A ROBINSON R-44 HELI PLT HAS A CLASS C AIRSPACE INCURSION 3.5 NM E OF CHA; TN.

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FLT ORIGINATED FROM COLLEGEDALE ARPT (3M3). THE PURPOSE OF FLT WAS TO TEST XMITTING EQUIP IN A ROBINSON R-44 HELI THAT IS USED FOR TFC RPTING. THERE WERE 3 PERSONS IN THE ACFT: MYSELF AS PLT; ANOTHER COMMERCIAL/CFI PLT; AND THE TECHNICIAN THAT WOULD BE PERFORMING THE EQUIP TEST. I PROCEEDED WBOUND FROM 3M3 TO THE LOCATION THAT THE TECHNICIAN WANTED TO PERFORM THE XMITTER TEST. THE FLOOR OF CLASS C AIRSPACE IS 2600 FT OVER 3M3 SO I WAS CONDUCTING THIS FLT AT 1500-1700 FT MSL. BY REFING THE CURRENT SECTIONAL CHART FOR OUR AREA (ATLANTA); I DETERMINED THAT THE TEST LOCATION WAS GOING TO BE 5.5 MI FROM THE CHA ARPT AND THAT WE WOULD BE STAYING JUST OUTSIDE THE SURFACE AREA BOUNDARY AND 1000 FT BELOW THE OVERLYING CLASS C AIRSPACE. THE OTHER PLT ABOARD AGREED THAT IT LOOKED LIKE WE WOULD BE STAYING JUST OUTSIDE CLASS C SURFACE AREA; SO I OPTED NOT TO CONTACT CHA APCH CTL. WE ORBITED OVER THAT LOCATION FOR 10 MINS AND THEN RETURNED DIRECT 3M3. ON ARR AT 3M3 UNICOM INFORMED ME THAT CHA CTL TWR HAD CALLED AND WANTED TO TALK TO ME. I RETURNED THE CALL TO THE CHA CTL TWR AND SPOKE WITH THE TWR MGR. HE SAID WE WERE IN FACT ONLY 3.5 MI FROM CHA; NOT 5.5 LIKE I HAD THOUGHT; AND THAT I HAD ENTERED THEIR AIRSPACE BY 1.5 MI. A SUBSEQUENT CHK VIA GPS AT THE LOCATION WE HAD BEEN ORBITING SHOWED THAT I HAD INDEED BEEN WITHIN CLASS C SURFACE AREA BY 1.5 MI. THE CAUSE OF THIS AIRSPACE INCURSION WAS INCORRECT IDENT OF MY CURRENT LOCATION ON THE SECTIONAL CHART AND COULD HAVE BEEN AVOIDED BY A MORE CAREFUL EXAMINATION OF THE CHART AND BY FOLLOWING THE GOOD PRACTICE OF CONTACTING ATC WHENEVER YOU ARE OPERATING THAT CLOSE TO CTLED AIRSPACE.