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Re: st: RE: exact command for distance ?

From   Austin Nichols <[email protected]>
To   [email protected]
Subject   Re: st: RE: exact command for distance ?
Date   Fri, 11 Sep 2009 09:28:45 -0400

You don't actually need to download anything to solve this kind of
problem, or much harder similar problems, as illustrated by e.g.
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

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 <[email protected]> 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: [email protected]
> 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

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