Question about the accuracy of longitutes and latitudes

[bigscoop]

I'm sure there are several factors involved but during the New World seafaring how accurate were the longitude and latitude readings? Plus or minus a hundred miles or was it more accurate then this, or worse?

 

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[Diver_Down]

With the recent anniversary of Juan Ponce de Leon landing in and declaring La Florida for the Queen of Spain, it was noted the last latitudinal reading was 30 degrees 8 minutes. This places his landing in the present day GTM NERR. It was interesting to note at the time that the Longitude concept was not yet established.

 

[bigscoop]

With the recent anniversary of Juan Ponce de Leon landing in and declaring La Florida for the Queen of Spain, it was noted the last latitudinal reading was 30 degrees 8 minutes. This places his landing in the present day GTM NERR. It was interesting to note at the time that the Longitude concept was not yet established.

"GTM NERR", not familiar with this? I was just curious about the various shipwreck records, just how accurate these latitude readings were, or were not, during the period?

 

[GMD52]

I'm sure that with the advent of satilites, the accuracy of lats and longs has improved over a sailor with an astrolabe pitching on the deck of a sailing ship. I have read that they can be more accurate than GPS units, but sure that that technology has caught up.

 

[ropesfish]

GTM NERR: Guana Tolomato Matanzas National Estuarine Research Reserve

GTM Research Reserve protects 73,352 acres south of the City of Jacksonville (Duval County) in St. Johns County and Flagler County on the northeast coast of Florida, one of the fastest growing regions in the state. The populations of St. Johns County and the adjacent Flagler, Putnam and Volusia counties have grown 20 percent since 1990 and are projected to grow an additional 20 percent by 2010. More than 1.3 million people live within 50 miles of GTM Research Reserve, including the cities of Jacksonville, St. Augustine, Palatka, Daytona and New Smyrna. These protected areas provide habitat for a wide variety of fish and wildlife. A species list recently compiled for Guana River Marsh Aquatic Preserve indicated the presence of at least 44 mammal, 358 bird, 41 reptile, 21 amphibian, 303 fish and 580 plant species. It contains habitats essential to 48 protected animals and 8 protected plants. GTM Research Reserve is also important for the economy because these include 16 species that are fished or harvested commercially and 18 species that are fished recreationally. GTM Research Reserve is geographically separated into a northern and southern component, separated by the City of St. Augustine. The northern component (referred to locally as Guana) is associated with the Tolomato and Guana River estuaries and the southern component is associated with the Matanzas River.

 

[Mackaydon]

Prior to 1759 the determination of longitude was primitive, at best. But in that year, John Harrison devised a clock (his 4th attempt) that could be effectively used at sea to determine longitude.
If you are really interested in the subject, you'll enjoy the book "Longitude" by Dava Sobel ISBN 0-8027-1312-2
Don.......

 

[bigscoop]

Thanks Don. I'll look for the book. Any idea about the accuracy of latitudes?

 

[bigscoop]

Thanks Ropesfish!

 

[Mackaydon]

How accurate these instruments were, I have no idea, but the cross staff, astrolabe and quadrant preceded the sextant. Around 1730 the sextant was invented and it provided mariners with a more accurate means of determining the angle between the horizon and the Sun, moon, or stars in order to calculate latitude.

 

[ropesfish]

Interesting question- This may be of some help:
[h=1]The History of Longitude Navigation by Marine Chronometer
and Astronomical Methods[/h]
by H. Houtman

I INTRODUCTION

Since ancient times, navigation across open oceans was unreliable and dangerous, until latitude and longitude measurements were made directly from the moving ship. Coordinates of latitude and longitude are necessary for safe navigation on the ocean. Latitude is simply the angle north or south of the equator as measured using the center of the earth as the vertex. Latitude is quite easily determined from altitude measurements of the sun and/or stars using a sextant; in the northern hemisphere, the north star (Polaris) provides the observer’s latitude directly. But longitude remained unknown for many centuries, even after the Vikings and Columbus sailed across the Atlantic ocean.

Longitude is simply the difference between the observer's local (sundial) time and the local (sundial) time at Greenwich, England. As any sundial shows, the earth turns at 15 degrees per hour (360 degrees per day), so this time difference is easily converted into degrees of the earth's longitude. Both local and Greenwich time must be known onboard the observer ship to determine longitude, so finding longitude is much more difficult than finding latitude. Either a marine chronometer or various astronomical methods may be used to determine these two times, and therefore longitude.

To utilize accurate clocks for the measurement of longitude while sailing the high seas was first proposed by Gemma Frisius, in 1530. Christiaan Huygens’ tried his pendulum clock as such a marine clock in 1664, and in 1675 invented a marine timepiece with his spiral-spring balance wheel, but ocean tests showed that these lacked the accuracy required for marine use. With a highly refined version, which had technical improvements including a remontoire and temperature compensation, John Harrison demonstrated the first sufficiently accurate marine chronometer in 1762. We therefore will start with Harrison’s contributions, and follow with the previous and subsequent history of the three main methods of longitude measurement.
Sourced at: History of Longitude - Marine Chronometer - Astronomical Methods - History of Clock

Another interesting article here: Time in Motion: The Story of the Sea-Clock, or Harrison?s Chronometers. | "Not Yet Published"

In John Grissim's "The Lost Treasure of the Concepcion" he states that the pilots of the Concepcion believed the ship to be 300 miles east of it's actual position which resulted in the ship meeting it's fate on the Silver Bank. That was October, 1641.

Longitude...it's all about the time.​

 

[McKinney_5900]

I believe that many of the lat/long coords from the sats are purposely skewed, especially when government buildings are pointed toward. I don't think that Big Bro wants precision coords sometimes in this day of the world.

Back when I worked in oil exploration as a marine navigator back in 1980-81 and steered the very first GPS nav systems invented, the Syledis, the cuting edge at that time, had a guaranteed resolution to the customer of 3 meters. That was using only 3 satellites! Today with all the sats up there...they could nearly get to a knats hair if they wanted to. martin

Correction here though. The Syledis system we worked with weren't from satellites...there were 4 base station antenna systems, three being used during shooting for data. The fourth was for hop-scotching when moving down the planned exploration waters. Range was just 50 miles but it was point to point, not from actual satellites. I still think that the location precision capabilities today are down to inches, feet at the worst. Those may only be given to the military though. m

 

[Diver_Down]

GTM NERR: Guana Tolomato Matanzas National Estuarine Research Reserve

GTM Research Reserve protects 73,352 acres south of the City of Jacksonville (Duval County) in St. Johns County and Flagler County on the northeast coast of Florida, one of the fastest growing regions in the state. The populations of St. Johns County and the adjacent Flagler, Putnam and Volusia counties have grown 20 percent since 1990 and are projected to grow an additional 20 percent by 2010. More than 1.3 million people live within 50 miles of GTM Research Reserve, including the cities of Jacksonville, St. Augustine, Palatka, Daytona and New Smyrna. These protected areas provide habitat for a wide variety of fish and wildlife. A species list recently compiled for Guana River Marsh Aquatic Preserve indicated the presence of at least 44 mammal, 358 bird, 41 reptile, 21 amphibian, 303 fish and 580 plant species. It contains habitats essential to 48 protected animals and 8 protected plants. GTM Research Reserve is also important for the economy because these include 16 species that are fished or harvested commercially and 18 species that are fished recreationally. GTM Research Reserve is geographically separated into a northern and southern component, separated by the City of St. Augustine. The northern component (referred to locally as Guana) is associated with the Tolomato and Guana River estuaries and the southern component is associated with the Matanzas River.

Thanks, Ropefish. It is a common abbreviated reference for us on the Historic Coast and much easier than spewing out the entire name. I would have answered last night, but I had already retired for the night. You know, beauty sleep and all...

 

[ropesfish]

One of my personal mental peculiarities is that when intrigued by a question I can just beat it to death, so bear with me, please.

In John S Potter's "The Treasure Diver's Guide" pp 4-5 he talks about the inadequacies of the Spanish fleets longitudinal accuracy and mentions that the Spanish charts, by virtue of using Cadiz as a starting point, were 6° 20' different than those shown in modern charts.
This article from Wikipedia addresses the subject in the "Age of Exploration" section. History of navigation - Wikipedia, the free encyclopedia
This is educational and addresses the methods of the day: Marine Navigation in the Age of Exploration
Here are links to pdf copies of both volumes of "The Seaman's Secrets" a treatise on navigation first published in 1594, directly addresses the subject:
http://www.spirasolaris.ca/sbb9d1.pdf
http://www.spirasolaris.ca/sbb9d2.pdf
I am going to quit now...
but then I found this:
Locations and courses now had to be spatial: a navigator needed to locate himself on a grid of imaginary lines of latitude and longitude.
The Portuguese pioneered the method of navigating by latitude. Ships had to be equipped with instruments (astrolabes, cross staffs) to measure the altitudes of stars or the Sun. It was not difficult to determine one's latitude to within about a degree by this method. Longitude was, however, a different matter. Observations of the Sun and stars were of no immediate help: in order to determine one's longitude with respect to, e.g., Lisbon, one had to find out the difference in local times between one's location and Lisbon. No easy method that was sufficiently accurate suggested itself. The magnitude of the problem is illustrated by the voyage of the Portuguese navigator Cabral who, on his way to the East Indies, swung west in the south Atlantic in order to pick up favorable winds and ran into the coast of Brazil. Further, the world maps prepared in the sixteenth century erred widely in the longitudes of places. The east-west length of the Mediterranean was in error by 19°--about 1100 miles! The longitudes of China and Japan were off by much larger margins. For nations engaged in trade with the East and West Indies, finding longitude at sea was a matter of national interest. Late in the sixteenth century the Spanish Crown instituted a large prize in the hope of a solution. This initiative was followed by the French, Dutch, and English governments in the seventeenth century.
here: http://galileo.rice.edu/sci/observations/longitude.html

 

[bigscoop]

So I'm guessing it's safe to assume that during New World seafaring if a wreck was said to be at 30 degrees latitude then that wreck could easily be 100 miles north or south (or more) of the next navigator's reading. Probably best to assume that these readings were very generalized, at best? I guess "latitude" holds the majority of my curiosity since it is referenced most frequently in regards to early wreck locations.

Thanks ropefish, I'll check out those links.

 

[ropesfish]

All navigational positions are assumed to be approximate up until...today. Only the margin of error has changed. If you take a look at my last (edited) post, they had miscalculated the length of the Mediterranean by 1100 miles. Generally the degree of error has decreased, but individual navigators had different levels of competence and even the best were sometimes just wrong.
To address another question - GPS was once less accurate by virtue of "Selective Availability"
[h=2]Selective availability[/h]GPS included a (currently disabled) feature called Selective Availability (SA) that adds intentional, time varying errors of up to 100 meters (328 ft) to the publicly available navigation signals. This was intended to deny an enemy the use of civilian GPS receivers for precision weapon guidance.
SA errors are actually pseudorandom, generated by a cryptographic algorithm from a classified seed key available only to authorized users (the U.S. military, its allies and a few other users, mostly government) with a special military GPS receiver. Mere possession of the receiver is insufficient; it still needs the tightly controlled daily key.
Before it was turned off on May 2, 2000, typical SA errors were about 50 m (164 ft) horizontally and about 100 m (328 ft) vertically.[SUP][6][/SUP] Because SA affects every GPS receiver in a given area almost equally, a fixed station with an accurately known position can measure the SA error values and transmit them to the local GPS receivers so they may correct their position fixes. This is called Differential GPS or DGPS. DGPS also corrects for several other important sources of GPS errors, particularly ionospheric delay, so it continues to be widely used even though SA has been turned off. The ineffectiveness of SA in the face of widely available DGPS was a common argument for turning off SA, and this was finally done by order of President Clinton in 2000.
DGPS services are widely available from both commercial and government sources. The latter include WAAS and the U.S. Coast Guard's network of LF marine navigation beacons. The accuracy of the corrections depends on the distance between the user and the DGPS receiver. As the distance increases, the errors at the two sites will not correlate as well, resulting in less precise differential corrections.
During the 1990-91 Gulf War, the shortage of military GPS units caused many troops and their families to buy readily available civilian units. Selective Availability significantly impeded the U.S. military's own battlefield use of these GPS, so the military made the decision to turn it off for the duration of the war.
In the 1990s, the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own radio navigation systems. The amount of error added was "set to zero"[SUP][7][/SUP] at midnight on May 1, 2000 following an announcement by U.S. President Bill Clinton, allowing users access to the error-free L1 signal. Per the directive, the induced error of SA was changed to add no error to the public signals (C/A code). Clinton's executive order required SA to be set to zero by 2006; it happened in 2000 once the U.S. military developed a new system that provides the ability to deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems.[SUP][7][/SUP]
Selective Availability is still a system capability of GPS, and could, in theory, be reintroduced at any time. In practice, in view of the hazards and costs this would induce for U.S. and foreign shipping, it is unlikely to be reintroduced, and various government agencies, including the FAA,[SUP][8][/SUP] have stated that it is not intended to be reintroduced.
One interesting side effect of the Selective Availability hardware is the capability to add corrections to the outgoing signal of the GPS cesium and rubidium atomic clocks to an accuracy of approximately 2 × 10[SUP]−13[/SUP]. This represented a significant improvement over the raw accuracy of the clocks.[SUP][citation needed][/SUP]
On 19 September 2007, the United States Department of Defense announced that future GPS III satellites will not be capable of implementing SA,[SUP][9][/SUP] eventually making the policy permanent.[SUP][10][/SUP]
Source: Error analysis for the Global Positioning System - Wikipedia, the free encyclopedia