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

To |
statalist@hsphsun2.harvard.edu |

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

Date |
Fri, 11 Sep 2009 09:28:45 -0400 |

Laura-- You don't actually need to download anything to solve this kind of problem, or much harder similar problems, as illustrated by e.g. http://www.stata.com/statalist/archive/2009-07/msg00261.html http://www.stata.com/statalist/archive/2007-01/msg00098.html and similar posts. I particularly doubt the final claim in the help file for -distmatch- in the paragraph "Distance matching is computationally intensive. Observations of 3,000 may take several minutes to complete. Other methods typically take days if not weeks and requires extensive user-involvement." But are you using the centroids of the bodies of water? That seems like a strange measure of the distance to water! You could try: use farms, clear local nf=_N g double mindist=. merge using waterbodies local R=6367.44 qui forv i=1/`nf' { local x1=farm_Y[`i'] local y1=farm_X[`i'] local x2 wat_Y local y2 wat_X g double L=(`y2'-`y1')*_pi/180 replace L=(`y2'-`y1'-360)*_pi/180 if L<. & L>_pi replace L=(`y2'-`y1'+360)*_pi/180 if L<-_pi local t1 acos(sin(`x2'*_pi/180)*sin(`x1'*_pi/180) g double d=`t1'+cos(`x2'*_pi/180)*cos(`x1'*_pi/180)*cos(L))*`R' su d, meanonly replace mindist=r(min) in `i' drop L d } drop _m waterbody_ID wat_X wat_Y la var mindist "Distance to center of nearest body of water" or adapt as appropriate... On Fri, Sep 11, 2009 at 3:23 AM, Laura Platchkov <LMP881@bham.ac.uk> wrote: > Dear Roy, > > My question was a general advice and also the approriate command. From what I understand from your message, you wrote a command called "dismatch" that I have to add to my STATA that do exactly what I need? It can be applied without any transformation to my case? > > After adding the plugin, I need to append the table with the waterbodies to the farms and simply write "distmatch, id(id) near(1) long(long) lat(lat), km" > > To adapt to my case, I need to write: > > farm_id (waterbod_id) near (1) farm_X (waterb_X) farm_Y (waterbod_Y), km > > Is that correct? > > Thanks a lot! > > Laura > ________________________________________ > From: Laura Platchkov > Sent: 10 September 2009 20:27 > To: statalist@hsphsun2.harvard.edu > Subject: exact command for distance ? > > Dear Statalist users, > > I was just wondering if someone would have an advice as to how to write a small program on STATA to compute distances between 2 datasets. > > I have 2 datasets (.dta files). The first, called farms.dta, contains 800 observations that correspond each of them to a farm,with, among others, the 3 variables: farm_ID, farm_X (longitude) and farm_Y (latitude). The second called waterbodies.dta contains information about the locations of the centroids of 135 waterbodies, more precisely 3 variables: waterbody_ID, wat_X, wat_Y. > > I want to calculate the distance of each of the farm to the nearest waterbody, in kilometers. > > Now, I need to write a small do file explaining to STATA to calculate the distance using the following Great Circle Formula: > > 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))), where x and y are the coordinates of the farms, and x2 and y2 are the coordinates of the bodies of water.... > > ...for each observation to each of the waterbodies, but I only need the smallest distance. > > 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 at the keep in 1same time and how to tell STATA to give me only the smallest distance. I want STATA to simply add the results as an additional variable in the first dataset (or create a 3d dataset with the farm_ID, farm_X, farm_Y and nearest distance if its easier). > > My idea is to do a write in the do-file, something like this: > > 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 ????? > > I guess there are some mistakes... Does anybody has perhaps a suggestion as how to improve the command? > > Thanks a lot! > Laura > * * 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: exact command for distance ?***From:*Roy Wada <roywada@hotmail.com>

**Re: st: RE: exact command for distance ?***From:*Austin Nichols <austinnichols@gmail.com>

**References**:**st: RE: exact command for distance ?***From:*Laura Platchkov <LMP881@bham.ac.uk>

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