# How to do a compass/heading indicator

I want to create a compass that is moveable around a sphere ("earth"). It's orientation should always be tangential to earth's surface, it's location the same as a second object and it should always point to a fixed point on the sphere ("magnetic north").

Background
I am trying to visualize how a compass/heading indicator behaves in order to display the difference between true (geographical) north, which is the axis of revolution of the sphere, and magnetic north for airplane navigation.

I set up an earth, a marker as a pole, an airplane, a housing and a compass needle. To be as close as possible to how this looks in the plane, I want the housing to contain directional information, whereas the 'needle' is fixed to the axis of the plane (compare https://en.wikipedia.org/wiki/Heading_indicator).

I managed to move the airplane tangentially to the earth by placing it on the "north pole" and then put the origin to the center of the earth. By now rotating it around the origin, it moves along the surface and stays tangential.

I'm able to keep the 'compass needle' aligned to the airplane by using the airplane as parent.

My problem is to find the correct constraints to keep the housing with the directional information on it in the right spot (on the airplane) and with the correct orientation.

I tried to use the constraints "track to / locked track to" to keep it facing the pole-marker, which works as long as I do not couple it to the movement of the airplane.

Coupling the housing to the airplane did also work, but then I'm losing the facing.

I tried to:
- permutate the order of the constraints
- played around with world space/local space
- different combinations of placing the individual origins.
- use of armature/bones

I'm also quite new to blender (using version 2.76), so I'm pretty sure I did stupid things and am missing the easy solution. But nonetheless I decided it's time to ask for help :-)

If you have any ideas or comments on this - I'm thankful for every help.

• First, parenting the airplane (and compass) to a point in the center of the "earth" would make this much easier. Then you would only have to track to the pole while locking whatever the "up" axis is. – JakeD Aug 28 '16 at 17:27