Latitude shift

[Darren in NC]

Years ago, I read about a group that realized they were looking for a wreck in the wrong spot due to a latitude (or longitude) shift from the colonial period to modern day charts. I cannot find this info anywhere in my files. Does this ring a bell with anyone?

 

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[Cablava]

Darren

In the 1400's they knew where a lot of things were and drew on elipticle paper, in the 1500's they had laitude so they had the position more accurately, so its possible that the wreck you read about was pre latitude but in the era of when they had positions of things.

 

[Narked1]

Darren, All,
To keep it simple here, latitude is constantly and slowly changing as a result a combination of what is called "Polar Shift" and changes in the orbits of celestial bodies used to determine Lat. and Long. Both are predictable and as a result Navigators use a set of books known as the Nautical Almanac which was first published in 1767 to provide Navigators with an annual list of changes in latitude. The annual polar drift is 0.00000097222 degrees at the 65-degrees west meridian, or approximately 0.1' of degree arc at the poles. Over a period of a few hundred years, this can result in a large difference of recorded coordinates, and charts/GPS coordinates that we use today.

Another point that may cause a difference in recorded coordinates of many years ago and present-day coordinates is light refraction through the atmosphere. Before the advent of GPS technology, the sextant was the primary navigation positioning tool. Light refraction through the atmosphere has changed over the years due to global warming, pollution, the use of propellants in aerosols (darn hairspray manufacturers!) and may also have affected recording of Lat. and Long. during early surveys when celestial bodies were viewed through a sextant.

Mike

 

[chipveres]

Narked1 is absolutely right; but there may be a simpler explanation. In colonial days longitude was often reconed in degrees east or west of the capitol city of the country the ship came from. French ship = zero longitude at Paris. Spanish ship = zero longitude at Madrid. There is also the possibility of a ship using someone else's map, and using his longitude scale.

Chip V.

 

[Narked1]

Hi All,
Chip V. is also correct. Depending on the dates that the coordinates were taken, and by who, the wreck coordinates may appear different on different charts. I tend to get caught up in the science of Navigation since that is one of the things that I have been teaching the Egyptian Navy for the past 10 years. Depending on the date that the original coordinates of the wreck were taken, and again by who, the wreck could lie a hundred miles or more from where the wreck was recorded to be and where it actually is by using "modern" coordinates.

So, using Chip V.'s explanation of navigational plotting of wrecks, chart used, and depending upon the date of the wreck, one may or may not need to apply the polar shift and changes in celestial orbits. Kind of leaves one with the feeling of "well I know it's there.... somewhere" doesn't it? Remember, Navigation and plotting positions did not become an "exact" science of the day until the mid-to-late 1700's. Although there were great Navigators early on, they all maintained their own charts, relied on a bit of luck and guess-timization and held their charts very close for job security purposes. As a result, we have early charts of different scales with different interpretations of just how round the world really was at that time.

A sailing we will go......
Mike

 

[Mackaydon]

Another explanation is that the North Pole ('top of the world') is not the North Magnetic Pole (where you compass will always point to--absent other 'interference')--which wanders. For example, In 2001, the North Magnetic Pole was determined to lie near Ellesmere Island in northern Canada at 81.3° N 110.8° W. It was (later) estimated to be at 82.7° N 114.4° W in 2005. This deviation was not caused by an error in calculation but by the movement, itself, of the north magnetic pole.

The angular difference between magnetic north and true north (defined in reference to the Geographic North Pole), at any particular location on the Earth's surface, is called the magnetic declination. Most map coordinate systems are based on true north, and magnetic declination is often shown on map legends so that the direction of true north can be determined from north as indicated by a compass.

You can usually find this magnetic declination near or part of the Compass Rose on charts.
Don....

Source:Wikipedia

 

[mariner]

If Narked1's value of the polar shift is correct, then by my calculations that is a movement of only 135 feet in 400 years, which is much less than the inaccuracy you would expect in any latitude reading 400 years ago, or even two hundred years ago.

Mariner

 

[rgecy]

I would have to go with the magnetic deviation. (Uhm, Variation that is!)

Robert

 

[ScubaDude]

I think y'all are starting to confuse two separate things. Latitude and longitude don't change other than inaccuracies in old days navigation and chart making.

The other issue being discussed is Variation, deviation is the error within the compass or ship itself and has nothing to do with the magnetic poles moving. Variation is due to true north and magnetic north not being in the same place as was previously mentioned, and yes it changes over the years. Variation is also what is listed on the compass rose on navigational charts.

 

[rgecy]

Yes Brad. I mis-stated my part. The Variation is what I think is the culprit here.

 

[ScubaDude]

Darren part of the issue you might be alluding to would be due to magnetic bearings taken by the ship back in the day, either from sighting land based objects or signals from radio beacons. You would need to compensate for the change in variation from the time of the wreck to present and work it backwards to find your target, if that makes sense.

 

[jeff k]

Magnetic North Pole
Located more than 1000 miles (1600 kilometers) south of the geographic North Pole lies the magnetic North Pole at 78?18' North and 104? West, southwest of Nunavut, Canada's Ellef Ringnes Island.

The earth's magnetic pole is the focus of the planet's magnetic field and is the point that traditional magnetic compasses point toward. Compasses are also subject to magnetic declination which is a result of the earth's varied magnetic field. Each year, the magnetic North Pole and the magnetic field shift, requiring those using magnetic compasses for navigation to be keenly aware of the difference between magnetic north and true north. The magnetic pole was first determined in 1831, hundreds of miles from its present location.

The National Geomagnetic Program of Geological Survey of Canada monitors the movement of the north magnetic pole and most recently determined its precise location in 1994. They've determined that the pole is moving at approximately 9 miles (15 kilometers) each year.

The north magnetic pole moves on a daily basis, too. Every day, there's an elliptical movement of the magnetic pole about 50 miles (80 kilometers) from its average center point.

http://www.world-mysteries.com/sci_2.htm

 

[Darren in NC]

You would need to compensate for the change in variation from the time of the wreck to present and work it backwards to find your target, if that makes sense.

It makes perfect sense, Brad. Thanks to all for your insight. Even with today's technology, it's not easy to narrow a search area. Between sextant readings used until a short time ago, Loran A, conversion inaccuracies, etc. it's still an art and not a science. However, taking the shifts into account could save a lot of investor's money if a spiral grid is used for surveying. The more accurate you adjust for the shifts, the less likely you'll need to move further out for your target. Of course, this is providing you dive your hits as you go and not wait until a prescribed grid is complete.

 

[ScubaDude]

For the compass bearing stuff you've got to keep it in your mind that one degree of error over 60 miles equals one mile of course or target.

I'll usually finish a survey before we dive anything, especially if we're down to one boat. Part of the fun of surveying is wondering whats on the next lane. Last season was funny because it was always the last lane of the box that held the most promising targets.

 

[Mackaydon]

Suggestion:
If you have not yet read the book "Longitude" by Dava Sobel you are missing a great story about how the problem of accurately determining longitude was solved.

 

[ScubaDude]

That does sound like interesting reading. I've used a sextant some offshore and I am amazed how accurately people navigated hundreds of years ago.

 

[Darren in NC]

Thanks, Don. I've not heard of that book. I'll check it out.

 

[Real de Tayopa Tropical Tramp]

Gentlemen: no-one has mentioned the true secret of longitude, "TIME" Until accurate time pieces were developed, it was a matter of good educated guessing.

A $1:00 electromic wrist watch could out perform the best and would have been worth millions in those days..

Reatively exact time allowed an accurate noon day shot for Latitude.


Don Jose de La Mancha

 

[Mackaydon]

Real:
"Time" is, of course, the essence of the book 'Longitude', and Harrison's attempts-- and finally success-- in creating a clock that would withstand the rigors of the sea for months at a time--and still be accurate. Capt. Cook carried a Harrison-type clock on his voyage and his complements of that timepiece were exemplory. While Harrison, as a relatively young man, solved the problem of 'time' in calculating longitude, he was in his 70s before the king acknowledged his deed and awarded him a bountiful cash prize.
Don..
PS: Darren; Fair warning: You may be tempted to read 'Longitude' twice. The book was a bestseller about 12 years ago.

 

[mad4wrecks]

Longitude was a great read, it sure did teach me alot. Most large libraries have the book if you don't feel like buying it.