How accurate do you think your GPS position is?

[Todd727]

I think the accuracy over time is quite repeatable these days, if differentially corrected. Before differential correction, there were sources of error that might drift over time with the weather, clock drift, etc. But this is all corrected out real time now by the differential correction scheme. It is what allows a 787 to land itself day after day and week after week.

Just leave your GPS tracking on in the berth and see what the scatter is for a week.

Airplanes don't autoland off of GPS signals.

 

[Mlhess86]

So I'm anchored for the next day and a half waiting out a storm in the Great Lakes. I'm anchored in a very protected cove (more like a lake that's 2/10 of a mile in diameter). The water is 9' deep with a fair amount of weed on the bottom. Anchor was thoroughly buried from 25 knot gusts from the South this morning. Front came through, and the gusts are now out of the North.

So the boat swings in an arc and drags through the weeds. The anchor likely pivoted or maybe popped out and reset.

Problem is that the MFDs tell you to the foot (maybe less) how far you are from your MOB anchor mark. I have no worries about my anchor, rode, scope, and can drag a couple hundred feet without issue. What I'm interested in knowing is when the display tells me I'm 112' from my anchor drop, how accurate is the mark from yesterday's drop of the hook and this afternoon's position. Yes, I understand that my GPS antenna is 15' from the bow roller which makes it a 30' error in this situation.

So what do you think the accuracy (same GPS numbers for the same position, repeatability) of your unit is, 24 hours apart? Don't tell me what your owner's manual says, tell me based on your experience.

Ted

GPS accuracy can be a simple answer. With modern pleasure boat type equipment in good working condition. You should be accurate with in 3ft to 15ft 99% of the time.

As mentioned before you have momentary position readings that will be significantly further away to the point where you would not trust that location. This is caused by a number of things. Normally environmental conditions that create a very dense atmosphere. Essentially making the signals from Satellite to receiver slower or altered. There can also be calculation errors caused by the loss of time accuracy between satellite and your GPS. On very rare occasion it can also be caused intentionally by government agencies for various reasons relating national defense.

Todays current equipment will receive a signal from 4 to 20 satellites, depending the type of GPS unit you are operating and also the geographic location you are using, and time of day. There are 5 different GPS service providers that have there own satellite networks. And not all receivers communicate with all satellites. That is the basics. Mentioned above is also differential corrections of a DGPS unit. They operate just like a standard GPS. But each satellite adds a send leg of communication to a ground station to help for time accuracy. This service it typically not available on a pleasure boat GPS and is often a paid service. The accuracy will typically be within about 1 ft. These are systems used on commercial aircraft and commercial merchant ships that the Accuracy is required to maintain position and for government related assets.

Sorry about being long winded. But this truthfully is just the basics. My Garmin GPS on my Grand Banks does an amazing job in the PNW. I would say within 5 to 10 ft 99% of the time. If you are close enough to be worried about 10ft I would pick up anchor and find a better spot.

 

[O C Diver]

Sorry about being long winded. But this truthfully is just the basics. My Garmin GPS on my Grand Banks does an amazing job in the PNW. I would say within 5 to 10 ft 99% of the time. If you are close enough to be worried about 10ft I would pick up anchor and find a better spot.

Assuming then 2 positions a significant time apart, in a worse case scenario, the combined error coud be 20'. My concern was not about location, but whether the anchor was dragging or it was GPS error (which it turned out to be).

Ted

 

[Simi 60]

US GPS, not very good in the southern hemisphere
Our GPS only marine plotter drops out often enough that I bought a new, rather expensive SEIWA antenna and no change.

Russian Glonass, China BeiDou, Europe Galileo and US GPS combined in a $14 puck work a treat and never had a dropout.

 

[Benthic2]

I have an $85 Garmin running watch that matches perfectly with google satelite photos. I can see where I run along a fence until I get to a gate, and my line of travel will go exactly through the gate on the google sat photo. I can tell whether I was on the sidewalk or on the street.

I have always assumed there was a bit of logic built in. If 9 out of 10 readings say you are here, the gadget will just ignore the outlier. When you are moving in a predictable manner, it is even easier to identify the outliers, but when you are docked or anchored the logic becomes less reliable. I'd guess that the gadget uses a logic + data = position formula that increases the weight of the logic component when you are moving in a predictable way, and increases the weight of the data component when you are not moving in a straight line. To go back to my jogging watch, it is very inaccurate for the first 2 or 3 minutes, because I am not moving predictably. Sometimes that plot of my run starts in my neighbors yard, or my backyard...but within a few minutes it is dead on. I always assumed that is because it has figured out my direction and speed and can rely on that more.

 

[DDW]

Airplanes don't autoland off of GPS signals.

While the last bits of the approach are more commonly ILS, GPS is used to get to the ILS. And many airliners now have SLS (satellite based landing system) which does not depend on the ILS. GPS accuracy has been certified by the FAA for landings for some time now.

Mentioned above is also differential corrections of a DGPS unit. They operate just like a standard GPS. But each satellite adds a send leg of communication to a ground station to help for time accuracy. This service it typically not available on a pleasure boat GPS and is often a paid service.

In some of the world (like North America) differential correction is freely available to all and commonly used by pleasure boat systems. There are private beacons used by surveyors which can achieve 1 foot accuracy (actually down to about 1 cm).

 

[psneeld]

GPS accuracy can be a simple answer. With modern pleasure boat type equipment in good working condition. You should be accurate with in 3ft to 15ft 99% of the time.

As mentioned before you have momentary position readings that will be significantly further away to the point where you would not trust that location. This is caused by a number of things. Normally environmental conditions that create a very dense atmosphere. Essentially making the signals from Satellite to receiver slower or altered. There can also be calculation errors caused by the loss of time accuracy between satellite and your GPS. On very rare occasion it can also be caused intentionally by government agencies for various reasons relating national defense.

Todays current equipment will receive a signal from 4 to 20 satellites, depending the type of GPS unit you are operating and also the geographic location you are using, and time of day. There are 5 different GPS service providers that have there own satellite networks. And not all receivers communicate with all satellites. That is the basics. Mentioned above is also differential corrections of a DGPS unit. They operate just like a standard GPS. But each satellite adds a send leg of communication to a ground station to help for time accuracy. This service it typically not available on a pleasure boat GPS and is often a paid service. The accuracy will typically be within about 1 ft. These are systems used on commercial aircraft and commercial merchant ships that the Accuracy is required to maintain position and for government related assets.

Sorry about being long winded. But this truthfully is just the basics. My Garmin GPS on my Grand Banks does an amazing job in the PNW. I would say within 5 to 10 ft 99% of the time. If you are close enough to be worried about 10ft I would pick up anchor and find a better spot.

Some outdated info as another has posted.

DGPS was never within 1 ft as far as I know unless it was NOT the USCG DPGS and WAAS (available to all if the machine can receive it) while very accurate is not 1 ft predictable. DGPS by the USCG was never a separate charge except the chartplotter had to be able to receive. No longer an issue as USCG DGPS has been discontinued.

Also the Gov't has not used selective availability for quite awhile and promised to never reactivate it unless something very unusual comes up.

Localized degradation of GPS can happen for a variety of man made reasons...but again is rare for the average boater.

 

[JDCAVE]

Over my experience and somewhat considerable use of my vessel over the past 11 seasons, I am satisfied with the accuracy of my GPS units. All of them. When in a very narrow channel, on a good chart, the combination of radar and visual piloting, convinces me the accuracy of my GPS is fine.

My biggest concern in British Columbia is systematic error in the charts. Many of the soundings and shoreline features were “drawn” pre GPS. I’ve found whole shorelines out as mush as 50 metres. These remain a concern in more remote areas. So you really need to carefully monitor your radar and depth sounder in narrow channels and have a bow watch. I presume a side-scan sonar would be useful in these situations.

Jim

 

[fgarriso]

I presume a side-scan sonar would be useful in these situations.

Jim

I had a side scan sonar but didn't find it very useful. Its range was too short to be much help except when going at idle.

 

[RUM DOG]

When the early handheld gps units used by the Navy came out we used to setup a differential station to give a fixed point of reference to correct the SA offset.We also used to test the accuracy of the units .A Geospatial engineer from the Navy lab explained to me that you have to always account for the wobble of the earth and satellite track abberations.Looking at some of the track graphs we did over a week looked like a spirograph drawing.Having dropped and recovered numerous pieces of hardware on the seafloor over the past 30 years I can tell you that if you can get within 5 -10 feet of a waypoint repeatedly that is as accurate as possible on a moving boat.

 

[tiltrider1]

Over my experience and somewhat considerable use of my vessel over the past 11 seasons, I am satisfied with the accuracy of my GPS units. All of them. When in a very narrow channel, on a good chart, the combination of radar and visual piloting, convinces me the accuracy of my GPS is fine.

My biggest concern in British Columbia is systematic error in the charts. Many of the soundings and shoreline features were “drawn” pre GPS. I’ve found whole shorelines out as mush as 50 metres. These remain a concern in more remote areas. So you really need to carefully monitor your radar and depth sounder in narrow channels and have a bow watch. I presume a side-scan sonar would be useful in these situations.

Jim

This is my experience as well. I often find that in narrow channels it is not uncommon for the chart plotter to show my boat on land. Fortunately, I don't need perfect accuracy as I can see perfectly well where the land is. I also give rocks a wide berth to make up for any inaccuracy in the charts. If an area is strewn with rocks that require pinpoint accuracy, I will map it out with the dingy and depth sounder first.