Re: Trouble migrating formula from JavaScript to Ruby


#1

Ingo W.:

//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:

#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?


#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