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From |
Roy Wada <roywada@hotmail.com> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: exact command for distance ? |

Date |
Thu, 10 Sep 2009 16:13:36 -0700 |

> Now, I need to write a small do file explaining to STATA to calculate the > distance using the following Great Circle Formula: > > I thought a loop would be the best, but I have small doubts about how > to write the command on stata. In particular, I don't exactly know how > to tell STATA to use 2 different datasets ... > > using farms > forvalues j =1/800 > cross using waterbod > 3963*acos(sin(y/57.2958)*sin(y2/57.2958)+cos(y/57.2958)*cos(y2/57.2958)*cos((x2/57.2958)-(x/57.2958))),> name near_dist > sort near_dist > keep in 1 > label variable dist "nearest waterbody" > list farm_ID farm_X farm_Y near_dist > save ????? If you are asking for general advice on matching, those based on -merge- or -cross- will run very slowly, although not as slowly as one-by-one comparison within loops whose running time will increase exponentially with data size. If you are asking for a program that will do this in a reasonable amount of time, then -distmatch- is available from ssc. After appending your dataset, the syntax would be: distmatch, id(id) near(1) long(long) lat(lat) where id is a unique identifier that should be left missing (replace id=.) if the observation is to be a non-donor, which in your case would be the farm for which the water bodies would be matched. -distmatch- implements haversine formula, which is almost exactly the same as the Great Circle Formula and is more accurate when the distance is less than 1 mile. -distmatch- was first discussed here: http://www.stata.com/statalist/archive/2009-08/msg01536.html Roy _________________________________________________________________ Get back to school stuff for them and cashback for you. http://www.bing.com/cashback?form=MSHYCB&publ=WLHMTAG&crea=TEXT_MSHYCB_BackToSchool_Cashback_BTSCashback_1x1 * * 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: exact command for distance ?***From:*Laura Platchkov <LMP881@bham.ac.uk>

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