Created : Nov 2008
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GPS co-ordinate Resolutions

Here is a typical GPS co-ordinate : san francisco,ca (lat : 37.775196, long : -122.419204)
Ever wonder how much distance each digit represents? Here is a breakdown.

Note: The points compared are diagonal (bot LAT and LON are incremented). And these distances vary depending on the location.

Coordinate Position distance (meters) distance (km) distance (miles) distance (feet)
Reference Co-ord
37.0, 122.0
47.0, 132.0 10s 1,382,074   1,382   859   4,534,363
38.0, 123.0 1 141,847   141   88   465,377
37.1, 122.1 1st decimal 14,218   14   8.8 46,647
37.01, 122.01 2nd decimal 1,422   1.4 0.8 4,665
37.001, 122.001 3rd decimal 142   .1 0.09 466
37.0001, 122.0001 4th decimal 14   0 0 46
37.00001, 122.00001 5th decimal 1   0 0 3
37.000001, 122.000001 6th decimal 0   0 0 0

So 6 decimal resolution should serve us pretty well.

Plug the numbers in the calculator here : http://www.gpsvisualizer.com/calculators.
Here is the RUBY script I use for calculating 'great circle distance':

geopoint.rb: 

class Geopoint
  attr_accessor :lat, :lon
  
  def initialize(alat, alon)
    @lat = alat.to_f
    @lon = alon.to_f
  end
  
  
  # calculates the distance in meters
  def distance(other)
    lat1 = @lat
    lon1 = @lon
    lat2 = other.lat
    lon2 = other.lon
    
    return -1 if (lat2 == nil  || lon2 == nil)
    
    ## following is standard
     r = 6367000 # earth's mean radius in m
    lat1 = lat1 * Math::PI / 180
    lat2 = lat2 * Math::PI / 180
    lon1 = lon1 * Math::PI / 180
    lon2 = lon2 * Math::PI / 180
    re = 6378137 # equatorial radius (meters)
    rp = 6356752 # polar radius
    r45 = re * Math.sqrt( (1 + ( (rp*rp-re*re)/(re*re) ) * (Math.sin(45)*Math.sin(45)) ) ) # from http://www.newton.dep.anl.gov/askasci/gen99/gen99915.htm
    deltaLat = lat2 - lat1
    deltaLon = lon2 - lon1
    a = Math.sin(deltaLat/2) * Math.sin(deltaLat/2) +  Math.cos(lat1) * Math.cos(lat2) * Math.sin(deltaLon/2) * Math.sin(deltaLon /2)
    c = 2 * Math.atan(Math.sqrt(a) / Math.sqrt(1-a))
   dist = r * c
   return dist.round
  end
end

meter.rb: 

# Distance class, created using meters
#class Float < Numeric
class Numeric  
  def to_km
    return self/1000
  end
  
  def to_mile
    return self * 0.000621371192
  end
  
  def to_feet
    return self * 3.2808399
  end
  
  def pretty_print
    return (self*10).truncate / 10
  end
  
end

gps-resolution.rb: 

require File.dirname(__FILE__) + "/geopoint"
require File.dirname(__FILE__) + "/meter"

## also check : http://www.gpsvisualizer.com/calculators

ref = Geopoint.new(37.0, 122.0)

points =  Array [
  Geopoint.new(47.0, 132.0),
  Geopoint.new(38.0, 123.0),
  Geopoint.new(37.1, 122.1),
  Geopoint.new(37.01, 122.01),
  Geopoint.new(37.001, 122.001),
  Geopoint.new(37.0001, 122.0001),
  Geopoint.new(37.00001, 122.00001),
  Geopoint.new(37.000001, 122.000001)
  ]

points.each do |p|
  d = ref.distance(p)
  puts "#{ref.inspect}  #{p.inspect}  = #{d} meters, #{d.to_km} km,  #{d.to_mile} miles, #{d.to_feet} feet"
end

Output: 

#<Geopoint:0x25850 @lon=122.0, @lat=37.0>  #<Geopoint:0x2501c @lon=132.0, @lat=47.0>  = 1382074 meters, 1382 km,  858.780968812208 miles, 4534363.5239526 feet
#<Geopoint:0x25850 @lon=122.0, @lat=37.0>  #<Geopoint:0x25008 @lon=123.0, @lat=38.0>  = 141847 meters, 141 km,  88.139639471624 miles, 465377.2972953 feet
#<Geopoint:0x25850 @lon=122.0, @lat=37.0>  #<Geopoint:0x24ff4 @lon=122.1, @lat=37.1>  = 14218 meters, 14 km,  8.834655607856 miles, 46646.9816982 feet
#<Geopoint:0x25850 @lon=122.0, @lat=37.0>  #<Geopoint:0x24fe0 @lon=122.01, @lat=37.01>  = 1422 meters, 1 km,  0.883589835024 miles, 4665.3543378 feet
#<Geopoint:0x25850 @lon=122.0, @lat=37.0>  #<Geopoint:0x24fcc @lon=122.001, @lat=37.001>  = 142 meters, 0 km,  0.088234709264 miles, 465.8792658 feet
#<Geopoint:0x25850 @lon=122.0, @lat=37.0>  #<Geopoint:0x24fb8 @lon=122.0001, @lat=37.0001>  = 14 meters, 0 km,  0.008699196688 miles, 45.9317586 feet
#<Geopoint:0x25850 @lon=122.0, @lat=37.0>  #<Geopoint:0x24fa4 @lon=122.00001, @lat=37.00001>  = 1 meters, 0 km,  0.000621371192 miles, 3.2808399 feet
#<Geopoint:0x25850 @lon=122.0, @lat=37.0>  #<Geopoint:0x24f90 @lon=122.000001, @lat=37.000001>  = 0 meters, 0 km,  0.0 miles, 0.0 feet



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