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Regarding your questions on my third question, the data for the shapefiles 
and database files are taken from National Historical Geographic Information 
System (NHGIS), which in addition to providing these also provides census 
2000 data.  The polygons do, I believe, have vertices, and they are of 
varying sizes; this goes for tract polygons as well as county polygons.  The 
greater the population density the smaller the tracts/counties, the more 
sparse the population the bigger the polygons.  And, yes, I am trying to 
calculate xy coordinates of the centroids to match the xy coordinates of 
geocoded addresses of US postsecondary institutions.  As I am not too 
familiar with the various coordinate systems in use, I can't talk to that 
issue.  Suffice it to be shown that there is a huge discrepancy between 
(-681360.7, 575246.7) and (-86.16864, 32.38251), and I don't know how to 
make them equivalent in expression.

I hope this was a bit clearer than my previous post.

Cheers,
David Torres

----- Original Message ----- 
From: "Austin Nichols" <austinnichols@gmail.com>
To: <statalist@hsphsun2.harvard.edu>
Sent: Thursday, July 02, 2009 11:50 AM
Subject: Re: st: Nearest distance (spatial) and shp2dta question


> David Torres<torresd@umich.edu>:
> Each of (1) and (2) is easy to do (if time-consuming) with a single
> loop over observations.
>
> Note that the main trick in my code was to do
> a nonmatched -merge- to get both datasets in memory at once.  You will
> have to be a bit more clear about what you want to get more specific
> advice: "calculate distances between census tract and county centroids
> to the nearest [school] with population of the tracts or counties used
> as weights" sounds like you want the final dataset to have one obs for
> each institution with some kind of average distance to potential
> students, but (2) says you want one obs per centroid.  What are these
> calculations to be used for?  Maybe that answer will help clarify what
> you need.
>
> On (3), what data do you have?  Polygon vertices?  How big are the
> polygons (i.e. is the curvature of the Earth important)?  Is this for
> calculating centroids for use in (1) and (2)?
>
> On Thu, Jul 2, 2009 at 10:42 AM, David Torres<torresd@umich.edu> wrote:
> > Three comments/questions:
> >
> > 1. I was just browsing the web looking for something similar to Cox's
> > nearest .ado file and stumbled on the example Austin Nichols gave for
> > calculating distances between two different sets of xy coordinates,
> > originally from two different data sets. I am not familiar with the code 
> > he
> > gave so I have to ask: Is there an .ado file that can do all that work 
> > for
> > me? I mean, if I already have two data sets that I've merged, is there a
> > simple command I can input that will give me additional distance 
> > variables
> > to work with?
> >
> > What I'm trying to do is calculate distances between census tract and 
> > county
> > centroids (for the entire US, AK, and HI) to the nearest postsecondary
> > institution (of all types and by sector and control of institution: 
> > public,
> > private, proprietary, pub2year, priv2year, prop2year, pub4year, 
> > priv4year,
> > prop4year), with population of the tracts or counties used as weights.
> >
> > 2. I also would like to produce variables for the total number of
> > institutions that fall within a 10, 20, 30, 40, 50, and 100 mile radius 
> > of
> > each tract and county centroid.
> >
> > I can do all of this in ArcGIS, to be sure, but with eight years of 
> > data,
> > and ten different .dbf/.shp files per year, this would be a tedious 
> > chore.
> > I would prefer to spend an hour and write a .do file that will do in
> > minutes what it will take hours to do in ArcMap/ArcInfo.
> >
> > 3. The shp2dta command produces xy coordinates for area centroids that I 
> > am
> > not familiar with. Does anyone know if the code in the .ado file can be
> > changed to produce what I want--regular xy or lat/lon coordinates?
> >
> > With regard to the first two parts of my questions, here is Austin 
> > Nichols'
> > code, the first part of which I don't really care about:
> <snip> Any help anyone can offer is greatly appreciated.
> >
> > Thanks,
> >
> > David Torres
>
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