(green= island in 1807, brown = 1865, heavy black line = 2010)
This morning, for another project, I was browsing in the marvelous maps available at nyc OASIS and found an overlay of the 1609 Manhattan shoreline from the fabulous Manahatta Project which differs substantially from the shoreline drawn by the Commissioners on their 1807 map.
On the Commissioners' map, Tenth Avenue ends at 23rd Street, below which the 1807 shoreline is shown to be east of what a southward continuation of the avenue would be (see above map).
On the Manahatta Project map, the projected 1609 shoreline is west of Tenth Avenue all the way down to 13th Street:
Is one right and the other wrong? A simple bias towards current technology inclines me to believe that the Manahatta Project shoreline is correct, and that the Commissioners map shoreline is in error.
But the Manahatta Project shoreline is based on the 1782 British Headquarters map, adjusted to identifiable contemporary features with as estimated error of 40 meters or about half a block, which is about the size of the discrepancy between the two shorelines. (This isn't to fault the Manahatta Project in any way: for Manahatta Project's incredible purposes an error factor of only 40 meters is as good as perfect — the error, if indeed there is one, is only relevant to the very modest aims my Tenth Avenue "then & now" project on this blog and even so, it's only the very close proximity of this stretch of Tenth Avenue to the original shoreline — wherever, exactly, it may have been — that makes this of any interest).
And did the Commissioners undertake a whole new survey of the island, or did they rely, or how much did they rely, on previous cartographers' efforts, e.g., the 1872 map or derivatives of it?
And similarly with Vielé's map — although it represents a new survey of the island, its showing of the original shoreline, pre-landfills, must be derived from earlier, namely pre-landfill sources.
One might say, in favor the Commissioners, that it is unlikely that they would have stopped Tenth Avenue at 23rd Street if the actual shoreline would have permitted its extension down to 13th Street or even lower.
It's possible of course that the Commissioners' shoreline was wrong and that, therefore, their plan for Tenth Avenue was wrong also, insofar as in that case it could have, even then, been extended below 23rd Street. But how likely is this? Even without sophisticated high tech measuring devices, the distance from a known point to the shore could be established to within an inch or so even in 1807.
This argument seems compelling to me, though I'm no expert. So I'll give the nod to the Commissioners 1807 shoreline (as shown on my first map mash-up), while keeping in mind the possibility of error that arises from the British Headquarters / Manhatta shoreline (second map).