Notice: On March 31, it was **announced** that Statalist is moving from an email list to a **forum**. The old list will shut down on April 23, and its replacement, **statalist.org** is already up and running.

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

From |
Nick Cox <njcoxstata@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

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

Date |
Thu, 2 Feb 2012 16:23:21 +0000 |

This reference to my work is rather elusive as "presented here some time ago" includes what may be fairly described as several emails. However, I don't think I ever posted code that is smart about distances between (latitude, longitude) pairs. If anyone knows that I am an geographer they might be surprised to learn that I never do this, but it's true. More seriously: I want to emphasise, although it's elementary, that calculating distances using Pythagoras' theorem by treating latitude and longitude as if they were planar coordinates can be at best only an approximation within very small areas. It's not a calculation I'd recommend, not least because there are better routines out there. Nick 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? > * * 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/

**References**:**st: Calculating the shortest distances between observations (based on longitude and latitude)***From:*Rüdiger Vollmeier <ruediger.vollmeier@googlemail.com>

- Prev by Date:
**st: Calculating the shortest distances between observations (based on longitude and latitude)** - Next by Date:
**Re: st: Calculating the shortest distances between observations (based on longitude and latitude)** - Previous by thread:
**st: Calculating the shortest distances between observations (based on longitude and latitude)** - Next by thread:
**st: FW: VAR lag selection and lags autocorrelation** - Index(es):