Work the Shell - Calculating the Distance between Two Latitude/Longitude Points

To get everything to work well, I actually hacked and slashed at the original script to make it a bit more succinct and, of course, invoke the C “distance” program as shown in Listing 1. [Listing 1 also is available on our FTP site at ftp.linuxjournal.com/pub/lj/listings/issue188/10606.tgz.] Ready? It's surprisingly short:

#!/bin/sh
converter="http://api.maps.yahoo.com/ajax/
↪geocode?appid=onestep&qt=1&id=m&qs="

tmpfile="/tmp/bc.script.$$"

# Get lat/long for point 1
addr="$(echo $1 | sed 's/ /+/g')"
values="$(curl -s $converter$addr | \
  cut -d\" -f13,15 | \
  sed 's/[^0-9\.\,\-]//g;s/,$//')"

lat1=$(echo $values | cut -d, -f1)
long1=$(echo $values | cut -d, -f2)

# Get lat/long for point 2
addr="$(echo $2 | sed 's/ /+/g')"
values="$(curl -s $converter$addr | \
  cut -d\" -f13,15 | \
  sed 's/[^0-9\.\,\-]//g;s/,$//')"

lat2=$(echo $values | cut -d, -f1)
long2=$(echo $values | cut -d, -f2)

# Now we have the lat/long for both points, let's
# figure out the distance between them...
dist=$(./distance $lat1 $long1 $lat2 $long2)
echo "$1 to $2 is $dist miles"
exit 0

The script would be even shorter if we tweaked the C program to accept x,y location pairs, but I'll leave that one to you. Instead, let's do a few tests:

$ farapart.sh \
      "union station, denver, co" \
      "union station, chicago, il"
union station, denver, co to
    union station, chicago, il is 917.984 miles

Now, how about something a bit more ambiguous:

$ farapart.sh "long beach, ca" "boston, ma"
long beach, ca to boston, ma is 2597.53 miles

Well, darn it, that seems way too short. Let's see what Yahoo Maps reports as the distance between those two cities. Sure enough, it reports that the trip should be 3,015 miles, not 2,597 miles.

Somewhere there's an error that's giving us poor results. My guess is there's some sort of significant rounding error going on in the C program (because we can verify experimentally that the lat/lon information we're getting is valid, simply by plugging it in to a mapping app and seeing where it places us).

I'm all tapped out on this example, however. It turned out to be far more tricky than I anticipated, and I leave it as an exercise to you, dear reader, to see if you can figure out what's broken in the C program and report your fix to us. We'll publish the best of them next month! Meanwhile, next column, I'll get back to something that's more about the shell and less about mathematics. I mean, heck, I didn't like math when I was working on my computer science degree, so why am I playing with it now?