Trouble migrating formula from JavaScript to Ruby

ruby-talk
1

Ingo Weiss:

# //JavaScript:
# dlng = p2_lng - p1_lng
# dlat = p2_lat - p1_lat
# a = (sin(dlat/2))^2 + cos(p1_lat) * cos(p2_lat) * (sin(dlon/2))^2
# c = 2 * atan2( sqrt(a), sqrt(1-a) )
# distance = R * c (where R is the radius of the Earth)

···

#
#
# #Ruby:
# dlng = p2_lng - p1_lng
# dlat = p2_lat - p1_lat
# a = (Math.sin(dlat/2))**2 + Math.cos(p1_lat) * Math.cos(p2_lat) *
# (Math.sin(dlng/2))**2
# c = 2 * Math.atan2( Math.sqrt(a), Math.sqrt(1-a) )
# distance = 6373 * c #(radius of the Earth: 6373km
#
#
# Any idea why the Ruby one returns incorrect numbers?

1) can you post sample running codes on both? (eg what are the values of p2_lang, etc)

2) do you have test cases? (so we know what to expect)

kind regards -botp

2

Peña wrote:

Ingo Weiss:

# //JavaScript:
# dlng = p2_lng - p1_lng
# dlat = p2_lat - p1_lat
# a = (sin(dlat/2))^2 + cos(p1_lat) * cos(p2_lat) * (sin(dlon/2))^2
# c = 2 * atan2( sqrt(a), sqrt(1-a) )
# distance = R * c (where R is the radius of the Earth)
# # # #Ruby:
# dlng = p2_lng - p1_lng
# dlat = p2_lat - p1_lat
# a = (Math.sin(dlat/2))**2 + Math.cos(p1_lat) * Math.cos(p2_lat) *
  

is dlng a typo, the first formula uses dlon?

···

# (Math.sin(dlng/2))**2
# c = 2 * Math.atan2( Math.sqrt(a), Math.sqrt(1-a) )
# distance = 6373 * c #(radius of the Earth: 6373km
# # # Any idea why the Ruby one returns incorrect numbers?

1) can you post sample running codes on both? (eg what are the values of p2_lang, etc)

2) do you have test cases? (so we know what to expect)

kind regards -botp

3

Turned out that I needed to convert the numbers to radians. Here is the
code that worked:

#Calculate distance between 2 coordinate points (decimal lat/lng),
expressed in km
#This is an approximation since it assumes the earth to be a perfect
sphere
def calculate_distance(p1, p2)

  # radius_of_earth = 3963.0 #miles
  radius_of_earth = 6373.0 #kilometers

  #convert position to decimal and radians:
  to_rad = 180/Math::PI
  p1_lat = degmin_to_dec(p1.lat) / to_rad
  p1_lng = degmin_to_dec(p1.lng) / to_rad
  p2_lat = degmin_to_dec(p2.lat) / to_rad
  p2_lng = degmin_to_dec(p2.lng) / to_rad

  #calculate distance:
  distance = radius_of_earth * Math.acos(Math.sin(p1_lat) *
Math.sin(p2_lat) + Math.cos(p1_lat) * Math.cos(p2_lat) * Math.cos(p2_lng
- p1_lng))

end

Ingo

···

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