At 10:06 AM 07/07/1999 -0400, you wrote:
>Hello all,
>
>I'm trying to determine the centroid of population of our 2-County
>region. Our population data is broken down into Traffic Analysis Zones
>(TAZ) of which there are roughly 500 in the region. Is there any way to
>plot this centroid of population? My goal is to track the movement of
>this centroid over the years.
Yes. Do a weighted average of the long/lat coordinates. For instance, query
your set of 500 centroids and multiply centroidx(obj) * population in each
one, then divide by the total population. Do the same thing with the
latitude and you will come up with a single weighted population (x,y) that
will be what you want.
For instance, I have a table with the USA by county. When I perform the
query...
Select sum(Population_70), Sum(Population_95) from USA_by_County into Selection
Browse * From Selection
...I get...
203293816 262748737
_____________________________________________
Then run the query...
Select sum(CentroidX(obj)*Population_70),
sum(CentroidY(obj)*Population_70),sum(CentroidX(obj)*Population_95),
sum(CentroidY(obj)*Population_95) from USA_by_County into Selection
Browse * From Selection
... and get....
-18201208482.75 7820955814.87 -24043161734.83 9931834862.45
_____________________________________________
Divide the 1970 x/y's by the 1970 population total and get x = -89.5315403
y = 38.4711939, which is near Mascoutah, IL. Good way to check the data,
since I used to go to High School there, and the only claim to fame the
town had was it was the center of the USA's population!
For 1995 you get x = -91.5062885, y = 37.7997435, that is 116.5 miles WSW
of the 1970 centroid.
HTH,
Steve Wallace
GIS & Market Information Manager
Florida Farm Bureau Insurance Companies
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