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GPS & Estimated Position Error

Discussion in 'Boats and Motors' started by blance, Sep 7, 2005.

  1. I have a Lowrance LMS480 combo unit with the GPS module. After seeing myself trolling up the side of a mountain at Kinzua according to the unit, it led to an interesting discussion between some of my muskies Inc chapter members back at the ramp. We set the unit to display EPE (Estimated Position Error) and was quite suprised and the numbers I seen. I was averaging around 50 ft of error while in the mountains. At home, on Shenango Lake, I've seen it anywhere between 17 and 75 ft. Sitting in the driveway, I've moved the puck all over the boat (both high and low) and the closest I can get is around 35ft of error (with WAAS). I even went so far as to pull the cables out for the GPS puck and rewired power back direct to the battery to eliminate interference with no increase in precision. Also installed a software update (1.7.0) for both the head unit and puck with no change. Can't say if I've experienced this seemingly high amount of error prior to the last month or so, because I never checked it. Just curious what some others (especially Lowrance owners) are getting for EPE. Also anyone have any suggestions I can try to help make the gps in this unit more accurate? Seems kind of useless to try following breaklines using the Navionics maps when the best I can hope is to come within 15-30 ft of it without constantly staring at the sonar while trolling.


  2. I would email Lowrance ;)
  3. I have a Lowrance GlobalMap 3200. I mainly use it at Lake Erie and find it to be extremely accurate for the spots that I mark. Time after time I can go back to a hump, rock pile,etc usually find it right away.

    The breaklines on the Navionics maps aren't necessarily going to be dead on but the spots you mark should be next time you go to use them. I've experienced similiar problems depending on the lake you are fishing. I think some lake maps on the cards are just more accurate than other lakes. Lake Erie is probably pretty good because they knew it would be a popular seller and therefore more mapping and accuracy/effort went into it.

    Just my guess.. I don't recall what my EPE was but next time I'll have a look. I know this can change at any moment depending on how much the government is scrambling / detuning our gps signals.

    Kudos to Lowrance, I just received a brand new puck, cables and cigarette lighter adapter cable so I can use my GPS in my truck. I talked to them at the BassMaster's classic and they said they could send me one for FREE and did!!! I guess it pays to go to the show! :)
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  5. eh, I wacked that response. Figured, it was awfully long winded and not really appropriate for this forum. Basically, if you take a serial gps (which most are in some fashion or another including USB via a software bridge) and connect to it in hyperterm at 4800-8-N-1 (I think), you will get an ASCII stream per the NEMA protocol spec. You can read that stream in any app, even your own. There is a Rockwell binary format and usually most vendors like Delorme like to use that in their apps, but the hardware can usually be shifted into a NEMA mode as well through some means. In the case of my unit, and yours too maybe, it would need to be a strictly NEMA standard stream due to the ability to use the second set of data wire to connect to a VHF for sending your coordinates in a mayday call to the coast guard.

    What the scenarios boiled down to is that the EPE needs to be calculated from somewhere, somehow. Does the head unit take at lat/long it receives from the puck, convert it to an (x, y) screen coordinate, applying any correction factors you put in during GPS initialization to draw the screen point. Then use the screen point and GPS data to use either a least distance theorm between points or great circle navigation between lat/longs to determine EPE. There has to be some point of reference used in this calcuation otherwise how can you say there's a position error. If lowrance is storing everything in a unit-corrected (x, y) format, that would explain why you were seeing errors on trails and waypoints between different units because the likelyhood of them both containing the same gps corrections factors would have to be dang near zero.

    Another possibility is that the unit store unit corrected lat/longs or the raw uncorrected lat longs. If the uncorrected ones were stored, I would think that would provide maximum portability of data between head units at the expense of lots of cpu overhead. Storing corrected lat/longs would reduce the cpu overhead at the expese of portability. This lends itself back to the scenario you mentioned about sharing points between a friends unit and your own.

    If my theory holds true, any error you introduce via the initialize gps option should serve to influance EPE one way or another. What I need to do is devise a scenario where I can test this theory. And if true all my waypoints that are in the unit now should be all out of wack. Right now, when I try initializing my unit in my driveway, I think I'm introducing some unknow quantity of error into the unit because my drive way isn't listed on the map and best I can do is guess where it might be. If I can initialize it at some known street intersection on the map, I may be able to reduce the error that is influencing the EPE. This may also explain why your unit was so accurate on stoplights. the base map would give you a good shot and choosing a good initialization value on the street, but not so good once you get off the main roads and reattempt the initialization. Make any sense??? I'm thinking it may be worth a shot anyways. Just some thoughts I was having that proved to be quite a usefull distraction from doing any real work today. :D

  6. With that and a hot cup of coffee, you WILL boat more fish!
  7. I have the Eagle FishElite 480 GPS/Sonar. Got it this spring, and I have also found that, with the Navionics map, I am often shown to be on land when I'm a good 30 yards from shore (at Alum Creek Lake). Meanwhile, my position error has never been that bad according to the unit. I was thinking that it was the map, since, as others have said, I seem to be able to repeat my location pretty well going over an old track or to a waypoint I set myself.

    So, to me, it was more of a waste for the $200 Navionics map than a problem with the Eagle unit...
  8. There has been a ton of discussion on walleye central about this subject. Go there and do a search. I think the biggest problem is in the accuracy of the map. Depending on the map vendor and then their source for the map determines the accuracy of the map. Some companies have used the lousy ODR maps. You can imagine how in accurate that must be. I think Lakemaster is actually surveying some specific lakes now and then some selected fisherman to notify them of inaccuracies. Anyhowa lot good info on this subject at walleye central as I said. Just my opinion.