GPS Mapping Accuracy? Channel Markers:

Discussion in 'Boat Safety' started by blackwaterkatz, Jan 13, 2007.

  1. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    I'm curious about how accurate the mapping programs are for inland waters. Has anyone else seen a problem, either on santee cooper, or other lakes?
    I've owned a Garmin and now a Lowrance gps, but the channel markers on the Santee Cooper lakes, Marion and Moultrie, are not where the map shows them to be, sometimes they're off by 1/4 mile or more. When I had the garmin "Recreational Fishing with Hot Spots" program, I notified Garmin, and they said the mapping was done by someone else, and all they could do is forward the information to make them aware of it, and hopefully a newer version would come out with corrections.
    So, I was with a friend who had a "Lowrance with Navionics Hot Map Classic" software, which I now have, and his was the same way, at least in Lake Moultrie. I also have the "Fishing Hot Spots Elite" software for my Lowrance, and it is the same way, almost none of the channel markers are where the map says they should be, and the channel from the 'brickyard' area across the lake and down to Randolph's landing even has the colors (red/green) reversed. As far as contours go, they are fairly accurate. My salt water charts show the marker buoys in coastal waters just where they should be, too.
    Now, this may not be a concern to a lot of folks, but for those of us who travel santee cooper lakes, especially in foggy or night-time conditions, it's downright scary. I took the time when I had my Garmin to mark each one of the channel markers individually, so I knew exactly where they were at night, and I'm in the process of transferring those waypoints into my Lowrance lms-337c.
     
  2. dougc

    dougc Active Member

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    1,711
    State:
    Independen
    On one of my buddies new Lowrance (don't know model #) units, it will show you running up on the bank of the river!
     

  3. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    Doug, that's not too unusual, when using the mapping that comes included in most units. If you want better accuracy, it will take a mapping program with more detail. Most of them are really accurate, overall. The mapping that comes with most units is ok, as long as you don't need a lot of detail or accuracy.
     
  4. ShilohRed

    ShilohRed New Member

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    4,339
    State:
    West Tn
    My navionics Premium maps shows the Markers right where they are on Pickwick lake.
    Now my hot spots maps are way off. But not the Premium.
    Pete
     
  5. Bubbakat

    Bubbakat New Member

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    4,532
    State:
    McMinnvill
    I don't know what kind Bam uses but he and I were on the land between the lakes and his was within 20 to 25 feet. Give or take a couple feet. Pete might know what he runs but at night I was very impressed with its accuracy.
     
  6. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    Thanks, Doug, Pete and Willard. My Navionics is the Hot Map Classic. I haven't used the premium version. My reason for starting this thread is to try and determine if the errors are unique to certain lakes, certain software, or what, and then I plan to contact the folks that make those maps, try to determine where the incorrect info is coming from. In the case of santee cooper, it may be caused by bad or outdated info from the Corps of Engineers, who I think are responsible for those markers, not certain. Some of those markers were replaced after Hurricane Hugo in 1989, when the channels were cleared of trees and other debris, or at least that's what I've been told.
     
  7. Dreadnaught

    Dreadnaught New Member

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    5,444
    State:
    Henderson,Ky
    Heck, The Ohio River has been up and down so much lately, that the channel markers are either gone or moved down river a bit. There are a few in the area that are still in the same place but very few.
    The Army Corps of Engineers needs to get them back in place quick, before a barge gets into trouble somewhere!!!
    As for the Units you are talking about.... I don't use them!! Not that they wouldn't be nice to have!!!
     
  8. Mr.T

    Mr.T Active Member

    Messages:
    2,554
    State:
    MO
    I would *never* rely on a GPS map for navigation in the dark or in foggy conditions. The big notice that comes up when you power up the unit is there for a reason -- you're not flying an airplane with certified navigation systems; you're driving a boat using a cheap electronic device and a cheap map compiled from mostly unverified sources. Plus your GPS unit doesn't tell you when it falls below any particular threshold of accuracy; navigation systems in aircraft are required to warn the pilot if the system isn't providing accurate information. .

    As DougC mentioned, I've got the Navionics Premium map card in my LCX-25C and it has detailed maps of the Missouri River among other things. Unfortunately the river map is nearly worthless -- if I followed the marked channel on that map, I'd be on dry land much of the time. There's no telling what source the map data came from or how accurate it was when it was collected or even when it was originally collected. So it's just a thumbnail sketch of the actual river; most other maps are just the same -- they'll give you a reasonable idea of what the shape of the lake or river is and how deep it might be but there's no guarantee of acccuracy.

    The only condition that I'll use my GPS for with respect to navigation is following a trail I've made previously -- and I use that feature quite often to find my way back to the ramp at night, knowing that the path I took on the way out was safe and if I follow the little pink line back, it'll put me in approximately the same place. But even then, I've got the spot light running and I'm constantly checking my position shown on the GPS with where I am on the water. In foggy conditions, I wouldn't dare move at more than idle speed, GPS or not.
     
  9. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    Marty, thanks for your input, buddy. If that's the way you feel about things, that's fine. Everyone has different opinions of things. Mine is that I paid a lot of money for my gps units and the software, and I expect those charts to be accurate, so I can use them to full advantage. I understand the part of the warning that comes up. That's called "CMA", just like the warning on lot of products, etc.
    When I bought my gps units, I did so with the intention of getting the most out of them that they are designed for, and that's what I'm trying to do. If I didn't trust them enough to use them in the fog or dark, I would have saved a lot of money and bought a basic unit. I have, in fact, used mine several times to get home in heavy fog, and it did a great job. When I'm running the narrow creeks along the coastal areas, my NauticPath chart shows me what's ahead, and it's always been right. If I see a channel marker or buoy on that chart, I'm 99% sure, from experience, that it will be there. Now, once I get into rivers, that's different. I use topo software, and the river itself is right on, so far, at least, but it's not detailed as much as the lakes and other nautical charts are, of course.
    Now, I've learned to trust my electronics, although I understand that they can fail, or satellites can sometimes not be received clearly (and my Lowrance and Garmin do tell me when there is a problem with reception, and a reasonable accuracy circle), which is why there is a compass mounted on the console, a set of charts in the storage area, and, if I'm in unfamiliar waters, a back-up handheld gps unit.
    Most of the Navionics and Hot Spots mapping programs appear to be designed primarily for lakes and don't have a lot of river detail, except in areas where larger commercial vessels travel. I've used mine in the James River, VA, and Cooper River at Charleston, SC, and they've been quite accurate, channel markers included; can't say about any other large rivers, though. Those ships navigate with gps and charts, as well as radar.
    None of the maps or charts are any more accurate than the information used to produce them, of course, as you mentioned, which is why I brought up the discrepancy in locations of the markers. Channels in rivers change with conditions, but these markers are firmly anchored in the lake bottom. If I mark a waypoint on my gps, I can usually return to it with a 30-50' range, on average, from my experience, but these channel markers are off by 400-800 yards in many cases, and another thing that I should have included in my original post is that the channel in these lakes was cleared by divers so boats would have a safe channel through the lenght of both lakes. If you get out of that channel by even a few yards, there is a significant risk of hitting submerged timber.
    None of what I've written hear precludes prudent vessel operation and safety procedures. I, too, slow down to a safe speed in low visibility conditions, like most boaters, do, and I use spotlights and such as needed, even stop occasionally and listen for other boaters in the fog, but I still expect my electronics to offer a reasonable degree of confidence that they are doing what they should.
    This is my opinion only, of course. I'm sure there are others who might feel differently. :smile2:
     
  10. cat tamer

    cat tamer New Member

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    MO
    a
    Marty, I agree with how you feel about not relying on the gps, it is ok for a reference but to trust your safety to. with my system even following a previous track I can be as far a 100 yards off couse before it will show a deviation.
     
  11. troyedm

    troyedm New Member

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    83
    State:
    edmond oklahoma
    my garmin etrex is close. i use it to get me close to fishing coves when i was on lake mead and mohve in las vegas. it also helped find my way back to camp when it was dark. but all it would do was get you close so you could look and see where you needed to be. the one that runs on my laptop is a lot more accurate. i don't trust my sounder also, ive been in 2' water and had it show 6'. its a garmin 160.
     
  12. ShilohRed

    ShilohRed New Member

    Messages:
    4,339
    State:
    West Tn
    SAme one I have.
    Pete
     
  13. Mr.T

    Mr.T Active Member

    Messages:
    2,554
    State:
    MO
    I suppose most of my caution and reluctance in using a cheap consumer electronic device to guide me through hazardous conditions comes from being an instrument-rated private pilot who uses GPS for enroute and approach navigation, and understanding how those (incredibly expensive) units work.

    In an aircraft GPS navigation system, the whole system (electronics and digital maps) is certified to be accurate and (most importantly) to tell me when it's unable to provide guidance that is accurate enough to use safely. When the system works properly, it'll guide you right to the runway (something that still amazes me every time!)

    But the $700 Lowrance unit and the $150 Navionics map card I have are not certified for *any* type of navigation use -- as the warning says, "data is provided for reference only" -- and won't tell you if the unit is not performing up to par -- you could be limping along receiving only 3 or 4 satellites, giving you an EPE (estimated position error) of several hundred feet and not know it. If your unit has an option to "Require WAAS" and you're going to rely on your unit for navigation, I'd definitely recommend enabling that option; at least then you'll have a clue when the WAAS satellite isn't online and keeping the data from the other satellites in line.

    If you choose to bet your life (or, perhaps less dramatically, if you choose to bet your hull) that the information it's showing you is accurate, please be my guest.

    But all it would have to do is show me a single navigation marker or bridge or dyke that isn't where it's supposed to be and I'd lose all faith in that system for anything other than giving me a general idea of where I am and what's around me.

    And in my experience so far, I've seen my boat several hundred feet up on "dry land" according to the map, while I was really right in the middle of the river channel. Plus I've seen my boat, on the trailer at the ramp, shown as being a hundred feet or more out in the water. That's enough for me to know better than to trust the information I'm getting.
     
  14. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    I'd like to try one of those premium chips sometime, Pete. I think Doctor might have one, maybe I can try his if I make it to the Tennessee gathering in feb. What model/brand unit do you use?
    I have no problem with the accuracy of my gps, it's the accuracy of some of my map data I'm questioning. I'm using some location numbers that I've had for years, and I can go back within easy casting distance, or less, of any of them consistently. The only time I had any problem was when I had installed a new sonar with 4000 watts within inches of my garmin gps. Sometimes it would show me that the signal was lost or degraded. I relocated the sonar a few more inches away; problem solved.
    We were up a river a couple years ago one night, and as we got ready to head home, the fog settled in on us. I couldn't see either bank of the river, which was barely 100 yds wide most places, and the spotlight would just glare in the fog, almost useless. I couldn't run close to the bank because of log piles. I zoomed in to about 300' detail with the gps and followed my trail back for 8 miles (at idle). Took us 2 hours to reach the landing, but without the gps we would have spent the night. The only time I got off course was when I took my eyes off the gps for a minute, and we started showing really shallow water. I corrected per the gps trail and got right back on course.
     
  15. special liberty

    special liberty New Member

    Messages:
    295
    State:
    Maryland
    I got the Navionics Gold + for my Lowrance 332C for myself for Christmas. Been very happy with it on the Tidal Potomac River in MD/VA/DC. Channel markers, wrecks and other structure seem to be dead on. Wasn't cheap but was a huge improvment over the Map Create Ver 6 that came with the unit.
     
  16. Doctor

    Doctor Member

    Messages:
    378
    State:
    Springfield, Ohio
    When I was down at the National Gathering mine was dead on to the channel markers out on the lake, I have noticed on the river that they can be off a tad but when they pull those units they sometimes don't get the new one dropped back into the original area plus the channel seems to change after a major flood.

    I fished a tournament 2 years ago on the white river, man it was a twisty windy river we were about 10 miles down river and had been fishing most of the night when the fog rolled in we idled back to the ramp using the cookie crumb trail left on the map it was dead on also cause I had marked some hazards along the way and I told Lynn we should be coming up on a snag, I don't like to trust them but there are times when you are put into a position that you have to again I don't go flying like a bandit but just idling along feeling my way back, while motoring on the Ohio I always run down around the bank even in the dark the river can get you turned around add fog and it's even worse.

    Seems like sound travels even further in the fog, guys sitting on the bank on the opposite side of the river sound like they are right next to you, and if a barge is traveling they hit them air horns and Man just scares the bajeebas out of you.........Doc
     
  17. WylieCat

    WylieCat Well-Known Member

    Messages:
    7,177
    State:
    NC
    You are dealing with a margin or error TIMES two.

    You have the GPS margin of error that affected the original plotting of the markers. This may have been from 9-40 feet depending on how many satellites they were receiving when they entered the points. It also has to to if they took an exact reading or from a position near to the buoy.

    THEN, you have the margin or error that affects your machine on the day you are using it. You can pull up that information and see how far off it is for that particular day. Add those two together and the markers could be 10-75 feet from where they really are.

    Obviously the machines are not intended for precise navigation. The only thing I would rely on in fog or darkness would be radar, and then I would not be running faster than I could react to something in front of me.
     
  18. jim

    jim New Member

    Messages:
    2,579
    State:
    Jacksonville NC
    Interesting,totally agree with the caveats everyone else have stated.Dieter ,good point but that presupposes that they actually went out and GPS'd the markers instead of transposing from a chart.Tommy I think the 2007 Premiums will be much more accurate than the old "Hotspots".As many dimb a---- as have knocked down those markers I doubt the new ones are exactly in the same spot so some error has crept in.I would make a trail which is what I did but again whatever the satellite error is the day you make the trail is built in.My personal feeling is the good Lord has made a major investment in getting me to the state of perfection I'm in,so if I did something stupid and killed myself he would probably sentence me to fishing for eternity in a lake that only had BASS in it.:big_smile: :lol: :smile2: :eek:oooh:
     
  19. roundhill

    roundhill New Member

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    810
    State:
    kings mtn
    tommy if you pull it all the way in they are pretty close but like i said start at #1 and mark them it want take to long.
     
  20. crazy

    crazy New Member

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    2,090
    State:
    Kansas CIty, MO
    The map is just there for an idea from what I have learned. Other wise I'm going with the trail that I lay down. I don't care how accurate you think the map is. If my trail says i need to drive 30 yards up on bank to be in the river channel by god I'm going to put the boat 30 yards up on bank. On a side note you would think the map's by navionics would be accurate but the one I have is not. I mean heck the hard maps by the corps of engineers is accurate. The day markers don't move on the bank. Also every spring the coast guard comes in to lay down the channel marker buoy's. By using side sonar and what ever else technology they have on there big boats.