exercise, we'll use the points showing railway stations in Taiwan, and the polygons for county level units in Taiwan.
Our goal is to (a) calculate the number of railways station points in each county unit, and (b) create a thematic map of the results
In QGIS, Add Vector Data, set UTF8 encoding, and browse to part_three/tw_stations.shp
Add Vector Data, set BIG5 encoding, and browse to china/TW_POP.shp You should have two layers in your QGIS project:

From the main QGIS menu, go to Vector | Analysis Tools | Points in Polygon
In the dialog, make sure that the Input Polygon Vector Layer = TW_POP
Then make sure that the Input Point Vector Layer = tw_stations
You may change the "output count field name" or accept the default: PNTCNT
For output shapefile, browse to the folder you want to save the new layer, and enter the output filename, here, we have typed "tw_station_county" as the output file.
When you have completed the dialog, click OK

Checking and symbolizing results

You will be prompted to open the new layer in the TOC, click YES
Right click on the new layer (tw_station_county) and open the attribute table
Scroll all the way to the right of the table to make sure the PNTCNT field was created
Click on the field name cell, PNTCNT, to sort the resulting count of stations in Ascending or Descending order
Now we can see that the highest number of stations in a single county was 6

Close the attribute table, and right click on the layer to open the layer Properties
Click on the STYLE tab to view the symbolization options
On the upper left, change the Single Symbol drop-down option to GRADUATED.
In the Graduated dialog, change the COLUMN drop-down selection to PNTCNT
Change the MODE setting to QUANTILE, choose a color ramp, such as GREEN TO RED
Most importantly, you must click CLASSIFY at the lower left, or QGIS will not run the classification process!

After Clicking CLASSIFY, you should see the graduated symbols ramp
You may customize your classification scheme in the Symbol, Range, and Label grid.
For our purposes, we will accept the default scheme and click OK in the lower right.
Our thematic map shows the concentration of railway stations in each county.