Here, in Florida, it is almost impossible to have accurate maps of bodies of water. The weather changes so much. We have had a year or two that was so hot and dry that many of the small lakes got so low there was no (and I mean none) water at any access point. Then, it started raining and now the water on these lakes is waaaay out in the woods and up in peoples yards. Now, at what level was that map/chart made? There is no way of knowing. I use mine simply to show me where the deeper spots and contour lines are, not as an accurate depth. One lake in particular shows the deepest hole is 9'. Right now that hole is 13' deep.
Now, I talked to a Lorwrance dealer and asked how any of these lakes could all be charted. Some of them were charted by him. He was provided with a recorder on his boat. It recorded the depth of every piece of water he fished and he fished a lot, as will any pro. Now, if he, by some chance missed a small, deep hole it just will not be shown. So be it. You have to understand that to make an accurate map of a lake you have to criss-cross that entire lake in a grid pattern. Who is going to pay the price for that. Be really glad you have what you have.
You could go back to the old depth finders we had when I was a kid. You put the oar over the side of the boat and if you touched bottom, it was shallow, if you didn't, it was deep.