I have a parcel shapefile. Many of the parcels have been combined over time, but the shapefile still represents the original parcel boundaries. The attribute table has a field for the original parcel number as well as, for those that have been joined, the current 'parent parcel'. I created a new attribute that replaces the historic parcel id with the current parent. When I dissolve by this I get a shapefile of that represents the current geographic conditions, but my attribute data is not accurate. In some cases QGIS dissolves and retains the parcel record that was merged into (and not currently recognized) with no associated attribute data.

I could just join up the data again after dissolving; but there must be a way to tell the dissolve function to retain a specific 'primary' polygon. I have a dummy attribute marking the 'primary' as 1, but I don't know how dissolve chooses which record to retain; sorting doesn't seem to make a difference. Is there a way to change how dissolve decides which record to retain, and feed it my dummy? Is there an entirely different approach I should try? Or should I just shut up and rejoin all of the data?

I'm hesitant to rejoin, because there are half a million records, and most operations are pretty taxing on my poor computer.

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    For both our sakes, can you make a diagram of the data relationship? – BillW Jul 1 '15 at 3:10
  • You can't give one feature two different values for the same attribute. When you dissolve, you should have options on how to combine the attributes of the inputs. For numeric fields you can do math like sum or mean. For text you can do first/last, but that order is mostly determined by return from the data. You can't specify a 'primary' polygon - only with individual merges can you specify which attributes to keep (in Arc anyway). What you could do is select your primaries, convert them to points within (not necessarily centroids) and then spatial join those to your dissolved polys. – Chris W Jul 2 '15 at 23:31

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