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.