Geographic Linestring Length

geographic_length.py

from math import sin, cos, sqrt, asin, radians

RADIUS_EARTH_M = 6371200.0 #The Earth's mean radius in meters

def geographic_length(coords):
    '''Returns the length of a linestring in meters'''
    
    coords = ((radians(x), radians(y)) for x, y in coords)
    rad_dist = sum(_haversine_distance(*c) for c in pairs(coords))
    
    return rad_dist * RADIUS_EARTH_M * 2.0

def _haversine_distance(a, b):
    '''Assumes that a and b are radians and does not multiply by the diameter
    of the earth'''
    
    lng1, lat1 = a
    lng2, lat2 = b
    
    sin_dlat_over_2 = sin((lat2 - lat1) / 2.0)
    sin_dlng_over_2 = sin((lng2 - lng1) / 2.0)
    
    f = sin_dlat_over_2 * sin_dlat_over_2 
    f += cos(lat1) * cos(lat2) * sin_dlng_over_2 * sin_dlng_over_2        
    return asin(sqrt(f))

def haversine_distance(a, b):
    '''Operates on lat,lngs and returns the distance in meters'''
    
    a = map(radians, a) #lng, lat to radians
    b = map(radians, b) #lng, lat to radians
    return RADIUS_EARTH_M * 2.0 * _haversine_distance(a, b)

def pairs(iterable):
    '''Generate pairs of items in iterable'''
    
    i = iter(iterable)
    prev = i.next()    
    for j in i:
        yield prev, j
        prev = j