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This symbol is on the tarmac at Ramona, CA airport. I've seen it at other small GA airports from Google Earth but I don't know what it is. Can someone please explain?

Ramona, CA airport

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That is a Compass Rose, painted on a Compass Calibration Pad. It's used to mark a location on the airport surface that is suitable for calibrating the compass of an aircraft.

Here's one in a slightly different style:
Compass pad photo

and a more basic one from an FAA diagram:
Compass pad diagram

More information on the requirements for the compass rose / compass pad can be found in AC 150/5300-13, in Appendix 6.

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    $\begingroup$ Fun bonus fact: The compass rose in your photo was painted by The Ninety-Nines - they volunteer to do compass rose paintings at a lot of GA airports, and you can recognize their work by the 99s logo in the center of the compass rose. $\endgroup$
    – voretaq7
    Feb 8, 2016 at 20:08
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    $\begingroup$ Related: Marking the way. $\endgroup$
    – mins
    Feb 8, 2016 at 20:16
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    $\begingroup$ @voretaq7 nice trivia. The 99s logo confused me because the runway is 9-27 and I thought it might be related to the runway heading. $\endgroup$
    – PJNoes
    Feb 8, 2016 at 20:23
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    $\begingroup$ @meowsqueak Per the advisory circular: Magnetic surveys of existing compass calibration pads must be performed at regular intervals of 5 years or less. … Pads not resurveyed after 5 years or after nearby construction should not be used. - Whether this is actually done in practice varies, so it is prudent to check with the airport to determine the last time the compass pad was properly surveyed. $\endgroup$
    – voretaq7
    Feb 9, 2016 at 5:22
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    $\begingroup$ @PJNoes Remember we're calibrating a magnetic compass: If the compass rose were aligned to True North they would have to paint or other wise mark (with non-magnetic materials) the magnetic variation with the same level of precision - that means they still have to do the periodic surveys, even without regular repaints. They also need to accurately point the aircraft's nose (and its magnetic compass) at the appropriate angles accounting for that variation, which isn't easy without painted markings - at that point they may as well use a calibrated master sight compass rather than a compass rose. $\endgroup$
    – voretaq7
    Nov 24, 2016 at 5:41

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