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

[Carlos Cerdán]

Let's try:

(1). Be sure that in layer "voting unit" there is a field for the total
area of each polygon.
(2). In the "census tract" layer you must have the factor to multiply the
corresponding area of "voting unit"
(3). Make an intersection of "voting unit" as input layer, with "census
tract" as intersection layer.
(4). Use the field calculator in the resulting layer to add new fileds for:
(4.1) New resulting areas for intersection
(4.2) Percent of (4.1) respect to (1).
(4.3) Result of multiply (4.2) by (2) (This is what I think respond your
question).

Good luck

Carlos Cerdán


2013/11/24 Neal Traven <beisbol at alumni.pitt.edu>

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