Say you mark a small hump or weed clump (say 10 feet across). How easily can you return to within a cast away?

Most GPS units are very lucky to obatin an accuracy to within 3 meters or... 9 feet. Extra sensitive recievers can get a little better, but usually no more than 3 ft, more like 5 or so. Some units will actually show you this accuracy on the navigation pages. GPS capabilites are much greater, actually becomming to less than a foot, but those are mostly military capabilites and are not programmed into civilian units.

That being said... think of your gps in terms of the exact location of your antenna at the time you hit waypoint. If you mark a weed clump and do it right, by putting it right under your antenna when you mark the waypoint, than the accuracy of your unit is within a 9foot radius around that exact point. (Assuming the 9foot as the example I gave earlier) Now, assuming the next time you are on the lake you approach this waypont with the same 9 foot accuracy as before. It is possible the ring of accuracy for your initial reading was the furthest, most least accurate point 9 ft from your position. When you come to this you could read with the same 9 ft erroring and be a total of 18 ft away from the original mark.  Some units give the option of "averaging" a waypoint.  Doing this, the unit takes several position readings and averages them to make one final reading, in attempt to get less error.  Although it does help some, its not very plausible to attempt this in anything but extreme calm water where you can hold your position for a few seconds.

Now, if you marked the point the wrong way, which would be to mark the waypoint according to what the transducer said and not the location of your antenna, then more comes into play. If your antenna is on the front of your boat and the transducer in the very back, you would have another radial arm of error the length of your boat to factor in.

Most people give way too much credit to their waypoints... it will undoubtedly get you closer than guessing, but its important to remember there is still error involoved.

I guess to answer your exact question, something like a 10ft hump or weed clump is rather easy to return to within casting distance, especially if you were to attempt your marking in the center of it... its the smaller stumps and brush piles that become the problem.

I tried to attach a picture to illustrate my point better but its not accepting it, although below the size requirement... I'll put it up when I can.

Hope this helps.

TommyBass,

Thanks. That helps a lot. I've read the numbers, but didn't know how that might play out on the water -yet. I'll be sure to sit right on my target and just play with it and see what I get.

-Paul

Paul, I can only relay less technical results from my experience.  I agree with Tommy, in that waypoint is best if used to get you to the spot, and antennae location may be a factor.  My antennae and transducer are only about a foot apart (shoot thru hull puck).  The spot on the spot, however is better located with the graph, and be using a feel bait.  You can test this out using marker buoys in calm water, as noted.  You'd be surprised that accuracy varies day by day, hour by hour.  

One thing that might help that I do is drop a waypoint for the "spot" and one or more for boat location to cats to the spot.  This also works well for drifts out in Lake O., where its simply water everywhere.  You mark your structure, and mark a couple of points for your start and endpoints of the drift.

Thanks John.

I have the technical numbers, but wanted an idea of what to expect on the water. Your's and Tommy's experiences and opinions are just what I was looking for.

Paul, I can only relay less technical results from my experience. I agree with Tommy, in that waypoint is best if used to get you to the spot, and antennae location may be a factor. My antennae and transducer are only about a foot apart (shoot thru hull puck). The spot on the spot, however is better located with the graph, and be using a feel bait. You can test this out using marker buoys in calm water, as noted. You'd be surprised that accuracy varies day by day, hour by hour.

One thing that might help that I do is drop a waypoint for the "spot" and one or more for boat location to cats to the spot. This also works well for drifts out in Lake O., where its simply water everywhere. You mark your structure, and mark a couple of points for your start and endpoints of the drift.

Good points.  I wouldn't necessarily want to drive up directly over the top of my marked spot unless it was in pretty deep water.  I'd rather get within pretty close range and then switch to the TM and find the spot with the graph.  

I wouldn't necessarily want to drive up directly over the top of my marked spot unless it was in pretty deep water.

Exactly. I'm fishing shallow and clear water, so I need to stay back. I don't want to have to relocate with the graph either if I can help it. In many cases I need to stay a long cast away.

I presently use marker buoys, but am limited in the number I can carry -and they are a pain. I was actually hoping to lose the buoys. I'm hoping a waypoint in conjunction with a line to a distant object will allow me to mark spots without buoys. Possible?? Is the accuracy (variability) of satellites going to allow this?

Help me out with these waypoints... I haven't used them too much but have marked a few stumps so I can avoid hitting them.  On my Lowrance 520, these stump waypoints are designated as "007" or "008", ok great I know that means that's the 7th and 8th waypoint I've saved, but on my screen this 007 is huge and one stump in the lake is really small. There doesnt seem to be any kind of reference to let me know where the stump is in relation to the "007" waypoint I made. I mean, is the exact waypoint/stump before the 0, after the 7, above the 007, below the 007, right in the middle?? If there was a little dot next the waypoint number it would be easy to understand but there isnt.  How can I be sure I'm within 9 feet of my waypoint if the waypoint marker takes up 100 feet on the map?  Thanks.

Well, my unit isn't even wet yet but...

the manual says my unit has a distance to target number available on the screen, and an alarm. Will know more after I've used it.

Paul, why don't you just fire her up and mess around in the yard with the sonar off?

Exactly. I'm fishing shallow and clear water, so I need to stay back.

Once familiar with the bottom contour, you are going to want to turn off the sonar anyway.  Bass can and will respond negatively to the incessant pinging.

Paul, why don't you just fire her up and mess around in the yard with the sonar off?

I will. As soon as this front leaves! It's been rather rotten out there the last few days. I'm chompin' at the bit. I guess the next best thing is blabbing on about it! ;D

Gotcha!  We have had yo-yo temps the past week or so.  Almost 70° one day, snowing the next.  Pretty typical Rochester schizo weather, LOL.

OK, ran a dry land test.

Hiked to a meadow out back and set waypoints (WP) at a rock pile, a shrub, and a set of 3 stumps. Targets were roughly 8 feet across.

I set the proximity alarm to 0.1mi (~50ft) -lowest it would go and a reasonable casting distance from the target. The unit gave me EPEs (Estimated Position Errors) of ~20 to 45ft.

But I was able to receive the proximity alarm consistently within a cast away from the target. Now these are objects I can see, so who knows what it'll be like on the water. But since my waters are small I should be able to orient visually pretty well.

One neat thing that might pan out on the water is that the unit draws a line from me to my WP. The unit appeared to be pretty sensitive about my immediate position from that line. Trying to run this test blind I looked only at the unit and adjusted to the line and came up a cast away each time. This with a shoreline object to line up on should give me something to work with.

Because the fix bounces around I would slow down as I got within about 75ft (as per the unit) and move in slowly. This kept me from blundering into my safe casting distance.

WAAS (as promised) kicked in and out, and at one point appeared to throw everything off terribly. I set a WP under WAAS, and when the unit lost WAAS as I was approaching, it appeared to throw my WP way off. This only happened once though, so I'll just have to see whether that pops up again.

So...we'll see how it works out on the water.

Oh, btw, the unit is a Lowrance M68c.

WAAS works much better in the deep southern states.  It's going to be hit/miss the further inland you get.  With my old gps, I was lucky if I could find come back to within 50 feet of my waypoint.  I can usually get within a few feet of my WAAS unit.