E170 FLT CREW ACCEPTS ROUTING DIRECT TO CYHZ. THEY LATER DISCOVER THE ROUTE TAKES THEM BEYOND 50 MILES OFF SHORE.

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WHILE ENRTE FROM ZZZ TO CYHZ; WE'RE GIVEN OUR CLRNC AS FILED. WE WERE GIVEN SEVERAL SHORTCUTS TO VARIOUS FIXES AS WE NAVED OVER NEW YORK CITY. ABOUT 20-30 MI OR SO N OF NEW YORK CITY; WE WERE 'CLRED DIRECT DEST.' THIS 'CLRED DIRECT' PRETTY MUCH CLRED OUT A PAGE AND A HALF OF FIXES ON OUR RTE IN THE FMS. BOTH THE CAPT AND I WERE MONITORING ON THE MFD WITH THE TERRAIN FEATURE TURNED ON TO VIEW OUR DISTANCE FROM SHORE AS WE WERE NOW HEADING OVER THE WATER. HOWEVER; AT THE MOMENT WE WERE CLRED; WE HAD TO ZOOM WAY OUT THE VIEWING SCALE TO SEE OUR ENTIRE FLT PATH. AT THAT RESOLUTION; WE WERE CONFIDENT THAT WE WOULD BE WITHIN 50 MI OF SHORE. WE THEN ZOOMED OUR MFD'S BACK TO 50 NM ARC TO MONITOR. AS OUR FLT PROGRESSED; I TOOK MY MFD OFF OF THE MAP PAGE TO VIEW THE STATUS OF ALL THE SYS; SPENDING A COUPLE OF MINS ON EACH PAGE MONITORING THEM AND WATCHING FOR ABNORMALITIES. WHEN I CYCLED BACK TO THE MAP PAGE; BOTH THE CAPT AND I REALIZED THERE WAS ONLY A SMALL SPECK OF LAND IN THE 50 MI ARC AND IT WAS DISAPPEARING FAST. THE CAPT TOLD ME TO MAKE AN IMMEDIATE L TURN TOWARDS SHORE AT THE SAME TIME HE ADVISED ATC OF THE SIT. THEY CLRED US TO DEVIATE L AS NECESSARY AND THAT WE COULD RETURN DIRECT CYHZ WHEN ABLE. BECAUSE THE SHORELINE RECESSED WESTERLY; IT TOOK A FEW MINS TO GET WITHIN 50 MI OF SHORELINE. AND THIS; OF COURSE; IS ESTIMATED ON THE ACCURACY OF THE TERRAIN FEATURE ON THE MAP PAGE. IF I HAD TO GUESS; I WOULD SAY WE WERE NO MORE THAN 55 MI FROM SHORE WHILE DEVIATING W OF COURSE. AFTER ABOUT 5 MINS; WE HAD PLENTY OF SHORELINE TO OUR L AND THE R; AND WE PROCEEDED BACK TO DIRECT CYHZ AND LANDED UNEVENTFULLY.