[Qgis-user] newbie Q on applying polygon characteristics to intersecting polygons

[Neal Traven]

Greetings, QGISers!

I don't know whether my subject header really describes what I want to 
do (that's how much of a newbie I am). But let me try.

I have two polygon SHP layers that cover the same total area. Polygons 
in the "voting unit" layer are smaller than those in the "census tract" 
layer, but they don't necessarily share the same borders. In other 
words, a voting unit might lie completely within a tract or it might 
contain portions of two or more tracts. A table gives me a measure of 
per-capita income for the census tracts.

What I want to do is estimate the per-capita income in the voting units, 
modeling the estimate by weighting the tract values by the percentage of 
the voting unit's area within each of its intersecting census tracts. 
For example, if 40% of the voting unit's area is in tract A and 60% is 
in tract B, the estimated value for that voting unit would be [(tract A 
income)*0.4 + (tract B income)*0.6].

It seems like that should be a relatively simple operation to carry out 
in QGIS, but I have no idea how to go about it. I've never taken any 
academic courses in GIS ... I've just blundered about in it without 
quite knowing what I was doing. So please be gentle with me.

Thanks,
neal

-- 
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neal traven                              beisbol at alumni.pitt.edu
   "You're only young once, but you can be immature forever."
                                -- Larry Andersen, relief pitcher