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根据两点的经纬度计算两点之间的距离：

# converts degrees to radians

# params: degree to convert

# returns: answer in radians

def self.convert_to_radians(degree_num)

radians = degree_num.to_f * Math::PI/180

end

# calculate the distance between two zips using the great circle ormula

# params: orig latitude, orig longitude, to latitude, to longitude

# returns: distance in miles

def self.calc_distance_between(orig_lat, orig_long, to_lat, to_long)

# find the delta’s of latitude and longitude

delta_lat = to_lat - orig_lat

delta_long = to_long - orig_long

# compute the sin^2(delta_*/2)

computed_lat = Math.sin(delta_lat/2)

computed_lat = computed_lat * computed_lat

computed_long = Math.sin(delta_long/2)

computed_long = computed_long * computed_long

# find the inner part of the great circle

inner_equation = computed_lat + Math.cos(orig_lat) * Math.cos(to_lat) * computed_long

# find the outer part of the great circle

outer_equation = 2 * Math.atan2(Math.sqrt(inner_equation),Math.sqrt(1-inner_equation))

# find the distance in miles

distance = 3956 * outer_equation

end

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