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Re: st: spatial analysis - DISTANCE


From   David Torres <torresd@umich.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: spatial analysis - DISTANCE
Date   Sun, 30 Aug 2009 08:36:41 -0400

Thanks, Austin. Yeah, I figured I would need to do something like this, but after playing around a bit, I just thought I'd ask my statalist colleagues for a little help. I'm going to try this later this morning and make it work.

Ciao,
David D. Torres


Quoting Austin Nichols <austinnichols@gmail.com>:

David Torres<torresd@umich.edu> :
This is in reference to e.g.
http://stata.com/statalist/archive/2009-07/msg00083.html
http://stata.com/statalist/archive/2007-01/msg00098.html
I believe.

Note that standard Stata commands like -summarize- and -replace-
inside the loop do all the work of saving characteristics of the
nearest, or k nearest, observations (and whatever else, such as mean
of Z over those with distance less than 10 units).  No program like
-vincenty- (on SSC) does that; a -replace- command does it.
-vincenty- does not return "the min, max, mean, or whatever" and it
appears that -globdist- (which I found with -findit-) does only part
of what -vincenty- does (and the -globdist- help file contains the
caveat that "Testing of this program has been limited").  In other
words, add a -replace- to put the result you want in a variable.

Maybe you want something like:

g long rad10=.
g double dist1=.
g id1=""
g xy1=""
merge using univpoints00_0
forv i=1/`np' {
qui vincenty `=latitude[`i']' `=longitude[`i']' sch_lat sch_lon, hav(dis)
g within10=dis<10
su within10, meanonly
qui replace rad10=r(sum) in `i'
su dis, meanonly
qui replace dist1=r(min) in `i'
qui levelsof unitid if dis==dist1[`i'], loc(vs)
foreach v of loc vs {
 qui replace id1="`v' "+id1 in `i'
 su sch_lat if unitid==`v', meanonly
 loc y1=r(mean)
 su sch_lon if unitid==`v', meanonly
 loc x1=r(mean)
 qui replace xy1="(`x1',`y1') "+id1 in `i'
 }
drop dis within10
}

On Sat, Aug 29, 2009 at 4:26 PM, David Torres<torresd@umich.edu> wrote:
I am trying to calculate multiple distances from one set of xy coordinates
to another
using vincenty.  In addition to getting those distances, however, I need to
somehow list
the ids for the xy coordinates used to calculate the distance.

Here is the piece of code I'm working with:

code excerpt:

g long rad10=.
g double dist1=.
g id1=""
merge using univpoints00_0

forv i=1/`np' {
qui vincenty `=latitude[`i']' `=longitude[`i']' sch_lat sch_lon, hav(dis)
g within10=dis<10
su within10, meanonly
qui replace rad10=r(sum) in `i'
su dis, meanonly
qui replace dist1=r(min) in `i'
qui levelsof unitid if dis==dist1[`i'], loc(vs)
foreach v of loc vs {
 qui replace id1="`v' "+id1 in `i'
 }
drop dis within10
}

end excerpt

As you can see, I'm getting a count of all sch_lat and sch_lon coordinates
within a 10
mile radius of the other set of latitude and longitude points.  Then I
return only the
minimum distance.  Will vincenty allow for returning more than just the min,
max, mean,
or whatever?  Or is globdist a better choice for returning multiple
distances and the ids
attached to the point the distance to which is being calculated?

Thanks

David Torres
UMich

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