PVT PLT OF AN SMA SINGLE ENG LAND ACFT INADVERTENTLY UNDERSHOT DEP CLB ALT ASSIGNMENT DUE TO MISREADING THE ALT.

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I WAS PIC OF AN SMA ON AN INST FLT PLAN IN VFR CONDITIONS. I HAD JUST DEPARTED SRQ RWY 32 ON A CLRNC OF: 'CLRED TO MCN AS FILED; AFTER DEP MAINTAIN A HDG OF 360; 1600 FT; EXPECT 7000 FT 10 MINS AFTER DEP; DEP FREQ 119.65; SQUAWK XXXX.' UPON REACHING AN ALT OF 600 FT MSL; THE R SEAT PAX; MY WIFE AND A STUDENT PLT WITH 21 FLT HRS IN THE SAME TYPE ACFT ASKED; 'AREN'T YOU AT YOUR ALT?' SHE HAD INCORRECTLY INTERPRETED THE ALTIMETER; I INCORRECTLY VERIFIED THE ALT; AGREED; AND LEVELED OFF AT 600 INSTEAD OF 1600 FT MSL. APPROX 2 MINS PASSED AND WE WERE 5 NM DME FROM SRQ. HAVING NOT BEEN TURNED OVER TO DEP ON THE SOMEWHAT BUSY TWR FREQ; I REQUESTED AND RECEIVED PERMISSION TO CHANGE FREQ TO TAMPA DEP. ON MY INITIAL CALL-UP I RPTED BEING LEVEL AT 1600 FT. THE REPLY FROM TAMPA WAS THAT MY MODE C WAS RPTING INCORRECTLY AND I WAS REQUESTED TO TURN MY MODE C OFF. I WAS NEXT CLRED TO 4000 FT AND SHORTLY THEREAFTER TAMPA REQUESTED MY ALT; WHICH I BELIEVE I RPTED AS 1900 FT; CLBING TO 4000 FT. I WAS ACTUALLY PASSING THROUGH 900 FT MSL. APPROX 1 MIN LATER; ANOTHER SMA PASSED W OF ME GOING THE OPPOSITE DIRECTION; APPROX 200 FT BELOW AND 2000 FT BESIDE ME WITHOUT BEING RPTED TO ME BY DEP. I QUERIED TAMPA DEP; WHO REQUESTED MY ALT (MODE C STILL TURNED OFF). THIS TIME I RPTED IT CORRECTLY AS 1100 FT CLBING TO 4000 FT; REALIZING MY ERROR FOR THE PAST 3-5 MINS. TAMPA DEP RPTED THAT THE PASSING SMA HAD ME IN SIGHT AND MAINTAINED VISUAL SEPARATION THE WHOLE TIME SO IT WAS NEVER A PROB. SHE ALSO CLARIFIED THAT; SINCE I HAD RPTED 1900 FT AND THE OTHER SMA WAS AT 900 FT (I BELIEVE THAT WAS THE ALT SHE GAVE) THE TFC WAS NOT RPTED TO ME. WE AGAIN COMPARED MY MODE C TO MY RPTED ALT; WHICH TAMPA DEP CONFIRMED WAS WORKING CORRECTLY. I CONTINUED MY CLB TO 4000 FT FOR THAT SEGMENT OF THE FLT. THE REMAINDER OF THE FLT PASSED WITHOUT MISHAP. THE CAUSE OF THIS INCIDENT WAS CLRLY THAT I MISINTERPRETED THE ANALOG PRECISION ALTIMETER. I TOOK A FEW MOMENTS TO ADD THEM UP; BUT THERE WERE SEVERAL CLUES EARLY ON THAT I WAS AT ONLY 6600 FT VICE 1600 FT MSL (ARPT ELEVATION 28 FT MSL). MY WIFE AND I WERE REMARKING THAT WE LOOKED LOWER THAN WE ARE USED TO FLYING; EXCEPT IN TFC PATTERNS. WE PASSED APPROX 1 NM W OF A RATHER TALL ANTENNA TWR (LATER FOUND TO BE 649 FT MSL); REMARKING IT ODD TO BE BEING VECTORED SO LOW WITH TALL ANTENNA TWRS IN THE AREA. WE HAD NOT HAD ANY PROBS PREVIOUSLY WITH THE MODE C; SO THE DISCREPANCY THAT TAMPA DEP SAW WAS THE FACT THAT WE REALLY WERE TOO LOW. I WILL BE IMMEDIATELY PULLING OUT MY PVT AND INST EXAM STUDY MATERIAL TO RE-STUDY ALTIMETER INTERPRETATION TO BE SURE THIS NEVER HAPPENS AGAIN! THIS BASIC INST CANNOT BE TAKEN FOR GRANTED.