通过转换为utm计算纬度/经度距离与使用近似方法得出的结果不同

Question

I've got two methods for calculating the distance between georeferenced coordinates in python:

from pyproj import Proj
import math

def calc_distance(lat1, lon1, lat2, lon2):
    """
    Calculate the great circle distance between two points
    on the earth (specified in decimal degrees)
    """
    # convert decimal degrees to radians
    lon1, lat1, lon2, lat2 = map(math.radians, [lon1, lat1, lon2, lat2])

    # haversine formula
    dlon = lon2 - lon1
    dlat = lat2 - lat1
    a = math.sin(dlat / 2) ** 2 + math.cos(lat1) * math.cos(lat2) * math.sin(dlon / 2) ** 2
    c = 2 * math.asin(math.sqrt(a))
    km = 6371 * c

    return km

def calc_distance_convert_utm(lat1, lon1, lat2, lon2):
    myProj = Proj("+proj=utm +zone=42, +north +ellps=WGS84 +datum=WGS84 +units=m +no_defs")

    # convert to utm 
    utm_x1, utm_y1 = myProj(lat1, lon1)
    utm_x2, utm_y2 = myProj(lat2, lon2)

    diff_x = abs(utm_x1 - utm_x2)
    diff_y = abs(utm_y1 - utm_y2)

    distance = math.sqrt(diff_x**2 + diff_y**2)

    return distance

Which I call with the following values:

lat1 = 34.866527
lon1 = 69.674606
lat2 = 34.864990
lon2 = 69.657655
print "approximation method: ", calc_distance(lat1, lon1, lat2, lon2)
print "converting to utm method: ", calc_distance_convert_utm(lat1, lon1, lat2, lon2)

However, If I compare the results, I get two different values:

approximation method:  1.55593476881
converting to utm method:  1928.21537269

Note, that the first method returns the distance in kilometers while the second returns it in meters. I've compared the result with distance calculators which you can find online, and it seems that the first method (approximation method) is the "more correct" answer as this is the value most online calculators return. I wonder, why the second method (converting to utm first) does not return a more similar result (something like 1555.9347...). I have a difference of almost 0.5 km which seems pretty much to me.

Did I do anything wrong? Any help is appreciated! Thanks

Answer 1

I've found the error ... In the utm converting method I've switched the lat/lon values in the conversion process. It should be:

utm_x1, utm_y1 = myProj(lon1, lat1)
utm_x2, utm_y2 = myProj(lon2, lat2)