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<div class="moz-cite-prefix">On 11/07/2012 03:34,
<a class="moz-txt-link-abbreviated" href="mailto:rhoskins@u.washington.edu">rhoskins@u.washington.edu</a> wrote:<br>
</div>
<blockquote
cite="mid:alpine.LRH.2.01.1207101734350.13769@hymn14.u.washington.edu"
type="cite">
<pre wrap="">I need to estimate the population of post codes by doing an areal overlay with smaller census tracts over the
postal codes.
How is this done in QGIS?
</pre>
</blockquote>
This kind of aggregate job is probably best done right in a
database. You can import both vectors into a spatialite database,
add a column for the population in the post_codes table, then run a
SUM query such as:<br>
<font face="Courier New, Courier, monospace"><br>
.loadshp 'post codes' post_codes LATIN1 <your coordinate
system SRID> ;<br>
.loadshp 'census_tracts' census_tracts LATIN1 <coordinate
SRID> ;<br>
<br>
ALTER TABLE post_codes ADD COLUMN population integer;<br>
<br>
UPDATE post_codes SET population=SUM(census_tracts.population)<br>
WHERE Intersects(post_codes.Geometry, census_tracts.Geometry)<br>
GROUP BY post_codes.zip;<br>
<br>
</font>If you have a large number of census tracts, you'll most
likely want to add a Spatial Index and use it in the query to speed
things up.<br>
<br>
HTH,<br>
Micha<br>
<br>
<br>
<blockquote
cite="mid:alpine.LRH.2.01.1207101734350.13769@hymn14.u.washington.edu"
type="cite">
<pre wrap="">
Thanks -
R Hoskins
Anchorage, AK
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</pre>
</blockquote>
<font size="+1"></font><br>
<br>
<pre class="moz-signature" cols="72">--
Micha Silver
052-3665918
</pre>
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