[OSM-dev] Question about Calculating Radius from GPS cord

Fire Girl firegirl at amorous.com
Fri Aug 8 17:10:26 BST 2008


Hi Rogier, I wanted to try this forumla out. 

Could you possibly help me by spelling out the formula in more word
terms, like

i.e. Take initial latitude number, and divide it by X then divide that by
the cosign of the XX ... which should result in XYZ

something like that.  Basically I hope to generate a bounding box based
on a Lat/Lon coordinate :)

I am on a steep learning curve, and not too sure when I need to divide or
where the numbers come from :) :)

  ----- Original Message -----
  From: "Rogier Wolff"
  To: "David Earl"
  Subject: Re: [OSM-dev] Question about Calculating Radius from GPS
  cord
  Date: Fri, 8 Aug 2008 17:44:11 +0200


  On Fri, Aug 08, 2008 at 04:02:10PM +0100, David Earl wrote:
  > On 08/08/2008 14:30, Fire Girl wrote:
  > > I am working with OSM data, and would like to be able to spec
  > out 5 mile > bounding boxes from certain GPS points.
  > > > After research into this problem, I am to understand that
  > each degree of > latitude is approximately 69 miles (111
  > kilometers) apart with a slight > variance (68.703 - 69.407
  > miles) between the equator and the poles, and > that each degree
  > of longitude is widest at the equator @ 69.172 miles > (111.321
  > kilometers) and gradually shrinks to zero at the poles. : ) :)
  > > > So what does this mean? If I want to take a input point, like
  lets say,
  > > > 167.9 lat
  > > -29.1 lon
  > > > or
  > > > -63.1
  > > 18.1
  > > > Can someone say with authority, what the 'calculus' would be
  > to > definitivly construct a NSWE bounding box with a 5 mile
  > radius around > those points?.... that would be basically close
  > enough or accurate? :)

  A degree longitude is about 40000km / 360 * cos (lat).
  A degree lattitude is about 40000km / 360.

  So 5 miles would be in longitude:
  5 / (40000 / 1.609 / 360 * cos (lat))
  km/circle degrees/circle
  mile km/mile

  and about

  5 / (40000 / 1.609 / 360)
  km/circle degrees/circle
  mile km/mile

  lattidude. This comes to about 0.0725, 0.0725/cos(lat) degrees for
  5 miles (lat, lon).

  This defines an almost-square where the circle would be almost
  completely inside.

  This especially doesn't work near the poles.

  Roger.

  --
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