Dease and Simpson, who had mapped the southern coast of what was then still called the "Polar Ocean," added their own errors to the charts; their latitudes were accurate enough, but their longitudes seemed to drift with the tides; they misaligned the coast of King William Land with that of the mainland, and misstated the longitude of their farthest point, just beyond the "Castor and Pollux River" (Dease and Simpson gave the river's longitude as 68°28′ N., 94°14′ W. whereas in fact the longitude is 93° 53′, an error of roughly eight miles too far west (Dr. Rae, though he confirmed Dease & Simpson's latitude, got a longitudinal reading of 93° 15′, which is an additional 16 miles off!). These errors could be due either to extreme refraction of the sun at the horizon, or to inaccurate chronometers -- but what was most misleading was Dease & Simpson's claim that the coast tended to the east as far as they could see (shown on the map as a dotted line), whereas in reality it soon turns almost straight north. There was no wide passage there, although had Franklin's men made it so far, they might have stumbled upon the Bellot Strait.
With these charts as their guide, it's little wonder that Franklin's men, on their death march along the southern coast of King William Island, failed to cross Simpson's Strait at its narrowest point; they may well have planned to continue by land as far as the coast of the Gulf of Boothia. Perhaps they were making for Ross's old Victory harbor; perhaps they hoped to launch their boats into the Gulf of Boothia and make their way to northern sea-lanes as had Ross. In any case, they never made it.