Point Geo Fencing - Sample Code

Point Geo Fencing is easier than polygon Geo Fencing.

As described in my first blog (read it here), you just have to calculate the distance between the point and your current location, and compare it with the radius of your point (circle).

First we need an object for representing our Point Geo Fence:


When your radius is defined in meters, you will need the Haversine formula.  This formula will calculate the distance between two points (in meters) while taking into account the earth curvation:


Now, we can calculate if we're inside the Geo Fence or not:


X and Y are your current longitude and -latitude.
The method will return true, when we're inside the Geo Fence and false when we're not.

More information can be found here:
- http://en.wikipedia.org/wiki/Haversine_formula
- http://stefanbangels.blogspot.be/2012/12/for-several-years-now-i-have-been.html
- http://stefanbangels.blogspot.nl/2013/10/geo-fencing-sample-code.html

Odo Meter: Behind the Magic

An odo meter is a clever component that measures the total distance travelled (like the mileage counter in your car).

In Mobile software development, we also use an odo meter for calculating the travelled distance in:

• track-and-trace applications
• mileage administration applications
• sports tracker applications (like Endomondo, RunKeeper, Strava...)
• bike computers
• taxi meters
• ...

There are multiple strategies for calculating the total distance travelled.

STRATEGY 1: Calculate distance using delta time and current speed

Imagine a process that fetches the current GPS speed at a fixed interval of x time.

At each interval, we can calculate the distance by multiplying the delta time with the current speed:

d = v * t

for example (at an interval of 5 seconds, the current speed is 10 meters/second):

d = 10 meters/second * 5 seconds = 50 m

The problem with this strategy is that, in order to have an accurate result, time (t) has to be very small. In other words: you have to fetch the GPS speed a lot (draining your device battery).

A second problem is: sudden (short) accelerations, can cause an huge increase in distance, even if this speed has only occurred for just a little while... causing the result to be less accurate.

STRATEGY 2: Calculate distance between two points, at a fixed interval

Instead of fetching the GPS speed, we can also use the GPS position (longitude/latitude).

At each interval, we calculate the distance between the previous position and the current position.

As with the previous strategy, the results become more accurate when the interval becomes smaller. But the results aren't affected by speed fluctuations. The problem with this strategy is: we don't drive a straight line.  If we were driving circles, passing the same location at each interval, we could get into a situation where the calculated distance is zero.

STRATEGY 3: Calculate distance between positions, at a dynamic interval

When calculating the distance between positions at a dynamic interval instead of a fixed interval, we could decrease the problems of the previous strategies.

The interval can increase or decrease due to criteria like:

• movement detected by an hardware acceleration- or gyro-meter.
• ignition detected by an input port, canbus or sensing power drop.
• an on/off switch.
• increased speed levels at the previous interval.
• distance threshold (for example, see Android LocationManager).
• course changes (like a compass).
• maps street information.
• ...

Conclusion

Applications that operate in environments with a fairly stable speed (like space applications, airplanes, machine measurements, ...), strategy 1 (calculate distance using delta time and current speed) will be suitable.

But for most applications, strategy 2 (calculate distance between two points, at a fixed interval) will be the best solution.

Just keep in mind, when you require more control (to save battery life or in environments with repetitive reoccurring locations, ...), strategy 3 (calculate distance between positions at a dynamic interval) can also be a solution.