As a practical matter, the most accurate GPS location for a particular site is the one YOU mark when your bottom finder indicates the "spot".
Each GPS unit seems to be a little different than the next but tends to be the most accurate when returning to a spot recorded by itself. GPS charts are only as accurate as the unit recording the site. Some charts have GPS numbers that have one less digit than your unit is capable of. Try adding the number 5 in the third decimal place--sometimes this average number will help.
Also, some sites like wrecks are large enough to get several "hits" on as you pass over and watch the bottom finder. Mark several...usually one will find the spot better than the others.
When approaching your destination, slow the boat down but don't stop--it screws up the readout and cursor (pointer) direction. Don't immediately react to a sudden cursor change--it may revert back in a second. Watch the feet to your target. Once it starts to get larger you are past and you should spin around.
Remember, most small boat bottom finders are attached to the bottom of the rear transom of the boat. That could be 10-15 BEHIND the position of the GPS. The net result is that you might read right on and the bottom finder shows nothing...yet. On a large dive boat, the bottom finder transducer may be amidships through the hull--this can make a big difference.
Once you find the site, head into the current, wait for a "hit" and heave the anchor in front of the boat, letting it free fall as quickly as possible. If you have a winch, this might be a problem...some winch down rather than free fall.
Grapple anchors are best for recks or reefs--then tend to fall more quickly and straighter than "Dansforth" type which seem to flutter and can bounce off the deck of a wreck. Personally, I like to have an extra lenght of chain with a SS caribiner to lock the anchor to the wreck. It's a real bummer to return to no anchor.
Take into account wind drift just like water current. Your boat may be "crabbing" to one side or the other even while being carried by the water current.
Once you are tied off onto the site, wait a few minutes and check the distance to your target. If it is relatively constant, great but if you see it getting larger, find the strongest guy on the boat and assign him to pull the stinking anchor back up!
I carry a Garmin GPS Map 75 . It has downloadable charts and is a great back up unit and perfect for "stealing" secret spots.
Ialso have an old Garmin 48. Both can be hooked up to an external antenna. The 75 monitors up to 12 channels and has WASS.
My buddies who have "combined" GPS/bottom finders don't like them too much. Until recently, you could only get the GPS screen OR the bottom screen but not both at the same time. At the Miami Boat Show last week, I saw a Garmin with a split screen that showed both simultaneously.
Finally, if your targets are within site of shore, line-ups are even more accurate than GPS. I know a bunch and can hit a target on the first pass instead of circling with the GPS and then heading up current. This won't work out of sight of land with no widely separated points. The farther apart two alligning points are, the more accurate the line-up is. You can also use the width of you finger, two fingers, etc to "space" the line-up targets. Do this with an outstreatched arm for consistency. Line-ups need either two sents of line ups to triangulate OR one line up and an known bottom depth that is specific to that site. You go to the depth, run that depth line till the line-up is coincident.
Weather may also alter GPS readings. Sometimes, they just seem to wander. If the differential is off it's harder and if the WASS is down it's harder. It used to be that in times of national crisis, these methods of accuracy were turned off...I guess they figured an A bomb wouldn't do as much damage if it hit 50 yards from dead center.
At the boat show, they said they don't do this anymore. The military GPS signals are on a different frequency than civilian units and are more accurate.
Finally, a true story...My garage was broken in to several years ago and my dive gear stolen. Several days later a detective knocked on our front door and asked if we had been robbed? My wife said "Yes." It seems, that he followed the waypoint "HOME" I had programmed into the unit back to my house. Most of my stuff was returned because they busted the fence.
Carry an extra set of batteries. My "back-up" GPS has saved more than one dive day. Sometimes the boat unit just takes a dump. Also try to get a unit that allows the "Track-Up" function...this way the cursor is pointed in the direction of travel rather than always pointed north. This makes changing direction much easier.
Good luck navigating
Larry Stein
