Suppose one wants to make polygons A and B, and their respective attributes to be one unique polygon C. What is this GIS operation called?

Is there any difference in GIS terminology about "joining" and "merging" geometries? Or does the terminology expand to different concepts besides geometries?

As an example, I could not find the answer reading tag wikis in GIS SE.

Currently, the excerpt and wiki of says:

In the context of GIS, merge typically refers to either

  • combining two or more geometries into a new, single geometry
  • combining the records of two different tables into a single table (similar to append)
  • combining multiple raster files (stacked or adjacent) into one.

A specific type or method of merging is dissolve.

And the excerpt of states:

The process of connecting two or more datasets based on a common attribute or spatial location.

"Connecting two or more datasets based on a common attribute" seems to be the same as the second bullet in the merge wiki.


When to use join:

  • When there is a one to one relationship between two distinct tables or geometrics (based on join field or spatial relation)
  • Used to append new field/columns to table (result = table has same number of rows, and more columns)

enter image description here http://resources.arcgis.com/en/help/main/10.2/index.html#//005s0000002q000000

When to use merge:

  • To combine one to many tables with similar database schema
  • Used to combine additional rows to destination table (result = table has more rows, and potentially more columns)

enter image description here http://resources.arcgis.com/en/help/main/10.2/index.html#//001700000055000000

The bullets you listed are correct, joins are for adding additional information such fields or columns to an existing table, and merge is for creating a new table that has the combination of rows from table A, B, C..etc.

  • 1
    Also, the term grouping is often used within GIS applications for organizing layers within a legend or layers listed in a table of contents.
    – artwork21
    Jan 22 '15 at 16:33

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