CHGIS provides county level points but not county level polygons for all of China. If you would like to assocate county level data to polygons for Thematic Mapping, described above, you can use the County Points together with an encircling polygon to generate Thiessen Polygons. Thiessen Polygons essentially enclose the space around each point using an algorithm to calculate the location of a boundary mid-way between the available points. The resulting Thiessen polygons encompass all the space that lies less than half-way between the Thiessen polygon center point and any other Thiessen polygon center point. To do this, we have included an ArcView extension on this CD-ROM with the permission of the author, Prof. Francisco Olivera, Texas A & M University.

• first add the vector11.avx file to your ArcView Extensions folder as described on the CRWR Extension page.

• open the extension by selecting CRWR-Vector from the FILE | EXTENSIONS menu

• open a polygon layer and select a single polygon that you wish to be the outer limit of your Thiessen Polygons. In this example we choose the area of Tibet using the SELECT TOOL:
  

• the area of the selected polygon should be colored yellow:
  

• select CONVERT TO SHAPEFILE from the THEME menu

• choose a folder and filename to keep your outer polygon file

• when prompted to add new theme to view choose YES

• DELETE the original theme from the view

• zoom in on the new TIBET theme

• add the COUNTY POINTS theme (CCAP_PTS)

• choose THIESSEN POLYGONS from the CRWR-Vector Menu

• you will be prompted to select the FIELD in the points file to use as the link to the Polygons. Select SYSTEM_ID:
  

• you will be prompted to select the polygon theme to use as the outer boundary. Select your Tibet THEME from the list:
  

• you will be prompted to continue by discarding all the point features that lie OUTSIDE the Tibet polygon. hit YES to continue:
  

• you will be prompted to choose a new name for the output Thiessen polygons file. hit OKAY

• the new Thiessen polygon file should display on your View.
  

• now you can join your data to the Thiessen polygons and create thematic maps:
  

Linked below are some examples of Thiessen Polygons for Shanxi Province, using the CHGIS 1820 County Seats as the central points. After the Thiessen Polygons were created, statistics for Population and Changping Grain Quota were joined to the the counties and thematically mapped.

• Thiessen polygons for Shanxi Counties using prefectures for outer boundaries
• Shanxi Counties 1820 Population raw population for each county
• Shanxi Counties 1820 Population by Area population normalized by area of each county
• Shanxi Counties 1820 Changping Grain Storage Quotas quota figures for each county
• Shanxi Counties 1820 Changping Quotas by Area quota figures normalized by area of each county