Module: GeoCalculations

Extended by:
GeoCalculations
Included in:
GeoCalculations
Defined in:
lib/geo_calc/geo_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(point1, point2, options = {}) ⇒ Object

Bearing between two points on Earth. Returns a number of degrees from due north (clockwise).

See Geocoder::Calculations.distance_between for ways of specifying the points. 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/geo_calc/geo_calculations.rb', line 91

def bearing_between(point1, point2, options = {})

  # set default options
  options[:method] = :linear unless options[:method] == :spherical

  # convert to coordinate arrays
  point1 = extract_coordinates(point1)
  point2 = extract_coordinates(point2)

  # convert degrees to radians
  point1 = to_radians(point1)
  point2 = to_radians(point2)

  # compute deltas
  dlat = point2[0] - point1[0]
  dlon = point2[1] - point1[1]

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

  when :spherical
    y = Math.sin(dlon) * Math.cos(point2[0])
    x = Math.cos(point1[0]) * Math.sin(point2[0]) -
        Math.sin(point1[0]) * Math.cos(point2[0]) * 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(point, 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).

See Geocoder::Calculations.distance_between for ways of specifying the point. Also accepts an options hash:

  • :units - :mi (default) or :km



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# File 'lib/geo_calc/geo_calculations.rb', line 181

def bounding_box(point, radius, options = {})
  lat,lon = extract_coordinates(point)
  radius  = radius.to_f
  units   = options[:units] || :mi
  [
    lat - (radius / latitude_degree_distance(units)),
    lon - (radius / longitude_degree_distance(lat, units)),
    lat + (radius / latitude_degree_distance(units)),
    lon + (radius / longitude_degree_distance(lat, 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/geo_calc/geo_calculations.rb', line 128

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

#distance_between(point1, point2, options = {}) ⇒ Object

Distance between two points on Earth (Haversine formula). Takes two points and an options hash. The points are given in the same way that points are given to all Geocoder methods that accept points as arguments. They can be:

  • an array of coordinates ([lat,lon])

  • a geocodable address (string)

  • a geocoded object (one which implements a to_coordinates method which returns a [lat,lon] array

The options hash supports:

  • :units - :mi (default) or :km



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# File 'lib/geo_calc/geo_calculations.rb', line 53

def distance_between(point1, point2, options = {})

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

  # convert to coordinate arrays
  point1 = extract_coordinates(point1)
  point2 = extract_coordinates(point2)

  # convert degrees to radians
  point1 = to_radians(point1)
  point2 = to_radians(point2)

  # compute deltas
  dlat = point2[0] - point1[0]
  dlon = point2[1] - point1[1]

  a = (Math.sin(dlat / 2))**2 + Math.cos(point1[0]) *
      (Math.sin(dlon / 2))**2 * Math.cos(point2[0])
  c = 2 * Math.atan2( Math.sqrt(a), Math.sqrt(1-a))
  c * earth_radius(options[:units])
end

#distance_to_radians(distance, units = :mi) ⇒ Object 



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# File 'lib/geo_calc/geo_calculations.rb', line 221

def distance_to_radians(distance, units = :mi)
  distance.to_f / earth_radius(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/geo_calc/geo_calculations.rb', line 246

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

#extract_coordinates(point) ⇒ Object

Takes an object which is a [lat,lon] array, a geocodable string, or an object that implements to_coordinates and returns a

lat,lon

array. Note that if a string is passed this may be a slow-

running method and may return nil.



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# File 'lib/geo_calc/geo_calculations.rb', line 270

def extract_coordinates(point)
  case point
    when Array; point
    when String; Geocoder.coordinates(point)
    else point.to_coordinates
  end
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/geo_calc/geo_calculations.rb', line 139

def geographic_center(points)

  # convert objects to [lat,lon] arrays and convert degrees to radians
  coords = points.map{ |p| to_radians(extract_coordinates(p)) }

  # convert to Cartesian coordinates
  x = []; y = []; z = []
  coords.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/geo_calc/geo_calculations.rb', line 253

def km_in_mi
  KM_IN_MI
end

#latitude_degree_distance(units = :mi) ⇒ Object

Distance spanned by one degree of latitude in the given units.



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# File 'lib/geo_calc/geo_calculations.rb', line 26

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

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

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/geo_calc/geo_calculations.rb', line 34

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/geo_calc/geo_calculations.rb', line 260

def mi_in_km
  1.0 / KM_IN_MI
end

#radians_to_distance(radians, units = :mi) ⇒ Object 



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# File 'lib/geo_calc/geo_calculations.rb', line 225

def radians_to_distance(radians, units = :mi)
  radians * earth_radius(units)
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/geo_calc/geo_calculations.rb', line 212

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/geo_calc/geo_calculations.rb', line 232

def to_kilometers(mi)
  mi * mi_in_km
end

#to_miles(km) ⇒ Object

Convert kilometers to miles.



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# File 'lib/geo_calc/geo_calculations.rb', line 239

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/geo_calc/geo_calculations.rb', line 198

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