echomap 7c0dv quickdraw and waypoints about 30 ft off.

[mjk1210]

Hi guys. I've been using the quickdraw countours on my echomap 70dv and marking rock piles. I've shaded certain depth ranges on the local lakes I have been mapping and I consistently see that I am not in the same depths as the shaded areas on the maps I have drawn. It seems that I am usually about 30 ft or so southeast of the exact spot or underwater point that I have mapped previously. It makes it very difficult hit certain contours and drop offs/points or any target in general. Any ideas what I can do to correct this problem?

Thanks

[Larry3215]

Not sure I understand exactly whats going on. Are you setting waypoints on the rock piles as you map, and then when you go back they are in the wrong place?

If its always off in the same direction and distance, it could be you have the wrong GPS MAP Datum. See what its set to now. I think it needs to be WGS84.

You might try a test. Get near something you can use as a refference - the end of a dock, etc, and drop a waypoint. Then move away and try to navigate back to it and see if its in the correct spot.

[mjk1210]

Thanks. I will check that setting and basically yes, you understand correctly. The way points have all been in open water where I've seen rock piles so those are hard to find again but good idea at the end of a dock or a fixed object I can see. Mostly I notice that all my quick draw maps show depth shading that doesn't match the depth I'm in. For example, yesterday where I was fishing at an old strip mine lake there was a very long underwater point. It's only a couple feet over the "strip" and the depth falls off from the sides. I've mapped the whole area before with the quickdraw and wanted to fish the 5-8' depth zone that I have shaded as light green on my map. I zoomed in and even though the boat icon was clearly moving along the contours for that depth range on the screen the sonar was giving me depth of 14 and 15 ft . I calculated by the legend that I was off the contours I wanted by about 30 ft.

[skeeterbrain]

Looks like you are having multiple problems. Best thing is to call customer support. You may just have a bad unit.
CS is great, I had 2 units that had a problem after an update ( lost internal mapping) and they replaced both units.

[mjk1210]

Thank SB. I noticed this problem just a couple weeks ago and didn't want to send it in due to a tournament coming up. I'd rather have the maps and waypoints be off than have no maps or waypoints at all.

[Larry3215]

This could be a GPS location issue or a map datum issue or a depth recording issue - or, like skeeterbrain said, some combination of all three.

Try the test I suggested above near a dock, etc, and see what happens.

If thats working ok, then I think the next most likely issue may be with how you are doing your recordings. Are you running parallel tracks that are very close together or overlap?

A single pass with quick draw will only have accurate contours in the very center of that pass. The outer portions will not be accurate - especially in areas where there are rapid depth changes. You need to run several tracks side by side and very close together or preferably over lapping to some degree. If there are rapid depth changes, then a second set of tracks at 90 degrees will help to get more accurate detail.

The reason for this is the way QD records the contours. Unless you are using a PS30, the only accurate part of the recorded track is that portion directly under the boat. On any single pass, QD draws contour lines at 90 degrees to the direction of travel. So, if you are driving over a section of bottom that is 10 ft deep to the left, 15 ft deep under the boat and 20 ft deep to the right, QD will draw contour lines that are at 15 ft all the way across the path of the boat. You wont get accurate depths at the edges untill you make a second pass next to the first pass, and so on and so on. The outer edges of any QD path will always be suspect.