Work the Shell - Exploring Lat/Lon with Shell Scripts

(The invocation is substitute/old-pattern/new-pattern/.)

Now we've got what we set out to create initially. Let's try it with yet another address:

$ sh whereis.sh 1313 S. Disneyland Drive, Anaheim CA
33.814413,-117.924424

Yep, that's the parking structure for Disneyland in California.

Now comes the hard part of this, actually. We can get the lat/lon of any address we desire, but calculating the distance between two points is a bit more tricky, as the mathematics involved is rather hairy, because what we're basically going to do is measure relative to the circumference of Earth.

I found a formula in JavaScript on-line as a starting point:

var R    = 6371;        // kilometers
var dLat = (lat2-lat1);
var dLon = (lon2-lon1);
var a    = Math.sin(dLat/2) * Math.sin(dLat/2) +
           Math.cos(lat1.toRad()) * Math.cos(lat2.toRad()) *
           Math.sin(dLon/2) * Math.sin(dLon/2);
var c    = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
var d    = R * c;

In this case, the circumference is R, and it's 6,371km. Because Earth is an oblate spheroid, not a perfect sphere, I expect this will have some small level of error, but let's proceed and see where we get.

To accomplish any sophisticated mathematics in a Linux shell, we're pretty much stuck with bc, but it's plenty powerful enough for this task, even if it's a bit clunky.

As an example, here's how you'd set the value of pi within a bc script:

pi=$(echo "scale=10; 4*a(1)" | bc -l)

The first stumble we have is that bc wants to work with radians, not degrees, but the lat/lon values we're getting are in degrees, so we need to convert them.

But before we do that, here's the intermediate output we seek, as we now need to work with two addresses, not just one:

$ sh farapart.sh \
  "1600 pennsylvania ave, washington dc" \
  "1313 s. disneyland drive, anaheim, ca"

Lat/long for 1600 pennsylvania ave, washington dc

= 38.89859, -77.035971

Lat/long for 1313 s. disneyland drive, anaheim, ca

= 33.814413, -117.924424

Next month, we'll crack open the script to see how I am working with two addresses at the same time and splitting it into the four variables we'll later need. Then, we'll look at how to use bc to do the math.