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Bigbartguy

GPS accuracy question

17 posts in this topic

I have an H20 C and have been using it on the highway and have noticed that it (pretty much) NEVER shows me actually on the road. It often shows me 1 street over from the road that I'm actually on...a difference of 75-200 ft at least.

But I've used rental car Gps's before and they are dead-on accurate. To the point where they say 'turn here' etc. I can actually see which side of the road I'm on with these units. Very little accuracy issues.

does anyone know why this is? is it an accuracy issue with the H20 highway map? Or are the car gps units just that much 'better'?

and does anyone know if this accuracy issue carries over into 'fishing/lakemap' use?

I know that Govt. required a certain amount of error for security reasons, but the accuracy difference between the 2 gps types seems to be huge!

thanks for your input!

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My hunch is that it's the map that is off and not the GPS. I imagine the companies that make auto GPS's spend a lot more time getting the roads accurate while Lakemaster is less concerned with the roads than the accuracy of their lake maps.

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Bart, It probably wouldn't affect accuracy, but there was some hype about software patches for daylight savings time on units sold before March of this year. Does yours automatically switch or do you do it manually? Might be worth a check and/or an update!

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There should be a spot on your H20 that'll bring up what accuracy it's showing.

On my Garmin Nuvi 350, I can run it down to an accuracy of 18'.

Also, the mapping for most of the auto units (as of last Christmas) was done by the same company, except for Tom Tom, which used their own maps.

I would check your accuracy, if you can. Maybe you have a bad unit as well???

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ready for some knowledge???

Im a tutor for a forest feild skills class at VCC in Ely, MN. Historically speaking gps units were off due to gov and military reasons but thats a thing of the past. The best of the civilian versions are still gonna be on average 20-as much as 80 feet off and thats radius. So on your waypoint you could actually be anywhere within the epe( error) radius. My H2o Does the same and I found out my Ely quad topo is actually about 300 yards off according to my topo lat long utm tool and the actual gps corridnates. hope this helps

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It's almost certainly your Cartography, not the GPS. Have you uploaded a detailed map to your unit or are you relying on a basemap?

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MaGuyv - I havent loaded any updates, I'm not even sure if I can or how?

I'm using Lakemaster Promap 2007 software. I dont see any accuracy adjustments available but maybe I'm just not looking in the right spot. My EPE shows between 20-50, which I understand to be some kind of accuracy measurement.

it's still much further off than 20-50ft when driving down alot of roads. Has anyone else noticed my issue with their H20? I'm tempted to think it's the highway software that's included with Lakemaster?

I guess it really doesnt effect my overall use, as long as the lake software is much more 'accurate' than my current highway accuracy.

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My H20c has the navionics chip, so no fancy road maps. I just have the base Lowrance highway map which also shows similar road inconsisitencies. If I used it for highway navigation, I would also be disappointed. A 50 foot EPE could easily be off the roadway.

I use it mostly for fishing/boating and that seems to be very consistent. The EPE is estimated position error. It gives you a confidence interval on your present position accuracy. I often get 14 feet on the water which means I have a good line of site to the southern WAAS satellite. This means that the unit is showing current position somewhere between 0 and 14 feet of truth.

I've sort of verified this through experience finding small underwater structures. It gets me pretty darn close when I have low EPE.

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Does anyone know why the H2O C is always so far off. Mine seems like it's always in the 30 foot range.

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I think 20-30's is about the lowest you will get without picking up the WAAS satellite signal. Mine only gets this with a good line of sight to the south (with no interuptions).

Do you have the WAAS option "on"? If so, try going to an open field/lake with good southern exposure. Wait a few minutes and see what happens.

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I've wondered the same. I kept a close eye on EPE while driving in southern Minnesota. WAAS was more active and EPE was usually 15 feet instead of 30.

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My WAAS is on, must just be my location all the time. Thanks for the info.

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I wouldn't get too hung up on the roads; my Garmin Legend has always wavered around on the roads pulled from the Roads and Recreation software they put out. It's never so bad that I can't tell if I'm on a road or not, or which road I am on when they are close together. It'll get you where you need to go.

As far as the accuracy goes, It seems the more I use mine, the better it gets. I know that that cannot be the case, either it's good or it's not. When I first got it, the best I'd get is +/- 17' and often in the 30's, but I can now consistantly get +/-10' accuracy (according to the unit) or better, even in the woods at times. Mornings are better than afternoons, and the more I move the better it is. The best I've gotten is +/-6' with WAAS on and battery saver off; but 20' or better is all you could conceivably need to get you to the spot. A flasher or camera will get you to the "spot on the spot."

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My Garmin handheld (Rino120) rarely shows accuracy greater than about 10'. Most common is within 9' and occasionally as low as 7'. I use it in Canada and all over Minnesota, driving, fishing, hunting forest or field, and ATVing, with the same results. It doesn't seem to matter whether I am in battery saver mode either and I usually have WAAS enabled.

Maybe unit quality? grin.gif

Bob

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As has been mentioned, the cartography is only as good as the source data. If not from a third party transportation specialist, it's likely to be slightly flawed.

Another key problem with folks seeing poor road accuracy issues is the lack of an external antenna. Depending upon satellite positioning in the sky, just having it on your dash does create some line-of-sight issues with a few of the satellites. While an external antenna for the car will not allow you to surpass accuracies of ~2-3m, it'll put you in the 4-6m range depending on GPS type/brand.

I'm of the opinion that esp. with the roads databases, you could double the error and still be fine.

Joel

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When I first bought my H2Oc last winter, I walked out in front of the house while it initialized and I marked it as my first waypoint. Over the last year I go back to the spot with the GPS and have never been off by more then 10 feet. (yes I am easily amused by blinking lights) So i would say that the accuracy is pretty right on.

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Though not usually the case I have noticed myself leaving a roadway on the GPS when I am driving on a recently constructed/rerouted road. Sometimes the construction was over a couple years ago but the map was not updated.

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