A PIPER PA28 PLT ATTEMPTING TO LOCATE HAF ENTERED SFO CLASS B AIRSPACE WITHOUT CLRNC.

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I WAS ENRTE TO HALF MOON BAY ARPT FROM THE SE ALONG THE COAST. THE ARPT ITSELF AND AREA TO THE W WAS COVERED WITH A LAYER OF CLOUD OR FOG; TOPS ABOUT 1500 FT MSL; UNDETERMINED BOTTOM. NO OFFICIAL WX IS AVAILABLE FOR HAF. OTHER THAN THE COASTAL AREA; WX WAS CLR AND UNLIMITED VISIBILITY. I ELECTED TO CONTINUE IN CLR VMC AT 2000 FT MSL SLIGHTLY N OF THE ESTIMATED ARPT LOCATION; ALONG THE EDGE OF THE CLOUD AREA. I AM REASONABLY FAMILIAR WITH THE AREA; HAVING BEEN TO HAF SEVERAL YRS AGO ON MULTIPLE LIGHT ACFT FLTS. THERE IS OFTEN A 'HOLE' ALONG THE COAST; WHILE ALLOWS A LEGAL DSCNT; AND VMC FLT TO THE ARPT UNDER THE CLOUD LAYER. THE AREA FOR THIS MANEUVERING IS SOMETIMES FOUND JUST TO THE N; IN SFO CLASS B; SECTOR F. I BELIEVED THE CLOUD LAYER EXTENDED ONLY TO THE COASTLINE; AS OBSERVED WHILE ENRTE TO HAF. ACCORDINGLY; I FOLLOWED THE EDGE OF THE CLOUD SLIGHTLY (APPROX 3-4 MI) N OF THE ASSUMED ARPT LOCATION. AS I WAS READY TO TURN BACK TO THE HAF AREA; I SAW A SMALL TOWN AT MY 1-2 O'CLOCK POS; WHICH WAS PROBABLY ROCKAWAY BEACH. IF SO; I MAY HAVE INADVERTENTLY STRAYED INTO THE SFO CLASS B AIRSPACE; SECTOR A; AT 1500 FT OR 2000 FT MSL. I MADE AN IMMEDIATE L TURN (SW) AWAY FROM THE AREA AND DECIDED TO BEGIN MY VFR FLT BACK TO MY ALTERNATE ARPT; WATSONVILLE. I BELIEVE THAT THE PROB IS DUE TO A PRECISE ATC BOUNDARY WHICH MUST NOT BE CROSSED; BUT WHICH CANNOT BE PRECISELY DETERMINED BY THE PLT USING PILOTAGE METHODS. SFO CLASS B IS NOT ALONE IN THIS CIRCUMSTANCE. WHILE I BELIEVE THAT I DID NOT STRAY INTO THE REFED SECTOR; IT IS POSSIBLE THAT I DID. WHEN RADAR CONTACT WAS RESUMED A FEW MOMENTS LATER WITH ZOA FOR FLT FOLLOWING TO WATSONVILLE; I WAS '7 MI S OF SFO ARPT;' WHICH IS WELL OUTSIDE THE REFED SECTOR A.