GPS accuracy

[bcn]

The HiRes material from GE for continental areas are not satellite photos but geo-referenced Orthophotos from national cartographic agencies (in regions where those are available and free to license by Google).

This is why these are pretty accurate usually.

[DotDun]

Originally Posted by zeehag

AS this is happening with NOT JUST my garmin, includes all other methods of positioning i have used since and including san diego, usa, i would say it IS the gps system giving that loss of accuracy.
remember this is a military thing, not meant for our use. as the military says, it will NOT be exactly accurate. i read a lot, and remember what i read.
some info says CANNOT GUARANTEE POSITION WITHIN 3 MILES.... this is not the manufacturing company stating these facts, it is the folks who program, place and maintain the satellites.
the numbers were a lot farther off in coronado, sin diego, kalifornikasha, than here in beautiful downtown rural fishing villageville, mexico. figger that one and try to splain it in grown up terms...SPOT, garmin, android x 3, computers x 2.....
channel into the lagoon is only 20 ft or less in width.

If your GPS is less than 10 years old, and it's antenna is mounted such it has a clear view of the sky, then 99% of the reason your chart is off is due to the way the chart drawn and the registration of the chart to the geo-frame.

What you must understand is that the geo frame is a virtual grid overlaid on the earth's surface for the sole purpose of providing a reference point. Your GPS is very accurate in figuring out where you are on the virtual geo frame. Maps, charts, and sat photos are depictions of the earth's surface. "Registering" the map, etc. to the geo frame and getting the aspect ratio, size, and anchor points correct is where most of the errors take place. Remember, the author of the chart performs this, and with many different authors you get many varying levels of accuracy.

The GPS positions you on the geo frame, the chart plotter takes that position and shows you where you are on the map. Two different functions.

[bcn]

Another source of problems can be the antenna or in most cases the antenna cable. Will result in spikes or black-outs.
If we have an antenna (=coax) cable and not a serial connection. In the most cases we will find an integrated receiver - all the GPS mice - and not receiver plus antenna.

[Stu Jackson]

Originally Posted by DotDun

The GPS positions you on the geo frame, the chart plotter takes that position and shows you where you are on the map. Two different functions.

Thanks for that. Very clear.

I believe that the misunderstanding of this very basic concept leads people to ask these "accuracy" questions.

As I said earlier, it (accuracy) simply doesn't matter, as Mark pointed out a few pages back.

zeehag pointed out the REAL WORLD implications in her posts.

And this good one:

Originally Posted by Lepke

From the point of view of an old sailor that crossed many oceans, many times with a sextant. On a ship in mild weather with a 3 star fix, I was lucky to plot a triangle with sides less than a mile.
GPS isn't perfect, but neither are your charts. Most charts are based on old manual inputs. Sighting positions were probably off more than 15 meters. The field people didn't have anything as accurate as gps until recently.
Be happy with what you've got.

Simply recognize that GPS is another navigational TOOL that gets us closer.

"Accuracy" to what, is the question. In navigation, once you're that close, get your eyes off the screen and your head outta the boat.

[phiggins]

"And for sure going GPS based into a 15m channel is just unreasonable."

You know its easy for you guys sitting in your safe little harbours in the USA with all your free accurate NOAA charts and lights and buoys to say that but try going in somewhere where there is no reliable charts in the middle of the night knowing that if you don't you will be hit by the Northerner coming through before morning. If have done this many times not by choice and I followed the GPS and my GE charts. But I did it damn slow.


Sent from my iPad using Cruisers Sailing Forum

[DotDun]

Originally Posted by phiggins

"And for sure going GPS based into a 15m channel is just unreasonable."

You know its easy for you guys sitting in your safe little harbours in the USA with all your free accurate NOAA charts and lights and buoys to say that but try going in somewhere where there is no reliable charts in the middle of the night knowing that if you don't you will be hit by the Northerner coming through before morning. If have done this many times not by choice and I followed the GPS and my GE charts. But I did it damn slow.


Sent from my iPad using Cruisers Sailing Forum

I think you missed the point.

"free accurate NOAA charts" are not as useful in the situation you describe. Spot lights and eyeballs are the primary tools for that!

[bcn]

Originally Posted by phiggins

"And for sure going GPS based into a 15m channel is just unreasonable."

Sent from my iPad using Cruisers Sailing Forum

Paul, I meant here to go Autopilot like confident that the GPS will do the navigation for you. ("there can't be a breakwater, the chart shows it 25m aways")
A 15m channel and putting the GPS fix in mid of the channel you have 2.5m (or 8ft) on either side as margin assuming a 5m error.
And if your boat is 5m wide you might be at the edge assuming that the GPS is centered on the boat or it's position is corrected in the plotter.

If your eyeball navigation and the fix are saying the same then for sure the GPS is a big help. But you will be at the steering wheel and perhaps having somebody standing at the bow just in case...

[phiggins]

"Spot lights and eyeballs are the primary tools for that!"

True if there is something to see, but if the channel is through a reef that has only shallow water for miles, there is nothing to see!!!

[bcn]

Ah, we did that on the Garonne at very, very low water (and we draw just 90cm).
The very accurate French InlandENCs did save our ass - more or less.

[zeehag]

mebbe i should speak more clearly.
i was not only using my gps.
i was using a cell fone
spot tracker
samsung tablet
samsung galaxy prime cell phone
SPOT tracker unit.
samsung chromebook
hp chromebook


the samsung telcel units were closest, but all are off by minimum 50 ft. they are mexican. however the satellites are govt controlled, evennow, dont even think not. read the teeny tiny finest print.
the wifi generated signals showed my position as 60 feet from actual location, in mexico
2011-2012, in mexico, SPOT was 50 meters off. now, 4 years later, it is only 50 feet off. biig improvement.
2011, in san diego, the spot tracker gps showed me in a parking lot across the bay in the barrio, national city, when i was on a mooring in coronado.
garmin showed me middle of san diego bay. i was close to beach in coronado.
i know why the accuracy is off, and i totally understand it.
the distance of error seemed to appear greater when a navy vessel was in the bay of san diego. it also appears to fail some during increased level of national alert. hhmmmm.......
imagine that.
you who dont understand the reasons should really research better.
observations , when treated properly , as in researching a project, can illuminate one nicely, and teach exactly how free our freedoms are.

[bcn]

Originally Posted by zeehag

the distance of error seemed to appear greater when a navy vessel was in the bay of san diego. it also appears to fail some during increased level of national alert. hhmmmm.......

Yes, jamming might happen. I would not expect it in zones where safe marine traffic depends on GPS --> AIS - in "normal" times.
Anybody here with experiences near to the Capitol or the White House?

WiFi location techniques are different. Or GSM/Mobile Phone triangulation. Google, Apple & Cie or your mobile provider will know how accurate they can trace you down. 5m not yet....

[Snowpetrel]

http://news.utexas.edu/2013/07/29/ut...n-yacht-at-sea ... Serious spoofing.

I remember a brand new set of raymarine charts (2007) for the beagle channel, that at one point where about 3 miles out of position. GPS was spot on, the electronic charts were just crappily geo referenced.

Radar, eyes and commonsense still worked fine. And the charts could be offset to match the radar overlay. At least until we ran out of "offset" and the set wouldn't let us adjust it any further.

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[Nicholson58]

Originally Posted by phiggins

To check the the accuracy of my GPS I created a chart of the area around my house from GE using GE2KAP then displayed the chart on OpenCPN. It first showed the position of the GPS 3 meters away from its actual location. I left it for a while with the track enabled to see how it moved around. After a few hours OpenCPN created an oval pattern (see attachment) with the center of the oval at the actual GPS position.

My question is what does the pattern represent?

THE OVAL - in Engineering terms this is Hysteresis. Its a combination of various factors including the resolution of the signal and the ability of your device to discriminate a variation. Like a bathroom scale - you can only resolve the data into discrete digital increments. 'Are you 179.6 or 179.4 pounds? If the scale can only report whole pounds, one time you are 180 and another time you are 179. Add to this various other features of electronics and that is as good as we non-military folks will get. (Yup, they have better resolvers).

Another issue is that the background base map may be badly oriented. This is unrelated to your question but you should understand that on the water, many of the background maps were made by the British navy with a Sextant 200 years ago. In some locations, the GPS may show you sailing well inland. Even in the North Channel of Lake Huron, there are places where we sail through islands. KEEP EYES OPEN.

[DotDun]

Originally Posted by zeehag

mebbe i should speak more clearly.
i was not only using my gps.
i was using a cell fone
spot tracker
samsung tablet
samsung galaxy prime cell phone
SPOT tracker unit.
samsung chromebook
hp chromebook


the samsung telcel units were closest, but all are off by minimum 50 ft. they are mexican. however the satellites are govt controlled, evennow, dont even think not. read the teeny tiny finest print.
the wifi generated signals showed my position as 60 feet from actual location, in mexico
2011-2012, in mexico, SPOT was 50 meters off. now, 4 years later, it is only 50 feet off. biig improvement.
2011, in san diego, the spot tracker gps showed me in a parking lot across the bay in the barrio, national city, when i was on a mooring in coronado.
garmin showed me middle of san diego bay. i was close to beach in coronado.
i know why the accuracy is off, and i totally understand it.
the distance of error seemed to appear greater when a navy vessel was in the bay of san diego. it also appears to fail some during increased level of national alert. hhmmmm.......
imagine that.
you who dont understand the reasons should really research better.
observations , when treated properly , as in researching a project, can illuminate one nicely, and teach exactly how free our freedoms are.

Zee,

I can only say we have to agree to disagree.

I have a thorough understanding of the technology that each of the devices you list use for location discovery/determination. Except for the Chromebooks and maybe the Samsung tablet, all the rest ultimately use an on-board GPS. You may see large errors while the GPS finds itself, but if it has a view of the sky, they will fairly quickly settle in at 10m or less accuracy. You observation of errors beyond this are 99% mapping issues (1% atmospheric/RF issues).

What I don't agree with is your Gov't conspiracy theory. While I find that GPS jamming close to a Navy ship plausible, I'm not sure the reason if it is in fact intentional.

Please provide links to credible data supporting your argument.

[zeehag]

Originally Posted by DotDun

Zee,

I can only say we have to agree to disagree.

I have a thorough understanding of the technology that each of the devices you list use for location discovery/determination. Except for the Chromebooks and maybe the Samsung tablet, all the rest ultimately use an on-board GPS. You may see large errors while the GPS finds itself, but if it has a view of the sky, they will fairly quickly settle in at 10m or less accuracy. You observation of errors beyond this are 99% mapping issues (1% atmospheric/RF issues).

What I don't agree with is your Gov't conspiracy theory. While I find that GPS jamming close to a Navy ship plausible, I'm not sure the reason if it is in fact intentional.

Please provide links to credible data supporting your argument.

all i am able to do is roll on deck laughing my ass off at this insensitive and off base remark. i have posted much proof of this along the path of my adventures. might wanna look it up in search feature.
i am excited and blissfully happy that folks who are not reading the information on my gps and various other machines run features and placement by same is only subject to YOUR numbers, what happens in real world is apparently a lot different than what your arbitrary numbers choose to show.
that is problem with numbers.
they FAIL to show anything other than what the collector of data desires to prove. folks choose to skew data and choose to listen to false information, yet, when it comes to real world use, there is denial and balderdash and arguments.
ok
you win. there is no deviation and no inaccuracy in gps. none programmed in and no jamming and no anything to befoul your happy snotty yotty planning in virtual time.
i hope you land on a shoal you are not able to exit due to your insistence that your gps is totally perfectly accurate.
i presume you use yours in a vacuum.

i know all about charts admiralty and otherwise, as the newest info on charts states 1865 or thereabouts.
yes i read fine print and i have been sailing since age 7, which, to you, is forever, as i am OLD. i even know how to use a proper lead line and knot line.
rodlmffao.
feet not fathoms inland fool.