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From |
Austin Nichols <austinnichols@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Re: Nearest distance (spatial) and shp2dta question |

Date |
Fri, 3 Jul 2009 15:54:22 -0400 |

David Torres<writeon4truth2@msn.com> : It's still not clear to me what you hope to achieve. I also don't see any added value in Sergio's pointing you to the SSC program -vincenty- (though that program may help you later if you calculate a weighted average over all schools in the country, where distances involved may be both great and small). The example code you quoted at the outset gives the general method you need to adapt to your data, except that you need to calculate centroids of some existing polygons first, for which code follows. I am assuming that you are working with polygons small enough that you can assume that distances can be measured as if they lay in a plane; otherwise centroids will be difficult to calculate. Also note centroids need not lie inside a polygon, if polygons may be nonconvex, which may undercut your intuition about using centroids to measure distances between points and sets. See also appendices A and B in http://www.nber.org/papers/w13246 for discussion of distance-weighted measures. Here is a program for calculating centroids, and a couple of examples: clear all prog calcentr, sortpreserve version 8.2 syntax varlist [,by(varlist) Stub(string) Force] gettoken X Y: varlist if "`stub'"=="" loc stub "c" if "`by'"=="" { tempvar by g byte `by'=1 } tempvar t x y xm ym sx sy sa a b qui { g long `t'=_n su `X', meanonly loc xo=r(min) g double `x'=`X'-`xo' su `Y', meanonly loc yo=r(min) g double `y'=`Y'-`yo' egen long `b'=group(`by') sort `by' `t' if "`force'"=="" by `by': assert `x'[1]==`x'[_N] if "`force'"=="" by `by': assert `y'[1]==`y'[_N] by `by' : g double `ym'=(`y'+`y'[_n+1])*(`x'*`y'[_n+1]-`x'[_n+1]*`y') by `by' : g double `xm'=(`x'+`x'[_n+1])*(`x'*`y'[_n+1]-`x'[_n+1]*`y') by `by' : g double `a'=(`x'*`y'[_n+1]-`x'[_n+1]*`y')/2 egen `sa'=sum(`a'), by(`b') egen `sy'=sum(`ym'), by(`b') replace `sy'=`sy'/6/`sa'+(`yo') egen `sx'=sum(`xm'), by(`b') replace `sx'=`sx'/6/`sa'+(`xo') } if "`stub'"!="" { ren `sx' `stub'x ren `sy' `stub'y } end input x y .9 0 0 1 .9 2 1 2 .1 1 1 0 .9 0 end calcentr x y, f line y x || sc cy cx, aspect(2) name(outside) use http://pped.org/stata/usa.dta, clear calcentr _X _Y, by(_ID) stub(b) f line _Y _X if _ID==10, aspect(1) || scatter by bx if _ID==10 * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: Re: Nearest distance (spatial) and shp2dta question***From:*"David Torres" <writeon4truth2@msn.com>

**References**:**st: Nearest distance (spatial) and shp2dta question***From:*"Sergio I Prada" <sprada1@umbc.edu>

**st: Re: Nearest distance (spatial) and shp2dta question***From:*"David Torres" <writeon4truth2@msn.com>

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