PVT PLT OF AN SMA SEL INADVERTENTLY ENTERED CLASS B AIRSPACE WHILE ENRTE TO AN UNDERLYING ARPT.

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THIS WAS A SHORT (10 MINS); RETURN FLT TO AUBURN; WA; (S50) FROM PIERCE COUNTY (IS0). THE PURPOSE OF THE FLT TO PIERCE WAS TO WARM THE ENG AND REFUEL THE PLANE WITH 80 OCTANE. ON DEP AT PIERCE; RWY 34; I BANKED TO THE E UNTIL OVER THE VALLEY (THE VALLEY FLOOR IS 500 FT BELOW PIERCE). THEN I TURNED N AND CLBED TO 1300 FT. THIS ELEVATION IS BELOW THE 1600 FT FLOOR OF SEATAC CLASS B. AFTER LEVELING OFF; I MONITORED FLT WATCH XMISSIONS ON WX CONDITIONS TO THE N AND S AND ALSO ADMIRED THE SCENERY. I REMEMBER GLANCING AT THE ALTIMETER AND READ IT AT 900 FT. I THOUGHT I HAD LOST 400 FT. NO PROB; PATTERN HT AT AUBURN IS 1000 FT. ON A 45 DEG TO ENTER THE PATTERN FOR RWY 16; I CHKED THE ALTIMETER AND IT READ 1900 FT. PROB; HAD I VIOLATED THE 1600 FT FAN SHAPED FLOOR OF THE CLASS B? THIS IS A STUPID SIT TO BE IN. I GOT THERE BY MISREADING THE ALTIMETER AND BY BEING COMPLACENT. THE STAIR STEP SHAPE OF THE CLASS B IN THIS AREA IS ASKING FOR A VIOLATION IF YOU DON'T PAY CLOSE ATTN. TRANSIENT VFR PLTS FLYING TO THE N OR TO THE S ARE LIKELY TO VIOLATE THIS SMALL SECTION OF THE FAN SHAPED CLASS B AIRSPACE. SEATAC AND FEDERAL WAY ARE ON A LONG PENINSULA; 500 FT HIGHER THAN THE VALLEY. THE SEPARATION BTWN THE 2 IS ABRUPT; A CLIFF. THE HWY FOLLOWS THE VALLEY FLOOR NEXT TO THE CLIFF. VFR PLTS NAV OVER LANDMARKS. HWYS ARE GREAT LANDMARKS. IT WOULD BE NICE IF THE FAA COULD GIVE UP A SMALL PIECE OF THIS CLASS B FAN; CHANGING THE BOUNDARY TO RUN ALONG HWY 167. THE HWY FOLLOWS THE VALLEY FLOOR NEXT TO THE CLIFF. THIS WOULD RAISE THE CEILING TO 3000 FT; THAT OF THE SURROUNDING AIRSPACE TO THE E.