A PLT AND CTLR RPT ON THE PLT'S CONFUSION REGARDING AN AGL VERSUS MSL XING ALT RESTR.

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ACFT X CHKED IN WITH MDW TWR 25 MI S; VFR; TO LAND. PLT WAS GIVEN A XPONDER CODE. 10 MI S OF MDW PLT WAS RADAR IDENTED AND TOLD TO APCH THE ARPT FROM THE S; ASSIGNED RWY 22R. PLT WAS INSTRUCTED TO FLY OVER MIDFIELD AT 800 FT AGL AND ENTER R TFC FOR RWY 22R. PLT RPTED 2 NM S OF MDW AT 800 FT AND WAS TOLD AGAIN TO FLY OVER MIDFIELD AT OR ABOVE 800 FT AGL. CTLR ISSUED TKOF CLRNC TO AN ACFT DEPARTING RWY 22R. JUST AS THIS ACFT WAS PAST MIDFIELD ON DEP; ACFT X PASSED BELOW THE CTL TWR APPROX 100 FT AGL AT A HIGH RATE OF SPD XING AT MIDFIELD AND BEHIND THE DEP. CTLR ADVISED THE PLT THAT HE HAD VIOLATED ATC INSTRUCTIONS AND COULD HAVE CAUSED PROBS WITH DEPARTING ACFT OFF OF RWY 22R/L. PLT SAID HE WAS AT 800 FT BUT HIS ALT WAS MSL; NOT AGL; AS INSTRUCTED ON AT LEAST 2 XMISSIONS THAT HE HAD ACKNOWLEDGED. INFO WAS PASSED ON TO FSDO. PLT READ BACK INSTRUCTIONS BUT DID NOT COMPLY. SUPPLEMENTAL INFO FROM ACN 653261: I CONTACTED APCH 35 MI SE OF THE FIELD AND INFORMED THEM OF MY TAIL NUMBER; INTENTIONS AND THAT I HAD CURRENT WX INFO. I WAS ASSIGNED A SQUAWK CODE; ASKED TO DSND TO 2000 FT 12 MI FROM THE FIELD AND PREPARE TO ENTER THE FIELD FROM THE S FOR A MIDFIELD XING AT 800 FT AGL. READBACK WAS GIVEN AND I EXCLUDED 800 FT AGL. 5 MI FROM THE FIELD; I BEGAN TO DSND FROM 2000 FT TO 800 FT; AND 3 MI FROM THE FIELD I BECAME UNCOMFORTABLE WITH THE ASSIGNED ALT. I RADIOED THE TWR INFORMING THEM THAT I WAS 2.5 MI TO THE S AT 800 FT. THEY REAFFIRMED MY POS; ASKED THAT I CROSS THE FIELD AT 800 FT AGL AND INSTRUCTED ME TO BE ADVISED OF THE CRANES LOCATED ON THE FAR SIDE OF THE FIELD. AT THAT MOMENT; I CONCLUDED THAT MY POS AND ALT WERE ACCEPTED BY THE TWR; AND FURTHER ASSURED MYSELF THAT I WAS AT THE CORRECT ALT WHEN THE TWR ADVISED OF THE CRANES. AS I CROSSED MIDFIELD; THE TWR NOTIFIED ME THAT I WAS IN VIOLATION; AND THAT THEY HAD INSTRUCTED ME TO CROSS MIDFIELD AT 800 FT AGL. I NOTED THAT I INDICATED 800 FT AND CLRNC WAS GIVEN TO LAND. I WAS GIVEN INSTRUCTIONS TO CONTACT THE TWR BY PHONE; WHICH I DID. I WAS INSTRUCTED TO FAX MY DRIVER'S LICENSE AND PLT'S LICENSE; WHICH I PROVIDED. I CROSSED MIDFIELD AT 800 FT MSL. IN HINDSIGHT; I SHOULD HAVE REQUESTED THE TWR TO CONFIRM MY ALT AT THE TIME I FELT UNCOMFORTABLE WITH THAT ASSIGNED. FURTHERMORE; I SHOULD HAVE INFORMED THE TWR THAT I WAS NOT COMFORTABLE WITH THE ASSIGNED ALT AND REQUESTED ANOTHER OPTION. THIS IS THE FIRST TIME THAT I HAVE BEEN ASSIGNED AN AGL ALT. THIS IS NOT AN EXCUSE; HOWEVER; ALL OTHER ASSIGNED ALTS TO DATE; INCLUDING THOSE GIVEN BY THE TWR ON THE DATE OF THE INCIDENT; WERE MSL. I BELIEVE THAT THE SHIFT IN TERMINOLOGY FROM MSL TO AGL CAUSED THIS ERROR.