An EMS helicopter pilot suffered some loss of orientation due to multiple unaligned directional devices.

Narrative

After transporting a patient to a local hospital I departed the heliport on an easterly heading. The helicopter is equipped with a GPS; a gyro compass and a regular wet compass. After departure I noticed a difference of about 40 degrees between the gyro compass heading and the GPS course indication. It is common after landing at certain hospitals that the gyro compass would indicate a wrong heading for a short time which is caused by strong magnetic fields due to the use of medical equipment near or underneath the heliport. I therefore used the GPS indication for navigation. The ATC Controller instructed me to fly headings between 100 and 120 degrees which I initially tried to follow using the wet compass until I noticed that there was no fluid in the compass and that the compass did not match the GPS indications. I was then instructed to fly a heading of 180 degrees followed by cleared direct on course which I did using the GPS. The wet compass was replaced with a serviceable unit.