Hacking Google Maps and Google Earth (ExtremeTech)

272 Part III — Google Map Hacks

Note that even with this formula, the exact distance may be different because the earth is not a

perfect sphere (it is slightly “flat” and therefore wider at the equator than at the poles), but the

difference for most small routes is so small that it is not noticeable.

Rather than performing the calculation within JavaScript on the host, the information is deter-

mined by the backend script as it returns the route point information. The function accepts

four arguments, the long/lat pairs for the start and end points. This is called for each pair dur-

ing the point extraction loop in the getroute()function. By adding each point pair distance

together, the distance for the entire route can be calculated:

sub latlngdistance

{

my ($xdeg1,$ydeg1,$xdeg2,$ydeg2) = @_;

my ($x1,$y1,$x2,$y2) = (deg2rad($xdeg1),

deg2rad($ydeg1),

deg2rad($xdeg2),

deg2rad($ydeg2));

my $radius = 6371;

my $latdistance = $x2 - $x1;

my $lngdistance = $y2 - $y1;

my $a = sin($latdistance/2) * sin($latdistance/2) +

cos($x1) * cos($x2) * sin($lngdistance/2) * sin($lngdistance/2);

my $c = 2 * atan2(sqrt($a), sqrt(1-$a));

my $d = $radius * $c;

return $d;

}

The same process could have been carried out within the browser, which would have also

enabled the application to show a live distance calculation as the route was being calculated,

but this information is easier and more straightforward to calculate within the backend func-

tion and also takes some load off of the browser.

Using the Application

The best way to get to know the application in use is, of course, to try it out. The basics of the

system (saving a route, listing available routes, and then loading a single route) can easily be

shown, at least in terms of how they look.

First, Figure 13-3 shows the application in its initial state, showing the map (centered, for a

change, on Grantham).

Click New Route to enable routing and select the first point for your route. Click further

points until you have a route displayed onscreen, as shown here in Figure 13-4. Note that the

route title and a description have also been given here.

In Figure 13-5 you can see the final route when saved, here showing its final distance and the

start and end markers.

Figure 13-6 shows the list of available routes, including the one just created.