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Accuracy in 18th Century cartography

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#1
c.eugene

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I have a question regarding the accuracy of maps created by the 18th century British Navy.

I'm earning a Masters Degree in History and am researching a Revolutionary War fort that was burned by the British shortly after the Battle of Long Island. The fort would have sat in present-day Brooklyn in the neighborhood of Red Hook. I've taken coordinates from a map that was surveyed and created by British Naval Engineer Bernard Ratzer in the late 1760s and, after adjusting for scale, plotted them onto a modern day map of the city, using the same landmark location on both maps as a common anchor point from which to measure. The anchor point is about two miles away from the location in question. My resulting data put the fort approximately 550 feet (about 167 meters) away from the currently-believed location (and some distance into Buttermilk Channel. I repeated the procedure, using the 1776 Henry P. Johnston map and a few others, with similar results each time.

My question is, would this 550-foot discrepancy fall within the expected margin of error when using a point 2 miles distant for reference, considering the surveying methodologies of the time? Or would these results give reason to doubt the currently-believed location?

Attached is an early illustration. Thanks for any information you may have to offer.

C. Eugene
Brooklyn, NY

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#2
woneil

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No, that's a five percent error, which is an order of magnitude or more greater than could be expected with mid-C18 surveying technology. Although instrument scales still had to be hand divided, instruments good for five minutes of arc were commonly available and not unduly costly, making survey accuracies of better than 0.5 percent (<60 ft in 2 miles) very reasonable.

Of course it is possible that Ratzer simply was sloppy in his work, or had no reason to take care. But it seems improbable to me.

You might want to join the Sextants Group at http://groups.yahoo....group/Sextants/ (which I moderate). We have several members who take a deep interest in surveying history and know more of it than I do.

You might also think about joining the scientific instrument mailing list, Rete. See http://www.mhs.ox.ac.uk/join-us/rete/.

Will O'Neil

I have a question regarding the accuracy of maps created by the 18th century British Navy.

I'm earning a Masters Degree in History and am researching a Revolutionary War fort that was burned by the British shortly after the Battle of Long Island. The fort would have sat in present-day Brooklyn in the neighborhood of Red Hook. I've taken coordinates from a map that was surveyed and created by British Naval Engineer Bernard Ratzer in the late 1760s and, after adjusting for scale, plotted them onto a modern day map of the city, using the same landmark location on both maps as a common anchor point from which to measure. The anchor point is about two miles away from the location in question. My resulting data put the fort approximately 550 feet (about 167 meters) away from the currently-believed location (and some distance into Buttermilk Channel. I repeated the procedure, using the 1776 Henry P. Johnston map and a few others, with similar results each time.

My question is, would this 550-foot discrepancy fall within the expected margin of error when using a point 2 miles distant for reference, considering the surveying methodologies of the time? Or would these results give reason to doubt the currently-believed location?

Attached is an early illustration. Thanks for any information you may have to offer.

C. Eugene
Brooklyn, NY


Will O'Neil
Author and amateur cartographer

http://analysis.williamdoneil.com/w.d.oneil@pobox.com

#3
c.eugene

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Thanks very much Will. I'll definitely check out those groups. I might re-do some of the measurements again before posting. I'd hate for it to be a case of my own inexperience or a lack of precision to be at the root of it all.

Sincere thanks for your insight!

No, that's a five percent error, which is an order of magnitude or more greater than could be expected with mid-C18 surveying technology. Although instrument scales still had to be hand divided, instruments good for five minutes of arc were commonly available and not unduly costly, making survey accuracies of better than 0.5 percent (<60 ft in 2 miles) very reasonable.

Of course it is possible that Ratzer simply was sloppy in his work, or had no reason to take care. But it seems improbable to me.

You might want to join the Sextants Group at http://groups.yahoo....group/Sextants/ (which I moderate). We have several members who take a deep interest in surveying history and know more of it than I do.

You might also think about joining the scientific instrument mailing list, Rete. See http://www.mhs.ox.ac.uk/join-us/rete/.

Will O'Neil

I have a question regarding the accuracy of maps created by the 18th century British Navy.

I'm earning a Masters Degree in History and am researching a Revolutionary War fort that was burned by the British shortly after the Battle of Long Island. The fort would have sat in present-day Brooklyn in the neighborhood of Red Hook. I've taken coordinates from a map that was surveyed and created by British Naval Engineer Bernard Ratzer in the late 1760s and, after adjusting for scale, plotted them onto a modern day map of the city, using the same landmark location on both maps as a common anchor point from which to measure. The anchor point is about two miles away from the location in question. My resulting data put the fort approximately 550 feet (about 167 meters) away from the currently-believed location (and some distance into Buttermilk Channel. I repeated the procedure, using the 1776 Henry P. Johnston map and a few others, with similar results each time.

My question is, would this 550-foot discrepancy fall within the expected margin of error when using a point 2 miles distant for reference, considering the surveying methodologies of the time? Or would these results give reason to doubt the currently-believed location?

Attached is an early illustration. Thanks for any information you may have to offer.

C. Eugene
Brooklyn, NY






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