PLT OF A C206 ON A PHOTO FLT MISSION TURNED OFF VECTOR HDG DUE TO MISUNDERSTANDING OF HIS VECTOR CLB CLRNC CAUSING THE ARTCC CTLR COORD DIFFICULTY WITH THE TFC ON SID AND STAR TFC.

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FILED IFR FOR AERIAL PHOTOGRAPHY GRID PATTERN OF DALLAS. CTR CTLR AT THE TIME STATED THAT I SHOULD HAVE PRECOORDINATED THIS MISSION AT LEAST A DAY BEFORE. THE PAST 20 YRS OF THE COMPANY'S EXISTENCE; FILING IFR HAS SERVED AS A METHOD OF COMMUNICATING MISSION INTENTIONS. THE FILING OF IFR IS NECESSARY SINCE THE ENTIRE PHOTO MISSION IS CONDUCTED AT FL200. I RECEIVED IFR CLRNC; WHICH WAS COMMON; AND FOLLOWED VECTORS FOR SID AND THEN TRANSITION. UPON REACHING CLRNC LIMIT; I INQUIRED ABOUT ALT AND REQUEST FOR HIGHER. I WAS TOLD TO EXPECT HIGHER IN 8 MI AND CONTINUE PRESENT HDG. I WAS VECTORED FOR 10-15 MI ON A HDG OF 030 DEGS. IN THE CLRNC I WAS INSTRUCTED TO EXPECT FL200 10 MINS AFTER DEP. THE REST OF THE CLRNC WAS THEN AS FILED. IN THE GRID PATTERN; WE FILE FOR SPECIFIC POINTS OFF A PRIMARY NAVAID. THIS IS DONE TO ESTABLISH THE AREA WE WILL BE WORKING IN. AFTER THE VECTOR TO 030 DEGS AND ON ALT ASSIGNMENT OF 13000 FT; THERE WAS A CTLR CHANGE IN PROGRESS. THE NEW CTLR ASSIGNED HDG OF 300 DEGS; CLB AND MAINTAIN FL180. A FEW MINS PASSED AND I WAS GIVEN A CLRNC OF WHAT I THOUGHT WAS LEFT TURN TO 180 DEGS; CLB AND MAINTAIN FL200; UPON REACHING FL180 I WAS TO RESUME MY OWN NAV TO THE FIRST POINT OF THE GRID IN MY FLT PLAN. I WAS STILL IN THE PROCESS OF CLBING; UPON TURNING TO 180 DEGS; THE ALT READ 16400 FT MSL. AT THIS TIME I CALLED ATC TO VERIFY HDG ASSIGNMENT. I WAS THEN INSTRUCTED TO LEVEL OFF AND MAINTAIN PRESENT HDG. I WAS THEN INSTRUCTED TO CALL CTR UPON REACHING GND. AFTER CLRNC WAS RECEIVED IT TOOK APPROX 40 SECONDS TO MAKE THE TURN FROM 300 DEGS TO 180 DEGS. NOT MORE THAN 1-2 MINS LATER; I ASKED FOR CLARIFICATION OF CLRNC. THAT'S WHEN I WAS INFORMED OF AN ATC DEV. ANOTHER FACTOR WAS MY PAX (THE PHOTOGRAPHER) AND I HAD BEEN ON OXYGEN SINCE 8700 FT MSL. HE WAS ON HIS FIRST PHOTO FLT AND SUFFERING FROM SOME SYMPTOMS OF HYPOXIA AND COULD NOT CONTINUE. I ASKED FOR IMMEDIATE DSCNT AND CANCELED IFR AT 17400 FT MSL. WE PROCEEDED TO DALLAS LOVE FOR LNDG. 2 THOUGHTS: 1) I FEEL THAT THE CLRNC WAS CONFUSING AND THAT'S WHY I TRIED TO CLARIFY IT SHORTLY AFTER INITIAL TURN; AND 2) I MIGHT HAVE HAD HYPOXIA BUT HAD NO SYMPTOMS. I HAVE CONDUCTED THESE OPS FREQUENTLY IN THE PAST.