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GPS question

 
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rret



Joined: 01 Dec 2009
Posts: 9
Location: sherman CT

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GPS question
PostPosted: Fri Mar 19, 2010 7:00 am     Reply with quote
I am building a "GPS box" that will only open when brought to a known set location. If I want to get to within +/- 300 ft of a location, how many minutes is that?
mbradley



Joined: 11 Jul 2009
Posts: 118
Location: California, USA

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PostPosted: Fri Mar 19, 2010 11:32 am     Reply with quote
This can not be answered directly, as distance in minutes is closer at the poles than the equator, and only for lon, not lat (I believe the order).

What you need is to know the distance between coordinates.

Here is a super simple formula, and its not accurate, but, well within your 300ft criteria.

Polar Coordinate Flat-Earth Formula
Code:

a = pi/2 - lat1
b = pi/2 - lat2
c = sqrt(a^2 + b^2 - 2 * a * b * cos(lon2 - lon1)
d = R * c

Abstracted from here, 1/4 page down
http://www.movable-type.co.uk/scripts/gis-faq-5.1.html

Also, there was a similar project on hackaday.com, about a week ago.
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Michael Bradley
www.mculabs.com
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