A TB20 PLT VIOLATED P40 AIRSPACE BECAUSE HE MISINTERPED THE CONCENTRIC RINGS ON HIS GPS AS MILEAGE INDICATORS.

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DURING A VFR FLT AT 3300 FT MSL FROM ZZZ1 TO ZZZ2 I INADVERTENTLY ENTERED THE 5-10 MI RING OF THE TFR ASSOCIATED WITH P-40. THE EXPANDED TFR WAS ESTABLISHED BY NOTAMS. AT THE TIME I WAS MONITORING THE ZZZ2 TWR FREQ AND THE ASOS. I WAS NOT MONITORING 121.5. BEFORE THE FLT; I FILED A VFR ADIZ FLT PLAN TO EXIT AND RE-ENTER THE ADIZ WITH THE FSS AND OBTAINED THE NOTAM DESCRIBING THE TFR. CHAIN OF EVENTS: THE PROB AROSE BECAUSE I MISINTERPED THE 2 CONCENTRIC RANGE RINGS AROUND P-40 SHOWN ON MY GARMIN 430 GPS. I MISTAKENLY THOUGHT THE INNER RING WAS A 5 MI RING AND THE OUTER RING WAS THE 10 MI RING. ACCORDINGLY; RATHER THAN FLY DIRECTLY FROM ZZZ1 TO ZZZ2; I WAS MANEUVERING TO THE SW OF P-40 TO REMAIN ABOUT 3 MI OUTSIDE OF THE LARGER RANGE RING AROUND P-40. I THOUGHT THAT THIS SHOULD KEEP ME WELL AWAY FROM THE TFR. I DISCOVERED MY POS ERROR WHEN THE ZZZ2 TWR CTLR CALLED ME (TFR FREQ) AND TOLD ME I WAS IN THE TFR AND TOLD ME TO TURN TO THE L. I IMMEDIATELY COMPLIED WITH HIS INSTRUCTION AND EXITED THE TFR TO THE SW. HUMAN PERFORMANCE CONSIDERATIONS: ONE HUMAN FACTOR MAY HAVE CONTRIBUTED TO THIS EVENT. THE ORIGINAL FLT WAS PLANNED ONLY FROM ZZZ1 TO ZZZ3 IN ORDER TO EXIT THE ADIZ TO PERFORM SOME LNDG PRACTICE. SHORTLY BEFORE TKOF; ANOTHER PLT ASKED ME IF I WOULD TAKE HIM TO ZZZ2 TO PICK UP HIS PLANE THAT WAS IN THE SHOP. THIS LAST MIN CHANGE OF DEST MAY HAVE CONTRIBUTED TO THIS EVENT IN THAT I HAD ORIGINALLY NOT ANTICIPATED FLYING NEAR THE TFR IN THAT AREA. FUTURE PREVENTION: THERE ARE 2 THINGS THAT I WILL DO IN THE FUTURE TO PREVENT THIS EVENT FROM HAPPENING AGAIN. 1) WHEN FLYING NEAR P-40; I WILL USE THE GPS TO PLACE A CURSOR ON THE CTR OF P-40 SO THAT I GET A CONTINUOUS READOUT OF MY DISTANCE FROM THE CTR. I USED THIS TECHNIQUE ON MY RETURN FLT AND IT PROVED QUITE HELPFUL. 2) I WILL MONITOR 121.5 WHEN IN THE ADIZ AND NEAR P-40. IT IS POSSIBLE THAT I MIGHT HAVE BEEN WARNED BY POTOMAC APCH ON 121.5 AS I NEARED P-40 IF I HAD BEEN MONITORING THIS FREQ. A THIRD ACTION MIGHT BE SUGGESTED TO THE GPS MANUFACTURERS TO HAVE THEM PLOT A DOTTED CIRCLE AROUND P-40 AT THE 10 MI RADIUS AS AN AID TO NAV IN THAT AREA. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT VIRTUALLY EVERY PLT HE HAS SPOKEN WITH THOUGHT THE CONCENTRIC CIRCLES ON THE GPS DISPLAYS WERE 5 AND 10 MI. IN FACT THEY ARE ABOUT 3 AND 5 MI RESPECTIVELY. THE RPTR'S LCL FSDO HAS CONTACTED HIM AND IS PROCESSING THE VIOLATION. THE RPTR SAID TYPICALLY THE FAA WILL SUSPEND LICENSES 30 DAYS IF ENFORCEMENT IS WARRANTED.