SIMPLE DISSOLVE

Dissolve Simple is a customized operation used to dissolve adjacent polygons that have the same value for a specified set of one or more attributes.  As an example, suppose a Deforestation map with the following atributes shown bellow and two polygons with their values for illustration only. They are organized into Scenes and each scene is subdvided into cells. Cells can be mapped simultaneouly by  different interpreters  - cell_oid is distinct for the green ones and two others beside it - the limit of the cell_id is the horizontal line.)

attributes

a) The dissolved adjacent polygons with the same value of 'class_name' (desmatamento) and  'scene_id' will became just one polygon at the output (union). 

original polygonsdissolve by class and Scene
Original Polygons                                                      Output Layer: class_name, scene_id

- b) The dissolved adjacent polygons with the same value of 'class_name' (desmatamento) and  'cell_oid' will became just one polygon at the output (union). 

dissolved by class and cell_oid 
Output Layer : 'class_name', 'cell_oid', 'scene_id'.

Observe that the ligth green ones are not dissolved because the 'cell_oid' is distinct in this case.  The set of attributes specified to be used to dissolve polygons are 'class_name' and 'cell_id'. The output attributes are 'class_name', 'cell_oid' and 'scene_id'.  

It is accessed through:

   Processing > Vector Processing > Dissolve Simple...

  1. Select the Input Layer to Apply the dissolve.
  2. Select one or more Attributes that will be used to dissolve objects with the same value.
  3. Select from the Input Attributes all the attributes that will be part of the output layer and move them to the Output  Attributes using the green arrows grren arrow or green arrow one.
  4. From the Output Attributes list, select the set of attributes that will be used to dissolve polygons
    • In the example (a) the set is:  'class_name' and 'scene_id' (use ctrl to select more them one).
      • the Output Attributes  can be:  'class_name' and 'scene_id' 
    • In the example (b) the set is: 'class_name' and ' cell_oid' 
      • the Output Attributes can be:  'class_name' , cell_oid, 'scene_id'
  5. Inform the Output Repository by clicking on:
    • (...) to select the output directory and also inform the new layer name to store the result, or
    • database icon to select the Data Source.
  6. Inform the new Layer Name to store the result, if the repository is a Data Source (PostGIS)
  7. Click on the green arrow to perform the operation or close the interface.
  8. Wait until the pop-up window indicating Sucess appear -> Click OK
  9. Click on Close
  10. Answer YES or NO to the question to add or not the output layer to your project.

Useful Tips:

Some attributes are default: FID and OGR_GEOMETRY

Be careful when defining output attributes. Selecting certain attributes can prevent polygons from dissolving (in the examples do not select task_id).

Input Attributes                                        Output Attributes

input x output