SMA MAKES INST APCH BELOW MINIMUMS RPTED.

Narrative

APCHING SE IL APPROX 60 MI FORM DEST I ASKED FOR WX AT DEST CPS. VISIBILITY WAS CALLED TO BE 3/4 MI IN FOG. I REDUCED MY SPD FOR REMAINDER OF FLT BECAUSE IT WAS GND FOG; LATE MORNING; AND I SUSPECTED THAT BECAUSE IT WAS DUE TO COOL AIR OVER WARM GND; THE UNDERCAST OVER WHICH I WAS FLYING HAD HOLES (IT WAS NOT A CONSISTENT FOG) AND THAT THE SUN WOULD BURN OFF THE FOG; I WOULD ARRIVE AFTER THE VISIBILITY HAD COME UP ABOVE THE 1 MI MINIMUM FOR THE APCH. AS I WAS BEING VECTORED FOR THE APCH; I CHKED IN AND FOUND THAT SPIRIT OF ST LOUIS ARPT WAS ABOVE MINIMUMS AND TOLD THE CTLR I WOULD GO THERE IF I MISSED THE APCH INTO CPS (IT WAS STILL RPTED AS 3/4 MI VISIBILITY). AT 450 FT AGL I BROKE OUT OF THE CLOUDS; SAW THE APCH LIGHTS AND RWY IN FRONT OF ME AND TO THE R (THE LOC IS OFFSET). I WAS SURPRISED -- I HAVE SHOT ILS APCHS DOWN TO 1/2 AND 3/4 MI VISIBILITY IN THE PAST -- THE VISIBILITY WAS BETTER THAN THAT. IN THE TIME I HAD TO LOOK AROUND; I ESTIMATED THE VISIBILITY TO BE A MI OR BETTER. I GAVE A CALL TO THE TWR THAT I HAD THE RWY AND VISIBILITY WAS A MILE TO A MILE AND A QUARTER; AND THEN LANDED. THE NEXT MORNING; ON DEP; I FELT THAT THE TWR UNDER-RPTED THE VISIBILITY ONCE AGAIN. I HAVE HAD SOME EXPERIENCE ESTIMATING VISIBILITIES AND ALSO KNOW THAT TWRS USE LANDMARKS AT KNOWN DISTANCES TO RPT VISIBILITY; WHICH IS A GOOD PROC. HOWEVER; FROM TIME TO TIME; I HAVE OBSERVED CTLRS WHO DO NOT UNDERSTAND VISIBILITY RPTED AND RPT ERRONEOUS DISTANCES.