Time to Buy a Sextant ?

[GordMay]

Originally Posted by Surveyor

... I am a surveyor who earns a living using GPS as well as other conventional methods and instruments to fix position and/or solve problems...

What horizontal (position) and vertical (elevation) accuracy and precision do you expect from GPS positioning?
What do (did) you expect from chain & transit?
(a) Mountainous terrain?
(b) Flat Terrain?

[rtbates]

I've had a sextant for years before GPS. SatNav, Omega, Loran, etc. And I'll keep it for sure, but not because I think the US gov might disable the system. GPS is used for military purposes..And even if they dither the signal to increase error, it'll still be more accurate than any sextant fix anyone can take from a small boat at sea. The best use for a sextant aboard these days is to determine angles between objects..

[hellosailor]

"What do (did) you expect from chain & transit?"
I've seen the USGS make errors of 1/3 mile in "well surveyed" flatlands with those. Of course, if you're dropping a 20-megaton bomb that error really IS insignificant.

What matters more, the size of the circle of error? Or KNOWING the size of the circle?

[Hiracer]

Dumb question of the day. Has the military ever scrambled GPS, say during the gulf wars?

What's the actual history on this?

I figure if there is ever an actual all out war, then the satellites will be the first thing blown up for obvious reasons. On both sides.

Some say, well, navigation will then be the least of your worries. But actually I think otherwise, as I'm gonna wanna head to wherever the war is least active and need to be able to miss reefs, etc., to get there.

I still think the greatest risk to GPS is loss of onboard electricity, say, through a lightning bolt or electrical short. But the loss on the transmit side is an interesting question too. Some say it will never happen because society is too dependent on it. But if the risk of an all out war becames real, I think the first thing the military will do is scrabble the transmission real bad because preservation of military power becomes paramount over all else, civilian domestic needs be damned.

[GordMay]

Originally Posted by hellosailor

I've seen the USGS make errors of 1/3 mile in "well surveyed" flatlands with those...

That seems unlikely to me.

The Federal Government has defined the following standards of accuracy for published maps:

United States National Map Accuracy Standards

1. Horizontal accuracy. For maps on publication scales larger than 1:20,000, not more than 10 percent (90% precision) of the points tested shall be in error by more than 1/30 inch, measured on the publication scale;
for maps on publication scales of 1:20,000 or smaller, 1/50 inch.
These limits of accuracy shall apply to positions of well-defined points only. Well-defined points are those that are easily visible or recoverable on the ground, such as the following: monuments or markers, such as bench marks, property boundary monuments; intersections of roads and railroads; corners of large buildings or structures (or center points of small buildings). In general, what is well-defined will also be determined by what is plottable on the scale of the map with-in 1/100 inch. Thus, while the intersection of two roads or property lines meeting at right angles would come within a sensible interpretation, identification of the intersection of such lines meeting at an acute angle would not be practicable within 1/100 inch. Similarly, features not identifiable upon the ground within close limits are not to be considered as test points within the limits quoted, even though their positions may be scaled closely upon the map. This class would cover timber lines and soil boundaries.

2. Vertical accuracy, as applied to contour maps on all publication scales, shall be such that not more than 10 percent of the elevations tested shall be in error by more than one-half the contour interval. In checking elevations taken from the map, the apparent vertical error may be decreased by assuming a horizontal displacement within the permissible horizontal error for a map of that scale.

➥ Map Accuracy Standards

[slomotion]

I agree with Randy. I grew up using a sextant. I'm old and that's all there was except for some RDFs which we never had. The math is easy once you really understand it. But, you are unlikely to get anything close to GPS accuracy when taking sightings on a small boat. It takes practice and it's not like riding a bicycle - you have to keep practicing. When we were practice sailing and getting ready to go cruising, I tried to fix our position with a sextant several times - showing off for the wife. The closest I got (by far) was about 10 miles off and without GPS I would never know a good sight from a bad one. We didn't bring the sextant.

[Hiracer]

Originally Posted by Hiracer

Dumb question of the day. Has the military ever scrambled GPS, say during the gulf wars?

"Last month [September 2007] the U.S. Department of Defense announced that it intends to stop procuring global positioning system (GPS) satellites with the capability to intentionally degrade the accuracy of civil signals. This capability, known as selective availability (SA), will no longer be present in the next generation of GPS satellites."

GPS III Permanently Discontinues Selective Availability

I find this news significant, even if dated.

I think it's safe to say just as accurate clocks and wristwatches fundamentally and permanently changed navigation so has GPS. The essential question is whether a back-up system is warranted. Again, I think the greatest risk to GPS is on the receiver side, not the transmit side. Which is not to say the risk is large, but just large enough not to ignore.

[hellosailor]

Gord-
The error of 1/3 mile was confirmed. For a rather large brick building in an area full of benchmarks, actually. But the topos in the area still use the 1927 survey, and the marine charts of course have been updated to GPS. On the topos, the "real" GPS positions simply don't line up but in 1927, the work was good enough since mariners and hunters were the main users--and they didn't have GPS to mislead them.

There is (or was) a rather famous hiking club reprint of a USGS topo map section from the Catskill Mountains (NYS) in the area of Slide and Cornell "mountains" which quite literally said "NOTE TO ENEMY BOMBERS:...." in Cold War humor. That entire section of NYS being of little interest to the federal government, there were some MAJOR mislocations because the original surveys were, well, good enough for government work.
Then there's the case of New York City, which has all of the city maps benchmarked to an old Dutch Nieu Amsterdam coordinate system. The US Census wasn't aware of this and for many years the TIGER maps were all off by about 1/4 mile. The mislocation varied in each of the five boroughs (Dutch equivalent of County) within NYC, and it is only in the past five years that those errors have been corrected. Official NYC survey benchmarks have always been offset "correctly" but outside of NYC, apparently only the real pros knew of the real offset.
The land navigation bozos like NavTeq still have similar errors, often from similar problems, even in well-populated areas. After all, the locals KNOW about these things, it is only the "tourists" that have problems. And tourists, well, as long as they leave their money and then go home who cares about tourists anyway? Really?
Heck, last night I used a Nuvi (with current maps) to find a county building. The Nuvi faithfully took me a full 1/4 mile past it and said "Arriving at destination". I said, I don't think so...and yes, it was off by a full 1/4 mile. Yes, that's a geocoding error not a positioning error per se--but the errors are still endemic.

[hellosailor]

John-
Gulf Wars? Oh, you mean the Gulf Incursions. (There were no formal declarations of war.) Bear in mind that during the first one, GPS was still a RARE NEW THING and there were times you couldn't buy one because West and other vendors were out of stock--after servicemen wrote home and said "Can you send me..." for their personal use out in the desert.
There were, and are, two GPS data streams in the US system. One is military encrypted, the other civilian use. All the GPS receivers that you could/can buy on the retail market use the civilian data stream only. You *can* apply to the government for permission to use military-enabled receivers, and in theory once Uncle Sam has vetted out your neighbors and confirmed you are worthy, you'll be allowed to do so if you present any valid reason for it.
But the civilian signal was intentionally diddled, corrupted with bad data, to degrade the position accuracy it gave. That diddling was turned off (in the 90s?) and no longer is in use, but it CAN be enabled again at any time.
If you ask the government about when, how, or if they have intentionally diddled GPS signals for military purposes--you won't get a meaningful answer. That really is a matter of national security and no one will honestly discuss it with you, unless you've got the security clearance and Need To Know.
Assume that sometimes, somewhere, the GPS signals are intentionally diddled. Or, that idiots really are at the helm. One or both must be true. :-)

[GordMay]

Originally Posted by hellosailor

... There is (or was) a rather famous hiking club reprint of a USGS topo map section from the Catskill Mountains (NYS) in the area of Slide and Cornell "mountains" which quite literally said "NOTE TO ENEMY BOMBERS:...." in Cold War humor...

Yea, right. (the rare case wherein a double positive conveys a negative meaning).
And, I suppose,you have, or could locate, a copy to substantiate this nonsensical claim - or perhaps not.

[Hiracer]

Originally Posted by hellosailor

John-
Gulf Wars? Oh, you mean the Gulf Incursions. (There were no formal declarations of war.) Bear in mind that during the first one, GPS was still a RARE NEW THING and there were times you couldn't buy one because West and other vendors were out of stock--after servicemen wrote home and said "Can you send me..." for their personal use out in the desert.
There were, and are, two GPS data streams in the US system. One is military encrypted, the other civilian use. All the GPS receivers that you could/can buy on the retail market use the civilian data stream only. You *can* apply to the government for permission to use military-enabled receivers, and in theory once Uncle Sam has vetted out your neighbors and confirmed you are worthy, you'll be allowed to do so if you present any valid reason for it.
But the civilian signal was intentionally diddled, corrupted with bad data, to degrade the position accuracy it gave. That diddling was turned off (in the 90s?) and no longer is in use, but it CAN be enabled again at any time.
If you ask the government about when, how, or if they have intentionally diddled GPS signals for military purposes--you won't get a meaningful answer. That really is a matter of national security and no one will honestly discuss it with you, unless you've got the security clearance and Need To Know.
Assume that sometimes, somewhere, the GPS signals are intentionally diddled. Or, that idiots really are at the helm. One or both must be true. :-)

This is the first I've heard about two signal streams from GPS satellites. (OTOH, I don't know much if anything about GPS.) That's interesting. So the news article I linked above is pure disinformation because the implication is that selective availabilty will not be operable once all of the satellites have been replaced. But you're saying the government retains the ability to stop civilian use at will despite the change in satellites.

Gulf wars: Some of us define wars by the number of dead bodies, not government declarations. Vietnam was never a declared war either.

Grenada was an incursion.

[hellosailor]

John, I don't know what plans they have for new satellites wrt supporting one or both streams, but I would be surprised if they change that. I don't recall how it is implemented but basically, all you need to do is send encrypted data alternately with the "public" data, and not give the public the keys.

Some of the dead might well argue that calling what killed them a war, is a dishonor in itself. If the full might and will of the nation had committed to a true war effort, they might well be alive. However....that's a bit far afield from sextants. Albeit quite relevant to the map:

Gord-
I've seen the maps but that was in the 80's. I think the maps were originally noted up that way in the antiwar (Vietnam) spirit circa 1970. I'm sure that if you looked up Catskill Moutain hiking club trivia, you could find it online somewhere--you're an ace at online research. Why should you think it impossible for a private club to make a wry comment when they have private printing done? Bear in mind, a "NOTE TO ENEMY BOMBERS" on a map of a major metropolitan area, would be a very different remark from the same thing printed on a map of useless [sic] and unpopulated state forest.

[GordMay]

As interesting as this little diversion has been, my question to Surveyor (what accuracy HE expects) remains.

Originally Posted by hellosailor

... I'm sure that if you looked up Catskill Moutain hiking club trivia, you could find it online somewhere ...

It seems a little unreasonable, to me, to expect me to research and prove your assertion.

Notwithstanding, I found 3 separate unsubstatiated repetitions of the claim, like yours, none supported. While his does not disprove the story, I cannot believe it adds much credibility.

It should be noted that the USGS didn’t even have an accuracy standard untill about 1941; so an 1800's logging map might well be out by as much as 6/10 to 1 mile.

"... In 1941, the U.S. Bureau of the Budget issued the “United States National Map Accuracy Standards,” which applied to all Federal agencies that produce maps. The standards were revised several times, and the current version was issued in 1947..."
➥ http://edc2.usgs.gov/pubslists/factsheets/fs17199.pdf

Apocryphal Stories?

In the 70's the Adirondack Mountain Club maps for one portion of NY's Catskill Mountains used to literally have a "Note to Enemy Bombers" on one quad, apparently there was a mountain charted a good mile out of position.
➥ NavList Message

Ships do run aground and planes do crash due to "simple" datum errors, they are made even in the professional world. One of the private maps (Adirondack Mountain Club, if I recall) for hikers in the Northeast used to be copied from the USGS topos, which dated back to logging surveys in the late 1800's for that area (again, iirc). The private map for many years literally said "NOTE TO ENEMY BOMBERS:" in the border, noting that a certain mountain was 6/10's of a mile away from where it was shown on the map. Well, for logging maps from the 1800's, that was good enough. For a forested preserve area pre-GPS, that might also be good enough. Today? That could be a problem, as a SAR team goes "here" instead of "there". (I assume that quad has long since been corrected.)
➥ [aprssig] Re: TIGER maps what-s the problem?

Meanwhile...I don't recall the name, but there was a USGS topo quad for the Catskill Mountains, somewhere around Slide Mountain (a popular hiking area) where the hiking club reprints of the topo all used to say "NOTE TO ENEMY BOMBERS...THE POSITION OF xxxx MOUNTAIN IS OFF BY...." because the maps, in the 70's, were still using the survey information from the earlier 1800s.
➥ http://community.netscape.com/n/pfx/...ear&redirCnt=1

TIGER/Line* Map Accuracy
"... The positional accuracy varies with the source materials used, but generally the information is no better than the established national map Accuracy standards for 1:100,000-scale maps from the U.S. Geological Survey (USGS); thus it is NOT suitable for high-precision measurement applications such as engineering problems, property transfers, or other uses that might require highly accurate measurements of the earth's surface..."
➥ http://www.census.gov/geo/www/tiger/...r/tlrdmeta.txt

* Topologically Integrated Geographic Encoding and Referencing system

[hellosailor]

Well, Gord, there is only one real "Proof" and that would be to find a copy of the map in question. The next best proof would be an eyewitness who has seen the map--and that's me. Anything you can find online on the internet (which didn't exist in 1970) would be at best heresay.

So take it or leave it, because if those maps are still around, they're likely to be hard to find. I'm not motivated enough to contact the AMC and find out if they have one archived. After all, it could easily be a counterfeit. :-)

Bear in mind the context and the state of cartography circa 1970, with the Cold War still very much real. In either 72 or 73, NORAD actually sent the war alert codes out to the media and asked all radio stations in the US to shut down and make the public announcement to turn to emergency stations, I know this because I read it on the teletype machine as it came in, a 5-bell event, and the station down the hall got an FCC commendation for being one of the few that actually did go off he air.
As you may have heard from other sources, the Soviet cartographers, who had a tradition of being among the world's finest, had at that point been mislocating entire villages, rivers, mountains, by many miles in order to be sure that any maps obtained by US intelligence could not be used for an incoming strike.
That's history, those who remember it don't have to research it. Those who want to confirm it--should do so independently, so they can rely on their own research. (Counterfeiting is an easy business.)

[slomotion]

Originally Posted by hellosailor

That's history, those who remember it don't have to research it. Those who want to confirm it--should do so independently, so they can rely on their own research. (Counterfeiting is an easy business.)

Here are 2 that I remember but cannot document - and I don’t have to because, well ... I remember them:

1. In the early 60s hundreds of thousands of Chinese and Soviet troops faced off near the Mongolian border. It was all high tension, political/miltary, muscle flexing; but there were periodic skirmishes and significant casualties. And, the pretext was a cartographic/border dispute involving parallel mountain ranges.

2. In 1990 during the Gulf war/incursion government agents seized GPS receivers from early adopters in the yachting community in Florida.