# Java ME Compass

## Contents |

## Introduction

This article shows how we can make Java ME compass with built in magnetic compass & GPS. If we select GPS then it will start using the GPS and calculates the direction. If we select built in magnetometer then it gets the angle from built in magnetic compass (if available). The found angle is used to rotate the compass image.

## Device Orientation

Device orientation can be found by built in magnetic compass or by taking GPS location and calculating the direction of movement accordingly. The orientation angle can be used to rotate the compass image and then redrawn on the device screen. Following code shows how to calculate direction of movement using GPS location.

private void ProcessLocation(float latitude, float longitude)

{

float alpha; // result

// calculate vector coordinates

float y = latitude - preLatitude;

float x = longitude - preLongitude;

if (y < 0.00008 && x < 0.00008)

{

return;

}

// hypotenuse

float sqrtResult = 0;

sqrtResult = (float)Math.sqrt(x * x + y * y);

float angle;

angle = (float)Math.sin(Math.abs(x) / sqrtResult);

alpha = (float)(angle * 180 / 3.14); // angle from North in degrees

// correction

if (x > 0)

{

// I or IV quadrant

if (y < 0)

{

// IV quadrant

alpha = 180 - alpha;

}

}

else

{

// II or III quadrant

if (y > 0)

{

// II quadrant

alpha = -alpha;

}

else

alpha = alpha - 180;

}

preLatitude = latitude;

preLongitude = longitude;

AngleFromNorth = (int) alpha;

angleString = Integer.toString(AngleFromNorth);

}

## Test Application

In the test application, we can select GPS or Magnetic compass to find direction: File:J2meCompassegpsmagnet.zip