AN ACR FO PNF MISTOOK DEPARTING TFC AT LGA AS THE TFC HE WAS FOLLOWING IN ON THE VISUAL APCH. THINKING THE TFC HE WAS FOLLOWING WAS OVER THE THRESHOLD; HE ALLOWED THE PF TO TURN FINAL; THEREBY CUTTING OFF THE MD80 THEY WERE SUPPOSED TO MAINTAIN VISUAL WITH.

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WE WERE FLYING THE EXPRESSWAY VISUAL RWY 31 LGA AND WERE FOLLOWING TFC POINTED OUT TO US. PASSING OVER DIALS; THE CAPT CALLED FOR THE GEAR; FLAPS AND LNDG CHKLIST. ABOUT THE SAME TIME; TWR TOLD US WE WERE CLRED TO LAND AND THAT THERE WOULD BE A DEP OFF RWY 4 PRIOR TO OUR ARR. I FINISHED THE LNDG CHKLIST; EXTENDED THE RWY CTRLINE ON THE FMS; CHKED THE COMMERCIAL CHART PAGES TO SEE WHAT THE RECOMMENDED TURN TO FINAL ALT WAS. WHEN I LOOKED UP; I SAW WHAT I THOUGHT WAS OUR INTERVAL OVER THE THRESHOLD AND STATED; 'THIS WILL WORK OUT FINE' (FOR THE RWY 4 DEP). AS WE STARTED OUR TURN TO FINAL; THE TWR SAID WE WERE CUTTING OUT OUR INTERVAL AND FOR US TO CONTINUE TO LAND AND FOR THEM TO GO AROUND. IN FACT; WHEN I LOOKED WELL OFF TO OUR R; AN MD80 WAS TURNING A DEEP FINAL; NO EVASIVE ACTION WAS REQUIRED OTHER THAN THE GAR. THE ONLY LINK IN THE CHAIN THAT I CAN SEE THAT MIGHT HAVE PREVENTED THIS (AND IT'S A BIG ONE) WAS WHEN WE WERE GIVEN OUR TFC TO FOLLOW; I WAS SO BUSY I DIDN'T 'PROCESS' THE INFO. WHEN I LOOKED UP AND SAW OUR 'INTERVAL' OVER THE THRESHOLD; IT WAS A TURBOPROP. OUR REAL INTERVAL WAS AN MD80. A VALUABLE LESSON THAT SOME INFO; WHEN MISSED BECAUSE OF BEING LOADED UP; NEEDS TO BE ASCERTAINED RATHER THAN ASSUMING WHAT YOU 'SEE' MAKES SENSE.