GPS accuracy vs Precision, what the gps makers don't tell us.

I do not doubt that any of the Waas GPS units are accurate. How accurate really depends on the manufs measuring stick. The manuf are not consistent with these measurements, They kinda make up their own rules, bout like sonar transmit power. There is no consistency amongst the manuf. This maneuver is just to keep us confused so we keep on buying. In other words, a marketing tool.

What I keep asking (and keep getting the run around) is how precise is the device? What is the consistency of precision? After all, there are major differences between accuracy and precision.

In other words if I pass by a marker at a given distance from my boat, will that marker be at the same lat/long if I repeat the pass on the same track?
Several passes at 1mph? At 2 mph? At 3 mph?

You all need to try this out and see for yourself. Locate a river/lake marker and drive by it at different speeds following the same track. Don't mark it. Something to do when the fish ain't co-opin'. Watch your gps position indicator an/or turn on your "ring". Try it and log your results. Pencil and paper make good log devices.:)

This is good info to know about your own gps.

Let me know your results.

Thanks

Tom

Also , sure would like too see some input form the guys who claim their GPS is accurate at .2 mph or.4 mph using this technique. I am trying to chart some user data stuff.

CP- sometimes we can pull up to our mats and be dead on em. But some times they are up to 15-20ft off. Sidescan mark and original mark may not be the same but close.

Tom --I assume you have trans. on both ends of your boat--i.e. trolling motor and another on the transom
How many gps recievers do you use and where are they located ??
I have 2 chartplotters--one with internal and one on transom fot the console---it looks to me if I was to interlink them I would,nt know didly about the location or accuracy ????:confused::confused:

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Originally Posted by DTG

CP- sometimes we can pull up to our mats and be dead on em. But some times they are up to 15-20ft off. Sidescan mark and original mark may not be the same but close.

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That is why I am getting around to the precision part of it.

Tom my 76c for the most part allows me to do what your talking about. I can go by a tree sticking out of the water. And it always shows the same on it. but today when going 5 MPH. It was not showing the same Lad and Lan? Would not have thought that speed would be a factor?
And wonder if the unit was just not getting the signal it does most days?

Now my 987c-SI GPS does show the same bouey in the same place all the time. and its close enough to run by at night. But I always wonder if it may be off enough to cause me to run over a marker? So I for one do not put full faith in it yet.
Will check this again next day I am out on the water. and give you the findings of the Garmin 76C
Shoot Come to think of It I can also do it in the yard on the 4 wheeler also.

How is our buddy Todd doing? If my stress test goes well Friday. I need to get with him and go Striper fishing next week. He likes the pole dancers while striper fishing LOL
Pete

What speed does best for finding stuff consistently? This is what I plan to chart/compute. There should be a common denominator for all receivers.

Next time try different speeds and check the results with both gps units. Low speeds will cause errors.

If the boat moves 1 mph, it is moving 1.46 feet per second. Not enough to get it out of the 3 foot accuracy circle.

If the accuracy of the system is 1 meter (mighty iffy) then it takes a speed of 2+ mph to get out of the last area ping (3 feet circle).

You see the math?

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Originally Posted by frank lawhead

Tom --I assume you have trans. on both ends of your boat--i.e. trolling motor and another on the transom
How many gps recievers do you use and where are they located ??
I have 2 chartplotters--one with internal and one on transom fot the console---it looks to me if I was to interlink them I would,nt know didly about the location or accuracy ????:confused::confused:

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I got on up front and one at back and a portable. I use the portable to mark stuff. That is when i remember to do it. I got crap everywhere and nothing links up. I can link front and back to my laptop, but too much hassle.

This stuff just gives me something to do so I don't haveta paint and mow.:cool:

Hey Tom!

Here is a test I did a few years ago (see attached image). It is a rough test, but gives clues to precision. A word commonly used in high accuracy gps circles is "repeatability". GPS drift a key aspect of repeatability, which is simply variation of indicated position (vs. actual position).

Initially when folks see this kind of thing, they are rather disappointed. The key thing is to note the scale bar. Now imagine the average length boat overlayed on the plots. That makes things seem better and reinforce why the gps is useful. It is common to get within a boat length or slightly more of a previously saved waypoint. If one can get that close, the object of interest can usually be found quickly with the depth sounder.

Users of GPS units for agriculture were commonly interested in the "relative" accuracy of their hardware. A quality submeter gps would only drift 1 foot in a 10-15 minute period. If a farmer was spraying a field he might be getting <1 foot pass to pass accuracy as long as he didn't take longer than 10-15 minutes to make another swath. Now if he wanted to run the same recorded path again an hour (or hours) later, it is likely he might be off a few feet, even with a submeter gps.

Pat H.

I have Lowrance units. When finding brushpiles I always make sure I start at least 100' away to get a good straight line. I idle toward waypoint (2-3mph). When my distance shows 10' or less I drop marker. My antenna is on top of console so I drop marker beside it. Always drop marker as close to antenna as possible. I totally trust gps for this step and never rely on depth finder until after I drop marker. This method is so consistent for me I know which side of the marker depth changes are as well as different parts of brush pile.

Now here ya'll go taking the fun out of my fishing:(,At my age I feel lucky just to find the lake.:p

wow....some of you guys are dead serious...

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Originally Posted by HydroGrafx

Hey Tom!

Here is a test I did a few years ago (see attached image). It is a rough test, but gives clues to precision. A word commonly used in high accuracy gps circles is "repeatability". GPS drift a key aspect of repeatability, which is simply variation of indicated position (vs. actual position).

Initially when folks see this kind of thing, they are rather disappointed. The key thing is to note the scale bar. Now imagine the average length boat overlayed on the plots. That makes things seem better and reinforce why the gps is useful. It is common to get within a boat length or slightly more of a previously saved waypoint. If one can get that close, the object of interest can usually be found quickly with the depth sounder.

Users of GPS units for agriculture were commonly interested in the "relative" accuracy of their hardware. A quality submeter gps would only drift 1 foot in a 10-15 minute period. If a farmer was spraying a field he might be getting <1 foot pass to pass accuracy as long as he didn't take longer than 10-15 minutes to make another swath. Now if he wanted to run the same recorded path again an hour (or hours) later, it is likely he might be off a few feet, even with a submeter gps.

Pat H.

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Were you making passes on a fixed object when you recorded this data? What was your setup, speed, etc.?

Thanks for the info.

Tom

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Originally Posted by poppop

wow....some of you guys are dead serious...

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Yeah, they are drifting out of fishing into that dreaded area called WORK.

Some tech info for you electronic gurus.
These pages are currently under construction

http://www.gpsworld.com/gpsworld/art....jsp?id=388666

http://www.maps-gps-info.com/gps-accuracy.html

Of course it's all over my head, I just use my GPS 72 and GPSMap 76C to get me close and look for it with my 480.

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Originally Posted by DTG

CP- sometimes we can pull up to our mats and be dead on em. But some times they are up to 15-20ft off. Sidescan mark and original mark may not be the same but close.

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Yep. I have had this happen a lot.

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Originally Posted by SteveJ

Some tech info for you electronic gurus.
These pages are currently under construction

GPS Accuracy: Lies, Damn Lies, and Statistics - GPS World

GPS Accuracy - How Accurate is it?

Of course it's all over my head, I just use my GPS 72 and GPSMap 76C to get me close and look for it with my 480.

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I read all this stuff Steve. Can't trust any of the manufacturers to be honest. They want show raw data or performance data such as Hydrographics just put up. That is what I want to collect, not somebody bragging how good or accurate their gps is at working.

I know sub meter GPS units are dead accurate (agriculture, surveying, etc, but that is a sophisticated and expensive technology. I could get a sub meter receiver, but wow what an expense.

If I am slow trolling and try to cover a "cluster" of close proximity brush piles, I never know what to expect. One minute it (the GPS) is showing stuff (w.points)to be in front of me, then next it is behind me. Really aggravating, especially in open water. I know I could mark them with buoys, but the units specs say less than 3 meters. Bull. Well, it is always 3 meters away from where it thought it should be.:(

I believe most folks know what I am talking about here. There are always some that know more about this stuff than this old dumass.

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Originally Posted by ShilohRed

Tom my 76c for the most part allows me to do what your talking about. I can go by a tree sticking out of the water. And it always shows the same on it. but today when going 5 MPH. It was not showing the same Lad and Lan? Would not have thought that speed would be a factor?
And wonder if the unit was just not getting the signal it does most days?

Now my 987c-SI GPS does show the same bouey in the same place all the time. and its close enough to run by at night. But I always wonder if it may be off enough to cause me to run over a marker? So I for one do not put full faith in it yet.
Will check this again next day I am out on the water. and give you the findings of the Garmin 76C
Shoot Come to think of It I can also do it in the yard on the 4 wheeler also.

How is our buddy Todd doing? If my stress test goes well Friday. I need to get with him and go Striper fishing next week. He likes the pole dancers while striper fishing LOL
Pete

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Let me know. My 76 is pretty accurate. Too many trees for me to try the 4 wheeler, but that is a good idea. Lots easier than in the boat.

I tried trolling a grid with my Tracker. Mod Vs aren't good for this type work. I roamed everywhere along the grid marks. Drunk sailor.:p

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Originally Posted by Cane Pole

Let me know. My 76 is pretty accurate. Too many trees for me to try the 4 wheeler, but that is a good idea. Lots easier than in the boat.

I tried trolling a grid with my Tracker. Mod Vs aren't good for this type work. I roamed everywhere along the grid marks. Drunk sailor.:p

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I want one of these things.

Trimble - GPS Pathfinder ProXRT Receiver

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Originally Posted by Cane Pole

Were you making passes on a fixed object when you recorded this data? What was your setup, speed, etc.?

Thanks for the info.

Tom

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The areas of data in the graphics were not on a particular known point. I was just letting the various gps units remain stable (in my yard). The purpose simply shows the "drift" over time. Think of it as saving a few thousand waypoints on the same exact place in the world over a couple of hours.

Related to your comment in post # 17

"If I am slow trolling and try to cover a "cluster" of close proximity brush piles, I never know what to expect. One minute it (the GPS) is showing stuff (w.points)to be in front of me, then next it is behind me."

I don't know if you mean the location is jumping around or simply the screen. If it is the actual calculated location that is jumping, that is definitely frustrating.

I prefer to keep the map display in the "north up" mode and zoomed all the way in when trying to get on top of a saved point. If in the "track up" mode, the gps may perceive itself as aggressively changing direction when moving very slowly. If your position relative to the waypoint position is jumping around in the "north up" mode, then it is definitely gps inaccuracy.

I use north up. I don't particularly like the jumping around of the maps, besides, moving the maps takes more real time (processing time) and could possibly cause slower response of the NMEA data. I am not privy to the true info, but I understand how processors work. This is probably one of the reasons Humminbird is using dual processors.

If the fish aren't cooperating this weekend, I plan to document some data.

I have a GPS10, BU353 puck, GPS76, GPS276, and Hummingbird 997 to test. I doubt if I can get it all together in a couple of days.

I am not the only person that has the problem. I am the one that gripes about the problems.

Thanks for you time Pat.

Tom

I've not got the quality of equipment that you do, but it is a Waas handheld unit claiming 3 m accuracy. I suppose at times it is if my revisit of the wp is not delayed for very long.
Examples: I marked a brushpile, came back the next day and missed it by ten feet or so.
I marked another one that day, and didn't return to it until yesterday. I am just guessing, but I would say that it would be about a three month span or so. I bet I missed it by almost forty feet.
I had a timber hole marked once, and didn't come back to it until the next year, and missed it by 120+ feet.

This is a Magellen Sportrak. I have noticed this problem in just the last couple of years with this unit. The unit is about six or seven yrs old.

Too much big boat (cruisers) to test gps. Will try again. Probably wait til after Labor Day when the big boat traffic slows.

I have a hand held etrec, and to get it to be accurate i have to find a spot and use a triangluation method marking the spot 3 times with in 10 ' or so of each other. It seems that i can't get any closer than 10 or so feet, so the over lapping circles really help alot when locating a prev marked spot. I also rely on visual triangulation to get me close, old military trick, then dial in with the gps. sometimes i cant find the spot anyways, i believe that current has pushed the cover or that some one has moved it to a spot more to their liking!

I know that y'all are talking bout the upper end units but i thought this would be food for thought

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Originally Posted by stinkies daddy

I have a hand held etrec, and to get it to be accurate i have to find a spot and use a triangluation method marking the spot 3 times with in 10 ' or so of each other. It seems that i can't get any closer than 10 or so feet, so the over lapping circles really help alot when locating a prev marked spot. I also rely on visual triangulation to get me close, old military trick, then dial in with the gps. sometimes i cant find the spot anyways, i believe that current has pushed the cover or that some one has moved it to a spot more to their liking!

I know that y'all are talking bout the upper end units but i thought this would be food for thought

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I have 2 lowrance 5200c [actually 1-5200 and 1 5300] ---they are,nt any more accurate than my garmin map76 ---I use the garmin to get close to the spots most of the time

There are many variables to consider when using these things -

1. Processor speed - Canepole brought this up and I have seen this first hand. I had a Garmin 168 and it was slow to update. Bought a 178 and my accuracy improved as far as finding my beds. It has a faster, more powerful processor and is able to calculate the signal given to it by the sattellites quicker thus improving accuracy. The more expensive units come with better proccesers. Electronics have always been this way. Look at computers and compare prices. Why can you have two computers that look identical but one is more expensive that the other? Proccessor and memory.

2. You have to remember that a GPS is more accurate if your moving slowly in a straight line. You won't get the drift (as pointed out by Hydrogrfx) as bad if your moving in a straight line. (You still will have some variations, but they won't be as bad) Have you ever noticed that when you are moving towards your waypoint in a straight line, you will find it more often that you will if your just going around in tight circles? It seems that when you are going around in circles, you never can catch your waypoint. That's because the GPS is comparing where you are located now as to where you where located two seconds ago. If you haven't traveled very far, you haven't gotten out of that "accuracy circle" and the readings aren't that much different than they were on the first signal. (I know, this is confusing)

3. The time of the year also affects accuracy. We have found that we are able to find our beds much quicker and easier in the winter than in the summer. Some of the reason for this, I believe, is that in the winter there are no leaves on the trees to interfere with the signal. GPS is line of sight. If there is a heavy canopy on the trees the signal is degraded.

4. Cloudy Days - Does anyone have satellite TV? Have you ever lost the signal when a storm was moving in? Same thing happens with the GPS.

These variables in part can explain why you can mark a spot one day and it be off the next. The fact is if you want more accuracy your going to have to spend more money and go for the sub-meter units. And even then you will still have some of the variables I just brought up. As for me, I don't have the money so I will just use my less expensive units and combine my GPS with good ole fashion line ups (triangleation) when I can. I'm still glad we got these thing though. They have been a blessing and a curse. I can now find my stuff quicker but so can everybody else who has run across it and marked it. Also, electronics are constantly improving and becoming more affordable so we can only hope that someday a sub-meter unit will be within our price range.

With relation to LOS (Line OF Sight) You are correct as to why your GPS seems to funtion perform better in winter months than summer months. Your GPS/Satellite signal is surely being affected by the leaves on trees or whatever vegetation is present in your LOS. Specifically it is the water/moisture in the makeup of the leaves that causes what is referred to signal scatter the SATCOM world. Same thing happens when it rains and you lose your satellite TV until the wxr clears a bit. Water in any form will cause scatter to one degree or another more water = more scatter = weaker signal strength = poorer performance from your satellite signal dependent electronics. Hope this helps a little bit in terms of understanding why you're seeing what you're seeing.

PG24