Finding Sites Using Old Charts: UPDATE 6/12/08

[pcolaboy]

There are a few sites of interest that I'm looking for using mid-19th century charts. These charts were developed by the old US Coast Survey Office using the "Trigonometrical Survey" method. The charts do specify the major lat/lon lines down to minutes.

For a goof, I set about to try and calculate specific coordinates of these sites using a ruler and a calculator. As an example, lets say that a wreck is marked on this old chart that lies a certain distance north of the marked Latitude line of 15' and a certain distance west of the marked longitude line of 5'.

Here are some steps I took to try and get a fairly accurate set of coordinates - this is going to get confusing so maybe sketch out on paper what I'm talking about.

1. Printed the chart on a large plotter to preserve the original aspect ratio of height and width.
2. Got a large transparent ruler with millimeter markings

ASSUMPTIONS
1. The marked lat/lon lines on the old chart are reasonably accurate
2. The location of the wreck was marked with reasonable accuracy in relation to the lat/lon lines.

CALCULATIONS
1. Measured the distance in mm between two adjacent lattitude marks (let's call this Y) Y=103mm
2. Measured the distance in mm between two adjacent longitude marks (let's calll this X) X=95mm
3. Measured the distance in mm from the wreck mark to the nearest lattitude mark to the south (lets call this A) A=13mm
4. Measured the distance in mm from the wreck mark to the nearest longitude mark to the east (lets call this B) B=73mm
5. Calculated the proportion of A and Y (13/103) to arrive at .126 or 12.6% of 1 minute lattitude [7.57" (seconds)]
6. Calculated the proportion of B and X (73/95) to arrive at .768 or 76.8% of 1 minute longitude [46.08" (seconds)]
7. Took the nearest lattitude line (15') and added the calculated seconds to arrive at 15' 7.57"
8. Took the nearest longitude line (5') and added the calculated seconds to arrive at 5' 46.08"

Plugged the numbers into Google Earth and was pretty happy to see that the site was in the area I thought it would be. Then I opened up my collection of interesting sites I found on VirtualEarth's hi-res photos and was thrilled to see the object below in the EXACT location as my calculated coordinates.



I should point out that this is the only bottom feature visible within a mile or so - the surrounding area is sand only. I will try to get out later in the week to ground-truth the site. According to the scale on VirtualEarth, the dark round area is about 6 meters in width. Ballast pile maybe?

Has anyone used similar techniques on old charts to locate wrecks with any luck?

Pcola

 

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[Darren in NC]

Re: Finding Sites Using Old Charts

Pcola, normally I'd tell researchers to stop analyzing the charts to death and just get out there! But you did a great job of taking the tools and opening up new possibilities. Way to go, bro...sigh...now I'll be up all night checking this out

 

[ivan salis]

Re: Finding Sites Using Old Charts

yes -- converting old sea charts and the information in them to new ones is very productive -- even much older 1700 era maps can be made to pump out locations of their wrecks site onto modern maps -- via a method I call map matching or over laying-- just make a online copy of the old map on clear tranparency paper (like they use on a overhead projector) via your printer --just as you would do using paper ---make sure to use proper size of scale as the modern map you going to be using -- then over lay the old map on the new one lining up known areas --- degree marks ( or look for distinct items land "mark" items that stand out ) line them up and tah da -- look where the wreck site is "now" at on the modern map -- now go "ground truth" it

P. S.--- many of the areas on the old late 1800 era to early 1900 era maps marked "foul" or obstruction --are often unnamed / unknown past prior wrecks and are often worth looking at if they are in a good possible location for a shipwreck to be at

Ivan

 

[pcolaboy]

Re: Finding Sites Using Old Charts

yes -- converting old sea charts and the information in them to new ones is very productive -- even much older 1700 era maps can be made to pump out locations of their wrecks site onto modern maps -- via a method I call map matching or over laying-- just make a online copy of the old map on clear tranparency paper (like they use on a overhead projector) via your printer --just as you would do using paper ---make sure to use proper size of scale as the modern map you going to be using -- then over lay the old map on the new one lining up known areas --- degree marks ( or look for distinct items land "mark" items that stand out ) line them up and tah da -- look where the wreck site is "now" at on the modern map -- now go "ground truth" it

P. S.--- many of the areas on the old late 1800 era to early 1900 era maps marked "foul" or obstruction --are often unnamed / unknown past prior wrecks and are often worth looking at if they are in a good possible location for a shipwreck to be at

Ivan

Regarding your P.S. section - I have definitely found this to be especially true in my area. The surveys following the Civil War (over 10 years after this one) only designate this general area as an Oyster Bed or submerged piles. The very latest survey only shows a heightened relief off the bottom in this area. I guess I'll find out for sure this week some time - in fact I think I'll be taking tomorrow off if the weather holds.

Pcola

 

[pcolaboy]

Re: Finding Sites Using Old Charts

Pcola, normally I'd tell researchers to ......

Them's fightin' words varmint I swear between my boat breaking down and my wife wanting a break from our two little kids, I mind as well rent some space down at the library Not that I have anything against researchers...but I'm more of an active participant whenever the three W's cooperate (Wife, Weather, Wallet).

Pcola

 

[RELICDUDE07]

Re: Finding Sites Using Old Charts

Great work pcola,now get your butt out there..

 

[pcolaboy]

Re: Finding Sites Using Old Charts

Great work pcola,now get your butt out there..

You mean OUR butts? I'm gonna drag you out there in case I get hung up in a shrimp net or something while diving it :-) Actually it's 12 feet or maybe a little less in that spot. What days can you get out during the week this week? I've got some vacation time to burn.

Pcola

 

[rgecy]

Re: Finding Sites Using Old Charts

OziExplorer has a neat feature that will allow you to scan the chart, set the coordinates of know locations, and use the old chart for gps. I did this with a fairly accurate chart from the 1700's. I picked a point of reference and went to my noaa chart (or google earth) and got the GPS coords, then went back and set the coord's on the old chart. You have to give at least 3 points of reference.

Works nice though! Here's a good example!

Robert

 

[pcolaboy]

Re: Finding Sites Using Old Charts

OziExplorer has a neat feature that will allow you to scan the chart, set the coordinates of know locations, and use the old chart for gps. I did this with a fairly accurate chart from the 1700's. I picked a point of reference and went to my noaa chart (or google earth) and got the GPS coords, then went back and set the coord's on the old chart. You have to give at least 3 points of reference.

Works nice though! Here's a good example!

Robert

Hey Robert,

I remember you recommending OziExplorer to me quite a while back but I never acted on it. I'll certainly give it a try as well.

Pcola

 

[pcolaboy]

I made a pretty snap decision to drive out to the site yesterday with my son. Of course about the time I launched the boat the wind changed to the east giving me a bumpy 7 miles of chop to the cooridinates. The bay was, and still is, filled with jellyfish and a few portuguese man-o-war so I knew I would be in electronic search mode instead of mask, fins, and snorkel. Once on site, I did pick up some bottom relief on the standard sonar but the chop in the bay made it very difficult to get a good sidescan. I will be heading out again over the weekend with the old fashioned panty hose anti-jellyfish suit. Anyone else out there ever go to this extreme?

Pcola

 

[RELICDUDE07]

Good chart work pcola,looks like the right spot

 

[stevemc]

Pcola, remember when plotting using charts that have Lat/Long, only Long is accurate to measure from, latitude varies, depending on where you are. So take your measurements off sides for miles etc, and you can use it up, down, sideways or diagonal now. If you take it off top or bottom, it will be inaccurate.

 

[pcolaboy]

Thanks Steve. I'll give that a shot and see what I come up with.

Pcola

 

[ScubaDude]

Actually it's the other way around, Latitude (or the N/S lines) is constant at 1 degree or 60 Min's = 60 nautical miles.

There are still some sizable errors in some of the old charts, and even larger ones involving coordinates and wrecks.

I've got some Bahamas charts based on the late 1800's British surveys, while most things are there, the coordinates are as much as 1/4 mile off. Interestingly everything on the chart was about the same distance and direction off so you could compare known non movable features and interpolate the numbers and find things that way.

 

[stevemc]

Yes I meant longitude the up and down lines,I think I said the sides or N/S. If you measure the latitude lines -W/E you will be off. Yes Brad, I have some present Bahamas charts and they say all data from 1880 datum, or something like that. Since there isnt 30 real channel markers in the whole Bahamas, besides a bare pole or iron stake here and there, and a few big bell bouys and nun bouys for big shipping and cruise ship channels, and maybe a still working light house, they dont need to change too much. It makes you wonder how much they are off. I use the new updated ones like the annual Bahamas Cruising Guide, or Steven Dodge's Guide to the Abacos. Off track there, but anyway....

 

[stevemc]

Brad, I see what you mean, use the lines of latitude -between each as that is even. I just go to the side of the chart, so I call it longitude. But you really are measureing between each latitude line.

 

[ScubaDude]

My bad, thats what I use to measure it so that's actually what I called it for 20+ years of boats and jets. Thanks for taking the time to expound. You are correct.

 

[stevemc]

Pcola, I use a dive skin, which is about the same as thick panty hose, but its a full body suit, like a wetsuit, but not hot. Protects against jellyfish and corals. Brad, sorry I was not too clear there. I know you know what you are doing. Kind of hard to explain with words, but yes you use the lines of latitude -between the lines of latitude for accurate measure, and these are marked on the longitude lines. confusing unless you know how to work it.

 

[FISHEYE]

Pcola,

jellyfish and man O war dont bother me.After living in hawaii for 12 years and diving just about every day and getting stung too many times to count them.Im immune to the stings.

 

[stevemc]

The man o War with the ballon, not the long tentacle jellyfish. If you can get one of those across the back and not yell underwater, I would like to see that! Even the big pink long tentacle ones are a bad sting. I dont get anything from the little ones and the big moon jellies, but I'll pass on the big pinks, the long browns, and a PMOW any day. I really dont fear anything underwater, but I give a wide berth to big long tentacled jelly.