A pilot on the SMX LOC DME BC A descended below the approach segment minimum altitudes and was advised as such by ATC but he disregarded the alerts because he had the runway environment in sight.

Narrative

I was flying a night IFR flight to Santa Maria; CA (SMX). SMX weather reported as clear and visibility 10 miles. After crossing RZS VOR; Santa Barbara TRACON asked me whether I wanted a visual approach or a LOC DME BC A approach to SMX. I requested the latter and was told to expect that. After crossing MADOO Intersection; TRACON issued me the following clearance: 'Cleared for the LOC DME BC A approach; cross KOAKS at or above 4;000.' I read back the clearance. I made note that the minimum altitude on the approach plate for the MADOO-KOAKS segment is 4;600 FT; so in an abundance of caution I descended to 4;600 FT and flew that altitude to KOAKS. Upon passing KOAKS; I began a descent to 3;000 FT which is the minimum altitude for the KOAKS-CAMCO segment of the approach. During this descent; TRACON instructed me 'Radar service terminated; contact SMX Tower on 118.3.' I read back the clearance; then contacted SMX Tower and reported '4 miles outside of CAMCO.' The Tower instructed me to 'Report CAMCO.' Upon crossing CAMCO (identified simultaneously by three different GPS navigation devices in the cockpit); I commenced a descent. At this point; the runway lights; VASI; and airport beacon were all clearly in sight; so I transitioned from instrument to visual references and descended on the VASI glidepath. As I was about to make my CAMCO report to the Tower; the Tower called me to say that I was not permitted to descend below 3;000 FT until crossing CAMCO. I replied that I was inside of CAMCO with the runway in sight. The Tower said they showed me outside of CAMCO. My three GPS navigators showed me clearly inside of CAMCO. Not wanting to get into an on-air debate with the Tower Controller over whose CAMCO was more accurate; or whose CAMCO was the one that mattered (given that CAMCO is a DME or GPS fix; not a radar fix); I simply reiterated to the Tower that I had the runway in sight. I continued my descent on the VASI glideslope. At approximately 1;500 FT; the Tower called me to say that I was below the 1;720 FT minimum altitude for the CAMCO-PATER segment of the approach. That was certainly true; but at this point I had ceased flying the published approach profile and had taken over visually and was flying the VASI. I reiterated to the Tower once again that I had the runway in sight. The Tower insisted that I was required to fly the published altitude whether or not I had the runway in sight. At this point; I canceled IFR and continued my descent on the VASI to a landing. I have flown this LOC DME BC A approach many times. In all those times I have NEVER crossed PATER at or above 1;720 FT except in those rare instances where I did not have the runway in sight upon reaching 1;720 FT. If one crosses PATER at 1;720 FT; one is too high to make a straight-in landing on Runway 30; which is why the LOC DME BC A approach has only circling minimums. I telephoned the Tower after landing to discuss this. I indicated my belief that once I had the runway in sight; there was no further requirement to observe the published IAP altitudes. He disagreed with me; and continued to say that 'ATC expects' that those altitudes will be observed; and that 'you're not supposed to' descend below those altitudes without canceling IFR. We agreed to disagree. I am still not certain which one of us is correct. I polled a group of very experienced instrument pilots and instrument flight instructors; and they were divided on the subject. I find it remarkable that I've been flying this approach the same way for years and this issue has never been questioned before. FAR 1.1 defines 'instrument approach' as follows: Instrument approach procedure (IAP) is a series of predetermined maneuvers by reference to flight instruments with specified protection from obstacles and assurance of navigation signal reception capability. It begins from the initial approach fix; or where applicable; from the beginning of a defined arrival route to a point: (1) From which a landing can becompleted; or (2) If a landing is not completed; to a position at which holding or enroute obstacle clearance criteria apply. My interpretation of this definition is that the IAP is executed 'by reference to flight instruments' and ends at 'a point from which a landing can be completed.' It was my understanding that the pilot could transition from instrument reference to visual reference at any time during the IAP at which the runway was in sight; and that it was not possible to transition from instrument to visual references 'too early.' Clearly; the Tower does not agree with me about this.