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RE: st: RE: exact command for distance ?

From   "Kieran McCaul" <[email protected]>
To   <[email protected]>
Subject   RE: st: RE: exact command for distance ?
Date   Sun, 13 Sep 2009 06:49:10 +0800


To quote GH Hardy: "I am reluctant to intrude in a discussion concerning
matters of which I have no expert knowledge,..."

But, it seems to me that for each farm, you could create a quick screen
that would remove the polygon points that were unlikely to be close to
the farm.  

If f_lat and f_long are the coordinates of the farm and the coordinates
for each polygon point are p_lat and p_long, then

gen fp_lat = f_lat - p_lat
gen fp_long = f_long - p_long
gen dist = sqrt(fp_lat^2 + fp_long^2)
qui sum dist, detail
keep if dist<`r(p5)'

If all your farms and lakes are in the UK, you would need to add a
constant to the longitudes before calculating the differences (modulo
180 or 360; I don't know how longitude is specified in the data).

Kieran McCaul MPH PhD
WA Centre for Health & Ageing (M573)
University of Western Australia
Level 6, Ainslie House
48 Murray St
Perth 6000
Phone: (08) 9224-2701
Fax: (08) 9224 8009
email: [email protected]
If you live to be one hundred, you've got it made.
Very few people die past that age - George Burns

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Laura
Sent: Sunday, 13 September 2009 12:59 AM
To: [email protected]
Subject: RE: st: RE: exact command for distance ?

Well, Roy, do you have any idea of how long it will take if i have one
dataset with 800 points  (farms) and the other, with 200'000 (points of
lakes lines) ???? Stata has been running the whole afternoon now, and I
am wondering whether I shouldn't maybe reduce the number of observaitons
of the second dataset...

Or Is there an amendment in the command that I could do in order for
STATA to compute only the distances from the farms to the nearest lakes,
and not from the points of the lakes to the nearest farms...?


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