IT'S MOST LIKELY THE PLT DEPARTED VGT ARPT OFF RWY 7 RATHER THAN RWY 25 AS STATED IN THE RPT. AFTER DEPARTING THE RPTR SAID HE MADE A R TURN TO A HDG OF 200 DEGS AND CLBED TO 3000 FT. USING HIS DME; HE BELIEVED HE WAS REMAINING CLR OF THE LAS CLASS B AIRSPACE.

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I TOOK OFF FROM VGT AFTER RECEIVING A LAS SQUAWK CODE AND FREQ FROM CLRNC DELIVERY. AFTER TAKING OFF FROM RWY 25; I TURNED R TO A 200 DEG HDG AND CLBED TO 3000 FT. VGT TWR APPROVED MY FREQ CHANGE REQUESTED AND I CALLED LAS APCH. LAS TOLD ME I WAS IN RADAR CONTACT BUT DID NOT CLR ME INTO CLASS B AS I WAS EXPECTING. WHEN MY DME SHOWED 6.2 NM FROM LAS; I STARTED A R TURN SINCE THE CLASS B FLOOR DROPS TO THE SURFACE AT 6 NM. AS I MOVED MY FINGER TO THE MIKE BUTTON TO REQUEST CLASS B CLRNC; APCH TOLD ME I WAS IN CLASS B. I ACKNOWLEDGED; CONTINUED MY TURN; CHKED MY DME WHICH SHOWED 6.1 AND I DOUBLECHKED MY CHART. I TOLD APCH THAT I WAS SHOWING THAT CLASS B FLOOR WAS 4500 FT WHERE I WAS. HE RESPONDED THAT I WAS IN THE AREA WHERE CLASS B FLOOR IS AT THE SURFACE. I APOLOGIZED AND FLEW OUT TO DME 7.5 AND FLEW A DME ARC UNTIL RECEIVING A CLASS B CLRNC. LATER I CHKED MY DME AGAINST KNOWN GND SOURCES AND IT APPEARED TO BE ACCURATE. I DON'T KNOW WHY THE POS DISCREPANCIES BTWN MY NAV EQUIP AND TRACON'S RADAR.