Module: Geocoder::Calculations

Extended by:
Calculations
Included in:
Calculations
Defined in:
lib/geocoder/calculations.rb

Constant Summary collapse

COMPASS_POINTS =

Compass point names, listed clockwise starting at North.

If you want bearings named using more, fewer, or different points override Geocoder::Calculations.COMPASS_POINTS with your own array.

%w[N NE E SE S SW W NW]
EARTH_RADIUS =

Radius of the Earth, in kilometers. Value taken from: en.wikipedia.org/wiki/Earth_radius

6371.0
KM_IN_MI =

Conversion factor: multiply by kilometers to get miles.

0.621371192

Instance Method Summary collapse

Instance Method Details

#bearing_between(lat1, lon1, lat2, lon2, options = {}) ⇒ Object

Calculate bearing between two sets of coordinates. Returns a number of degrees from due north (clockwise).

Also accepts an options hash:

  • :method - :linear (default) or :spherical; the spherical method is “correct” in that it returns the shortest path (one along a great circle) but the linear method is the default as it is less confusing (returns due east or west when given two points with the same latitude)

Based on: www.movable-type.co.uk/scripts/latlong.html



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# File 'lib/geocoder/calculations.rb', line 79

def bearing_between(lat1, lon1, lat2, lon2, options = {})
  options[:method] = :linear unless options[:method] == :spherical

  # convert degrees to radians
  lat1, lon1, lat2, lon2 = to_radians(lat1, lon1, lat2, lon2)

  # compute deltas
  dlat = lat2 - lat1
  dlon = lon2 - lon1

  case options[:method]
  when :linear
    y = dlon
    x = dlat

  when :spherical
    y = Math.sin(dlon) * Math.cos(lat2)
    x = Math.cos(lat1) * Math.sin(lat2) -
        Math.sin(lat1) * Math.cos(lat2) * Math.cos(dlon)
  end

  bearing = Math.atan2(x,y)
  # Answer is in radians counterclockwise from due east.
  # Convert to degrees clockwise from due north:
  (90 - to_degrees(bearing) + 360) % 360
end

#bounding_box(latitude, longitude, radius, options = {}) ⇒ Object

Returns coordinates of the lower-left and upper-right corners of a box with the given point at its center. The radius is the shortest distance from the center point to any side of the box (the length of each side is twice the radius).

This is useful for finding corner points of a map viewport, or for roughly limiting the possible solutions in a geo-spatial search (ActiveRecord queries use it thusly).



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# File 'lib/geocoder/calculations.rb', line 160

def bounding_box(latitude, longitude, radius, options = {})
  units = options[:units] || :mi
  radius = radius.to_f
  [
    latitude  - (radius / latitude_degree_distance(units)),
    longitude - (radius / longitude_degree_distance(latitude, units)),
    latitude  + (radius / latitude_degree_distance(units)),
    longitude + (radius / longitude_degree_distance(latitude, units))
  ]
end

#compass_point(bearing, points = COMPASS_POINTS) ⇒ Object

Translate a bearing (float) into a compass direction (string, eg “North”).



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# File 'lib/geocoder/calculations.rb', line 109

def compass_point(bearing, points = COMPASS_POINTS)
  seg_size = 360 / points.size
  points[((bearing + (seg_size / 2)) % 360) / seg_size]
end

#distance_between(lat1, lon1, lat2, lon2, options = {}) ⇒ Object

Calculate the distance between two points on Earth (Haversine formula). Takes two sets of coordinates and an options hash:

  • :units - :mi (default) or :km



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# File 'lib/geocoder/calculations.rb', line 47

def distance_between(lat1, lon1, lat2, lon2, options = {})

  # set default options
  options[:units] ||= :mi

  # convert degrees to radians
  lat1, lon1, lat2, lon2 = to_radians(lat1, lon1, lat2, lon2)

  # compute deltas
  dlat = lat2 - lat1
  dlon = lon2 - lon1

  a = (Math.sin(dlat / 2))**2 + Math.cos(lat1) *
      (Math.sin(dlon / 2))**2 * Math.cos(lat2)
  c = 2 * Math.atan2( Math.sqrt(a), Math.sqrt(1-a))
  c * earth_radius(options[:units])
end

#earth_radius(units = :mi) ⇒ Object

Radius of the Earth in the given units (:mi or :km). Default is :mi.



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# File 'lib/geocoder/calculations.rb', line 216

def earth_radius(units = :mi)
  units == :km ? EARTH_RADIUS : to_miles(EARTH_RADIUS)
end

#geographic_center(points) ⇒ Object

Compute the geographic center (aka geographic midpoint, center of gravity) for an array of geocoded objects and/or [lat,lon] arrays (can be mixed). Any objects missing coordinates are ignored. Follows the procedure documented at www.geomidpoint.com/calculation.html.



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# File 'lib/geocoder/calculations.rb', line 120

def geographic_center(points)

  # convert objects to [lat,lon] arrays and remove nils
  points.map!{ |p| p.is_a?(Array) ? p : p.to_coordinates }.compact

  # convert degrees to radians
  points.map!{ |p| to_radians(p) }

  # convert to Cartesian coordinates
  x = []; y = []; z = []
  points.each do |p|
    x << Math.cos(p[0]) * Math.cos(p[1])
    y << Math.cos(p[0]) * Math.sin(p[1])
    z << Math.sin(p[0])
  end

  # compute average coordinate values
  xa, ya, za = [x,y,z].map do |c|
    c.inject(0){ |tot,i| tot += i } / c.size.to_f
  end

  # convert back to latitude/longitude
  lon = Math.atan2(ya, xa)
  hyp = Math.sqrt(xa**2 + ya**2)
  lat = Math.atan2(za, hyp)

  # return answer in degrees
  to_degrees [lat, lon]
end

#km_in_mi ⇒ Object

Conversion factor: km to mi.



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# File 'lib/geocoder/calculations.rb', line 223

def km_in_mi
  KM_IN_MI
end

#latitude_degree_distance(units = :mi) ⇒ Object

Calculate the distance spanned by one degree of latitude in the given units.



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# File 'lib/geocoder/calculations.rb', line 28

def latitude_degree_distance(units = :mi)
  2 * Math::PI * earth_radius(units) / 360
end

#longitude_degree_distance(latitude, units = :mi) ⇒ Object

Calculate the distance spanned by one degree of longitude at the given latitude. This ranges from around 69 miles at the equator to zero at the poles.



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# File 'lib/geocoder/calculations.rb', line 37

def longitude_degree_distance(latitude, units = :mi)
  latitude_degree_distance(units) * Math.cos(to_radians(latitude))
end

#mi_in_km ⇒ Object

Conversion factor: mi to km.



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# File 'lib/geocoder/calculations.rb', line 230

def mi_in_km
  1.0 / KM_IN_MI
end

#to_degrees(*args) ⇒ Object

Convert radians to degrees. If an array (or multiple arguments) is passed, converts each value and returns array.



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# File 'lib/geocoder/calculations.rb', line 190

def to_degrees(*args)
  args = args.first if args.first.is_a?(Array)
  if args.size == 1
    (args.first * 180.0) / Math::PI
  else
    args.map{ |i| to_degrees(i) }
  end
end

#to_kilometers(mi) ⇒ Object

Convert miles to kilometers.



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# File 'lib/geocoder/calculations.rb', line 202

def to_kilometers(mi)
  mi * mi_in_km
end

#to_miles(km) ⇒ Object

Convert kilometers to miles.



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# File 'lib/geocoder/calculations.rb', line 209

def to_miles(km)
  km * km_in_mi
end

#to_radians(*args) ⇒ Object

Convert degrees to radians. If an array (or multiple arguments) is passed, converts each value and returns array.



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# File 'lib/geocoder/calculations.rb', line 176

def to_radians(*args)
  args = args.first if args.first.is_a?(Array)
  if args.size == 1
    args.first * (Math::PI / 180)
  else
    args.map{ |i| to_radians(i) }
  end
end