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From |
"Davide Cantoni" <davide.cantoni@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: Building a matrix of distances with sphdist |

Date |
Sat, 18 Oct 2008 11:37:08 -0400 |

Hello, I have a panel dataset with cities observed in different years, and I would like to construct a matrix of (great circle) distances of all cities to each other. I was using, up to now, Bill Rising's -sphdist-. So this is the way I was thinking to proceed. *** begin code*** levelsof cityid, local(allcities) foreach C of local allcities { sum latitude if cityid==`C', meanonly gen lat1 = r(mean) sum longitude if cityid==`C', meanonly gen lon1 = r(mean) foreach K of local allcities { sum latitude if cityid==`K', meanonly gen lat2 = r(mean) sum longitude if cityid==`K', meanonly gen lon2 = r(mean) sphdist if cityid==`C', lat1(lat1) lon1(lon1) lat2(lat2) lon2(lon2) gen(dist_`K') drop lat2 lon2 } drop lat1 lon1 } *** end code *** The problem is that -sphdist- does not accept something like "replace" but would only create new variables with the required "gen" option. So when the second city in `C' is processed, Stata complains that the variable "dist_1" exists already. What I want, though, is that for any city in my panel the variable "dist_1" indicates the distance to city #1, so that each variable "dist_1", "dist_2" etc fills up as Stata mills through the values of `C'. Alternatively, one could create NxN variables by replacing the -sphdist- command with sphdist if cityid==`C', lat1(lat1) lon1(lon1) lat2(lat2) lon2(lon2) gen(dist_`C'_to_`K') but then again Stata does not accept the large number of variables that corresponds to NxN. Do you have any suggestions on how to solve this? Many thanks for your interest. Best regards, Davide Cantoni * * 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/

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