Clip, Spatial Join or Union?

Discussion created by wsr102209 on Jun 25, 2014
Latest reply on Jun 25, 2014 by rfairhur24
Hi All
  I have a question about which geospatial process need to be selected in my case, I would be much appreciate if someone could guide me and tell me what their differences.

I have two maps in my case, one is world bounday map (with COUNTRY name) and another is world city map(Only polygon without COUNTRY name). both maps are polygon formats. In order to know how many countries are located within certain country, I need use some geoprocessing tools, such as Clip, Spatial Join and Union.

I just wounder which geoprocessing tools should be the best one if I want to link two maps and get the number of cities.

I had already tried these three functions, see as following:

Clip: I use cthis fucntion to clip those countries I need (According to the world bounday map by "Select by attribute") and output as a new file. Open the attribute table then the number of cities was there.

Spatial Join: World bounday map is input as the Target features and World city map as Input featres. Then, "Contains" operation has been used for linking these two maps and result obtained.

Union: Input these two maps and the output can also show the similar result as above mentioned.

However, These three results are not equal in terms of the city numbers. For example, Clip could be 2000, Spatial join (Contains) could be 1850 and Union could be 1992. I have no idea which one will be more accurate.

Please note: sometimes, some cities were located around the country bounday that may significantly affect the result of this geoprocessing since these cities might be counted.
If anyone who has any experience on this topic, Please give me some suggestions and tips. Thank you.