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What is the best method to convert a singlepart shapefile (i.e. one record that contains several non-connected polygons) into a mutlipart shapefile?

--EDIT

For clarification the original question asked how to produce a singlepart featureclass with unique IDs for each spatially non-connected polygons. The explode method in advanced editing or Multipart to Singlepart would give this result...

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What software are you using? – Aaron Sep 3 '12 at 19:57
ArcGIS 10.0/Quantum GIS/GRASS – BJEBN Sep 3 '12 at 20:25
It sounds like you may be confusing singlepart and multipart. Multipart is when you have several polygons that share the same attributes and are stored as one feature/one record in the attribute table. Singlepart is when each polygon has it's own record in the attribute table, even if some of the attributes are identical to other polygons. – GeoKevin Sep 4 '12 at 1:58

3 Answers

up vote 6 down vote accepted

If you would like to convert a polygon feature class with a single record to a polygon feature class with a record for each polygon, use Multipart to singlepart (Data Management) aka "explode".

To convert single-part polygons to multi-part polygons, you can use the dissolve tool in ArcGIS. Make sure to dissolve by OBJECTID and select "Create multipart features". ArcGIS help also has a section on creating a multipart feature.

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Unfortunately, dissolve is not the tool that I am looking for as this creates dissolves features into multipart features. I have nothing to dissolve and thus it simply creates a copy of the input file... – BJEBN Sep 3 '12 at 20:04
Instead of using the objectid field which is unique to each polygon you should be using a field that groups the polygons into the single multipart. – Hornbydd Sep 3 '12 at 21:20
There are no other attributes besides shape area and shape length... I was hoping that there was someway to make a multipart feature based on spatial extent of the polygons. – BJEBN Sep 3 '12 at 21:41
See my edited answer on how to use Arcgis to do what (I think) you need. – Alexandre Neto Sep 3 '12 at 23:57
+1 for multipart to singlepart (data management) – GeoKevin Sep 4 '12 at 2:00
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up vote 4 down vote

Using Quantum GIS, you can use "SingleParts to multipleparts" tool.

Vector > Geometry Tools > Singleparts to Multiplepart

Has you will notice, you can do the opposite in the same menu.

Edit:

Since you have access to ArcGIS you can enter edit mode, select your feature and in the advanced editing toolbar use the explode tool.

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I am not that familiar with Quantum GIS but when I use the singleparts to multiparts function I receive the error "At Least two features must contain the same attribute value. Please choose another field" I only have one feature in my featureclass... – BJEBN Sep 3 '12 at 20:08
I had no problem using the single part to multipart tool with a single feature file. – Alexandre Neto Sep 4 '12 at 14:49
up vote 2 down vote

To convert singlepart polygons with one record to multipart polygons, Advanced Editing in ArcGIS can be used. Start Editing session, activate the Advanced Editing and then use Explode Multipart Feature (Note: all the polygons need to be selected).

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