....How do you read depth?
First off, congrats on the switch to an AT Pro - wow, that was quick!! Careful...you're starting to show all the signs of full-fledged MD addiction
You'll know you've really got it bad when you start showing up to hunt with the ATP in one hand, the Ace in the other, and swingin' 'em simultaneously
Anyway, sorry I'm late to the party - I just saw this thread today. You've gotten some pretty good info already, but I thought I would add my 2 cents - the more opinions the better, and somewhere amidst it all you'll find what works for you.
I started with an Ace 400 and moved to an AT Max earlier this year. The ATM is much more similar than it is different when compared to an ATP. Both have proportional audio (assuming you're running in Pro mode) and depth depiction/pinpointing behave the same way for sure.
Unlike what some others have implied, I actually consider the depth indications on the Garrett machines to be quite accurate,
as long as you keep in mind the important caveats. First, like others have mentioned, the pinpoint feature is calibrated based on a coin-sized object - larger objects will register shallower than actual, smaller objects will indicate deeper. Second, the depth scale is really only accurate in pinpoint mode - this important characteristic isn't directly stated in the manual, only implied. These two fundamental qualities are often misunderstood and I think they are the main reason why so many people either get frustrated or simply call the depth meter "inaccurate". The confusion for a new user is certainly understandable - as you said, you swing over the target, and it says 6-8", pinpoint mode says 4", and out pops a bit of copper wire barely 2" down...what the?!?
Basically, I ignore the depth indicator when sweeping a target - that's where the proportional audio will give you a better initial impression of depth (softer/quieter signals
tend to be deeper). But the problem is, proportional audio suffers from the same calibration caveat as pinpoint mode - small, relatively shallow targets will trigger the softer audio while big, deep targets will sound off loud and proud. So, how do you tell what's what?
Personally, I use pinpoint mode’s fixed coin-sized calibration to my advantage - I'll use pinpoint mode to "x mark the spot" and note depth, but then I lift the coil a couple inches to pinpoint and note the depth again. Here's what happens...if the target is coin sized, and I lift the coil 2", the depth indication should increase by 2". If the target is noticeably larger than coin sized but deep, lifting the coil a couple or even a few inches won't impact the depth indication at all – you might even need to lift the coil half a foot or more before the depth indication changes! Lifting the coil this way literally only takes an extra second or two, but will save you buckets of time compared to the long minutes you might otherwise spend digging 10" down just to recover a rusty, cell phone sized steel plate that rang up in the upper 80s and indicated 4" down. If the target is small and shallow, lifting the coil should cause too large of a change in corresponding depth indication - but honestly, for some reason, that seems a little harder to notice. To guard against the shallow stuff fooling me, whenever I think I have a coin sized target, I cut the plug then sweep the hole and plug with my handheld pinpointer. If no joy at that point, I do a quick sweep with the detector to confirm whether it's in the plug or in the hole before I resume digging.
I've been learning to listen to signals, but I'm still unsure of what I hear, so I tend to look at all the info on the screen to help me determine what and how to dig.
Just like you mentioned, I also still tend to watch my screen pretty closely - maybe that’ll change as I gain more experience, but for now, as a second year detectorist, I feel that the VDI provides too much important information that tone alone can't give me on the Garretts I've used. I scratch my head a little when folks suggest that with experience you can tell a quarter from a dime just by the tone - maybe that's true on a machine with 50 tones, but with the AT and Ace series, a high tone is a high tone. 88 doesn't sound any different than 81 to my ears. But I guess for high tones, it doesn't matter so much - if the depth/size seems right, I'm usually digging a high tone. But for mid-tones, I don't see how you can decide to dig or not without evaluating what the VDI has to say.