title: Lat/lon distance calculation by converting to utm gives different results than using approximate method
body: <p>I've got two methods for calculating the distance between georeferenced coordinates in python:</p>
  
  <pre><code>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
  </code></pre>
  
  <p>Which I call with the following values:</p>
  
  <pre><code>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)
  </code></pre>
  
  <p>However, If I compare the results, I get two different values:</p>
  
  <pre><code>approximation method:  1.55593476881
  converting to utm method:  1928.21537269
  </code></pre>
  
  <p>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. </p>
  
  <p>Did I do anything wrong?
  Any help is appreciated! Thanks</p>