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From |
"P. Wilner Jeanty" <pwjeanty@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: RE: Calculating the shortest distances between observations (based on longitude and latitude) |

Date |
Thu, 2 Feb 2012 14:38:27 -0600 |

2012/2/2 Rüdiger Vollmeier <ruediger.vollmeier@googlemail.com>: > Hello guys, > > I want to calculate the shortest distances between observations based > on the coordinates (latitude, longitude). I have adapted a simple > version from N. Cox's nearest neighbor search which was presented here > some time ago. In contrast to that, I want to calulate not only the > shortest but also the second shortest (third, and so on) distances. > > Here is a simplified structure of the dataset: > > observation_type country year latitude longitude > 1 Albania 2010 42.07972 19.52361 > 1 Albania 2010 42.15028 19.66389 > 1 Albania 2010 42.01667 19.48333 > 2 Albania 2010 39.95 20.28333 > 2 Albania 2010 42.08417 20.42 > > I want to calculate the smallest distances for a given observation of > observation_type=1 to an observation of type=2 for a given year in a > given country. Here is the code (all variables are generated of the > form gen bank_1_dist_1 =.) > > * Shortest distance > local n = _N > forval i = 1/`n' { > forval j = 1/`n' { > if (`i' != `j') & (observation_type[`i']==1) & > (observation_type[`j']==2) & > (country_number[`i']==country_number[`j']) & (year[`i']==year[`j']) { > local d = (latitude[`i'] - latitude[`j'])^2 + (longitude[`i'] - > longitude[`j'])^2 > replace bank_2010_1_`j'=`d' in `i' > if `d' < bank_1_dist_1[`i'] { > replace bank_1_dist_1 = `d' in `i' > replace bank_1_id_1 = `j' in `i' > } > } > } > } > * Second shortest distance > local n = _N > forval i = 1/`n' { > forval j = 1/`n' { > if (`i' != `j') &(observation_type[`i']==1) > &(observation_type[`j']==2) > &(country_number[`i']==country_number[`j']) &(year[`i']==year[`j']) { > local d2 = (latitude[`i'] - latitude[`j'])^2 + (longitude[`i'] - > longitude[`j'])^2 > if (`d2' > bank_1_dist_1[`i']) & (`d2' < bank_1_dist_2[`i']) { > replace bank_1_dist_2 = `d2' in `i' > replace bank_1_id_2 = `j' in `i' > } > } > > } > } > > Here is the problem: The shortest distance seems to be well > calculated. However, the second smallest distance is not calculated > correctly (sometimes it takes on the same value as the shortest > distance and only sometimes it is the actual shortest distance). Do > you know why? Do you have any suggestions for improvement? > > Thanks in advance. > Ruediger For what Ruediger wants to do, his best bet may be -nearstat-. He can install -nearstat- by typing: ssc install nearstat, all Best regards, Wilner ------------------------------------- P. Wilner Jeanty Rice University Email: pwjeanty@rice.edu * * 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**:**st: Re: st: RE: Calculating the shortest distances between observations (based on longitude and latitude)***From:*Rüdiger Vollmeier <ruediger.vollmeier@googlemail.com>

**st: Re: st: RE: Calculating the shortest distances between observations (based on longitude and latitude)***From:*"Dimitriy V. Masterov" <dvmaster@gmail.com>

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