A C425 PLT; ON APCH TO OMK; WA; EXECUTED A MISSED APCH; CITING HE WAS UNSURE OF HIS POS WITH REGARD TO THE FINAL APCH COURSE.

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I WAS SHOOTING THE GPS APCH INTO OMAK; WA; WHEN I SAW A RIDGE THROUGH A HOLE IN THE CLOUDS THAT LOOKED TOO CLOSE AND MY RAD ALT WAS COUNTING DOWN THROUGH 600 FT. THIS CAUSED ME TO BELIEVE I COULD BE W OF WHERE I SHOULD BE AND I EXECUTED AN IMMEDIATE MISSED APCH TO ON TOP (7000 FT) INSTEAD OF THE PUBLISHED 5000 FT. I DID THAT BECAUSE I WAS UNSURE OF MY POS AND DECIDED THAT IF I WAS IN THE WRONG PLACE; THE PUBLISHED MISSED APCH PROC WOULDN'T GUARANTEE MY SAFETY. MY ALTIMETER WAS PROPERLY SET; MOVING MAP ON KLN 90B OPS AGREED WITH HSI THAT I WAS ON COURSE. I CLBED BECAUSE I THOUGHT TERPS WOULD GUARANTEE 1000 FT TERRAIN SEPARATION. DOES IT? SINCE THEN; I HAVE SUPERIMPOSED THIS APCH ON A SEATTLE SECTIONAL CHART AND FOUND TERRAIN IN EXCESS OF 3000 FT; BUT LESS THAN 3800 FT BELOW A SEGMENT OF THE APCH THAT ALLOWS DSCNT TO 3900 FT. THAT GIVES ME 900 FT TO 400 FT CLRNC BTWN ETORY AND CF35. IS THAT ENOUGH? CALLBACK CONVERSATION WITH A SAFETY INSPECTOR REVEALED THE FOLLOWING INFO: A SAFETY INSPECTOR WHO HAS FLOWN THE GPS APCH IN QUESTION MANY TIMES AND IS FAMILIAR WITH THE AREA REVEALED THAT THE RIDGE REFERRED TO BY THE RPTR DOES EXIST. WHEN THE APCH IS FLOWN AS DEPICTED ON THE GPS RWY 35 APCH CHART; A TERRAIN CLRNC OF APPROX 600 FT IS MAINTAINED WHILE PASSING OVER THE RIDGE. THE TERPS MANUAL WAS QUOTED AS STATING: ON NON PRECISION APCHS TERRAIN CLRNC IS PROVIDED TO 1000 FT DURING THE INITIAL SEGMENT AND TO 500 FT DURING THE FINAL SEGMENT OF THE APCH.