# Re: st: Nearest distance (spatial) and shp2dta question

 From "David Torres" To Subject Re: st: Nearest distance (spatial) and shp2dta question Date Thu, 2 Jul 2009 14:59:10 -0400

```Mr. Nichols,

postsecondary institutions to potential students.  I may add this approach
to what I'm doing, so any advice you can give on this and my current main
approach is greatly appreciated.  Learning Stata is fun, indeed.

DT

----- 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:
> >
> > 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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```