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.