I hope nobody reading this thinks we are crazy LOL. I'm only continuing this discussion because I feel like people are learning from our dialogue. If it helps someone than it's worth it.
Norman, To further what Doug stated. Sonar is a CONE it has a circle for a footprint. It has no perception of left or right, frontwards or backwards. For instance. I have TWO brushpiles in this image below. How do I know this? I put them there and they are approx 15ft apart.. I want you to put your cursor on the LEFT one? Now do you understand? You can't put your cursor on the left brushpile because the sonar doesn't know left from right it lumps them both together on the screen because they are both in view of the sonar cone. Down Imaging works similiar.
Side Imaging knows left and right and that is why you can use the cursor effectively. Keep in mind I said MOST accurate GPS coordinates with 200khz. I didn't say you can't mark a brushpile in any other beam cone to mark brush because you can. If you happened to run over it with the 200khz cone in dual beam then yeah it will be accurate. If you run over a 30ft tree and mark a waypoint then yeah you have a good shot at being able to snag the tree with your spoon. But will you be fishing the trunk or the top of the tree? But if the structure is off to the side of the 200kzh beam then it will be off some.
Here is the sonar shot with TWO brushpiles in the sonar image. Which one is the left one and which is the right one? Or is one in front of the other?? You have no way of knowing with sonar..
To further illustrate my point. You made the comment that the GPS marks the TOP of the water.
nwright - 8/3/2010 12:26 PM
Basically the GPS is marking the waypoint on top of the water, not in the water column.
You are correct and based on that fact look at these diagrams. The RED X under the boat marks the GPS points. Both of these trees are within the cone beam and show up on your sonar. Which GPS point is more accurate? The one directly under the boat in the 200khz or the one off to the side??