1796 American Coast Pilot

[pcolaboy]

Some quite astounding deviations in accuracy of published Lat and Long from 1796 can be observed by checking out page 128 of the document at this link. (you have to have Adobe Acrobat Reader installed).

http://docs.lib.noaa.gov/rescue/csc/...f/CSC-0011.PDF

On the dozen or so that I've punched into my charting program, they're almost all off on the longitude by a dozen miles or more. What gives?

[GOHO]

They used a completely different datum than we use today. Bernard Romans map of 1775 is the same way... he has the latitude for Indian River inlet at 27' 20" but today it's around 27' 30". I am not for certain the datum they used in 1796 but i am sure with a little research it could be found. The coast pilot should tell you what datum they are using....

[GOHO]

I think in 1796 that the most commonly used map datum was the Greenwich prime Meridian. Today we use WGS-84 which is based on a different location.

"The coordinate origin of WGS 84 is meant to be located at the Earth's center of mass; the error is believed to be less than 2 cm.[1]

In WGS 84, the meridian of zero longitude is the IERS Reference Meridian.[2] It lies 5.31 arc seconds east of the Greenwich Prime Meridian, which corresponds to 102.5 metres (336.3 feet) at the latitude of the Royal Observatory.[3][4]"

[aquanut]

GOHO,
I had no clue! And I'm sad to say I still don't. You've just impressed the hell out of me.
Aquanut

[itmaiden]

I didn't download the large file, but could it be that they fail to account for the difference between magnetic north and "True North" ?

We also have to consider that maybe someone was using "distance measurement" to make their chart and failed to consider the differences between Spain, England, and French.

itmaiden

Some quite astounding deviations in accuracy of published Lat and Long from 1796 can be observed by checking out page 128 of the document at this link. (you have to have Adobe Acrobat Reader installed).

http://docs.lib.noaa.gov/rescue/csc/...f/CSC-0011.PDF

On the dozen or so that I've punched into my charting program, they're almost all off on the longitude by a dozen miles or more. What gives?

[GOHO]

Maybe its has a little to do with magnetic north but the mercator projection that was used to take the positions is based on a different point than what we use today to plot the positions. Try ploting 3 or 4 points and compare the modern charts LL with what you get from the book. You might find the offset error and be able to apply it to the rest of the positions in the book.

[billinstuart]

Romans also shows the Sebastian River near Melbourne.

[ivan salis]

ah someone is thinking wisely and it shows --- measuring points and distances are not always what folks think they are -- the english leauge --3 nautical miles --the old spanish leauge of 2.6 miles --then theres other factors that must be taken into account as well.

[pcolaboy]

Thanks for the great information GOHO and crew. I literally ran across the link to that document by accident and was originally just going to pass it along until I saw the ports listed for Spanish Florida. My curiousity got the best of me and I certainly learned a good bit on the use of chart datums.

Thanks,

Pcola