Hello colleagues!
I am trying to setup an arduino with a HMC5883L regarding to your website example:
https://www.sparkfun.com/products/10530?
I used a modified sketch for the HMC5883L but the only differenc to your sketch is that I also calculate the heading. This code is from Arduino Cookbook 
But I found out, that I will get different kind of values, if I hold the HMC5883L a little bit down or tilt it to a side.
What I need is an device or a sample Code, that gives me the security, to get the correct heading data independent to the position of the compass.
I only need to now the actual heading, but i must works in all positions, like the compass in an aeroplane.
What can I do? I dont know 
Do you now a good tutorial for my requirements.
BR
Dieter
/*
Uses HMC5883L to get earths magnetic field in x,y and z axis
Displays direction as angle between 0 and 359 degrees
*/
#include <Wire.h> //I2C Arduino Library
const int hmc5883Address = 0x1E; //0011110b, I2C 7bit address of HMC5883
const byte hmc5883ModeRegister = 0x02;
const byte hmcContinuousMode = 0x00;
const byte hmcDataOutputXMSBAddress = 0x03;
void setup(){
//Initialize Serial and I2C communications
Serial.begin(9600);
Wire.begin();
//Put the HMC5883 IC into the correct operating mode
Wire.beginTransmission(hmc5883Address); //open communication with HMC5883
Wire.write(hmc5883ModeRegister); //select mode register
Wire.write(hmcContinuousMode); //continuous measurement mode
Wire.endTransmission();
}
void loop(){
int x,y,z; //triple axis data
//Tell the HMC5883 where to begin reading data
Wire.beginTransmission(hmc5883Address);
Wire.write(hmcDataOutputXMSBAddress); //select register 3, X MSB register
Wire.endTransmission();
//Read data from each axis, 2 registers per axis
Wire.requestFrom(hmc5883Address, 6);
if(6<=Wire.available()){
x = Wire.read()<<8; //X msb
x |= Wire.read(); //X lsb
z = Wire.read()<<8; //Z msb
z |= Wire.read(); //Z lsb
y = Wire.read()<<8; //Y msb
y |= Wire.read(); //Y lsb
}
//Print out values of each axis
Serial.print("x: ");
Serial.print(x);
Serial.print(" y: ");
Serial.print(y);
Serial.print(" z: ");
Serial.print(z);
int angle = atan2(-y , x) / M_PI * 180; // angle is atan(-y/x)
if(angle < 0)
angle = angle + 360; // angle from 0 to 359 instead of plus/minus 180
Serial.print(" Direction = ");
Serial.println(angle);
delay(250);
}