To work correctly, Android device must have built-in gyroscope, accelerometer, and 3-axis magnetometer. Also, auto-rotate of display must be disabled in settings.

To begin using, hold phone flat with the top pointing away from you and slowly move back and forth horizontally. Tilt phone up a small amount and rotate back and forth on the new imaginary plane. Continue tilting phone up after the completion of each arc. Phone must not tilt upwards more than 90 degrees. For more accurate results, phone can be mounted in mechanized harness.

Default pixel size is 15 -- can adjust with slider from pixel size of 1 to 30.

Colorization options:

Direction + Strength:
Field direction is indicated by colors: Red is north, blue is south, green is a perpendicular field. Field strength (relative to max) is indicated by pixel brightness.

Polarity + Strength:
Field polarity is indicated by colors: Red is north, blue is south. Field strength (relative to max) is indicated by pixel brightness.

Strength:
Field polarity is not indicated. Red is high field strength, blue is low field strength relative to max.

How it works:

As the phone is moved, the app uses the phone's built-in 3-axis magnetometer to record the field strength on each of 3 axis.