Unite several polygons that share boundaries in QGIS

Question

My study zone is in the middle of two geologic maps with two different shp files.

Is it possible to remove the boundaries of the polygons between these two maps that are created at the union of these in order to have only one polygon (per geologic unit)?.

In other words: to obtain an "unpartitioned" map (in QGIS).

Answer 1

Use Dissolve tool from Vector -> Geoprocessing -> Dissolve and choose the field that has the geologic name or id, the one you are using to style the polygons.

• Input layer: Select the polygon layer
• Dissolve field(s): Select the field that represents the geologic data
• Dissolve: Save the output final file into your hard drive

If the line did not disappear, zoom in into the line and check if there is a gap between the polygons.

Answer 2

If you want more control over the attributes of the merged feature than the Dissolve tool allows, use the Merge Selected Features tool in the Advanced Digitizing toolbar.

Manually select the features you want to merge into one feature. Click the Merge Selected Features button. Now you get a popup window, that allows you to set the attributes of the merged feature. For every attribute table field, you can choose from these options:

• Feature 1/2/3 etc (take the attribute from one feature, and discard the others)
• mathematical calculations for numerical fields: Count, Sum, Mean, Median, Standard deviation (pop or sample), Minimum, Maximum, Range, Minority, Majority, Variety, Q1, Q3, IQR
• concatenation for text fields
• Skip attribute (replace with NULL)
• Manual value (type whatever you want)

Obviously the downside of this tool is that you have to do each set of features separately, so it can be time-consuming if you have a lot of features. You can achieve a similar level of control over the attributes by first Dissolving, then running join attributes by location (summary).