Mega 360 Object Distance Interpretation

[SLYDoggie]

I've had my Mega 360 for a couple of months now and am still in learning mode with it. I'm using it for Striper Trolling so it's not really a "normal" use case but I'm finding it very effective.

One question I have at this point is how to interpret distance from the Mega 360 pod to a specific object. I've attached an image I captured with a couple of schools of small Hybrid Stripers in the water column and have added an arrow to the area I want to focus on.

You can see from the image that I'm trolling at 2.8 MPH and over a 37' bottom. I realize that the arrow is pointing at shadows being cast by the fish in the water column at about 11:00 but, for this discussion, assume those shadows are actual fish returns.

My specific question is, for the objects at the arrow on the 75' range ring, are they actually 75' from the Mega 360 Pod or would I need to subtract out the 37' water column "ring" to determine their real distance so they would actually be 38' from the pod? In general, I would like to have a group discussion on interpreting object distance with the Mega 360. Would love input from the experts who have been using the Mega 360 for a while.

FYI, my Mega 360 is mounted on a rail about 3/4 of the way back on the helm side of my 21' pontoon. My Downriggers are mounted on the front corners of the boat and, if you zoom in on the image, you can see my 2 downrigger balls, the right planer board (with wake), and the Mercury outboard lower unit with cavitation trail from the prop. The right downrigger ball is just to the right of the bow of the boat icon, the left downrigger ball about 10' to the left of the bow of the boat icon, and the motor is just to the left of the stern of the boat icon.

It's amazing the detail of what you can pick up with the Mega 360!

[Wayne P.]

Side Imaging (that is what the 360 is) does slant range not horizontal distance. Slant range is the distance between the transducer and subject in a straight line.
The closest way to get horizontal distance displayed is turn Contour Mode on, then you won't see the water column or what is in the water column displayed.

The arrow is pointing to the shadows of the fish that are displayed in the water column at 25' from the transducer, so those are not "objects". The fish are blocking the sonar pulses, so that blockage creates a shadow for each fish.

This is the same thing on a smaller scale:

[SLYDoggie]

Wayne, yeah, I realize that what I pointed out was shadows and I said to "assume" they were fish for the sake of my question. So, if the shadows at the 75' range line were actually fish, would they be 75' on a diagonal line from the Mega 360, or would you need to subtract out the water column distance when not in contour mode? I've always wondered that on SI as well... I've understood the diagonal line thing but have always wondered about the impact of the water column border. I typically fish in 60' to 80' depth so a lot of the image is water column.

[Wayne P.]

The shadows are 75' diagonally from the transducer. There is no horizontal calculation in that image. You would have to calculate the angle at 75' along with the distance from the 360 transducer to the bottom to get horizontal distance IF the bottom is flat.
With that stated do you know what the depth is were the shadows are, there is nothing that measures that. It could be deeper or shallower over there than what is under the transducer.

[Capw]

Just like on your side view mirrors....”objects are closer than they appear”, unless it’s really shallow.

[Wayne P.]

Just like on your side view mirrors....”objects are closer than they appear”, unless it’s really shallow.

[SLYDoggie]

Just like on your side view mirrors....”objects are closer than they appear”, unless it’s really shallow.

Yeah, that's really what I'm trying to get to. It just seems like the water column circle should have an impact on the actual distance things are. Totally familiar with the "diagonal" part if you were on a 25' bottom where the water column circle coincided with the 25' range ring and there was an object right at the 50' range line, is it really at a 50' diagonal from the transducer?

I understand that something in the water column at the 24' mark could be directly under the boat a foot off the bottom, to the surface at 24' over, or anything in between. That's where DI comes in handy to see if it also appears there.

[CV_X21]

Steve,

Assuming that the fish that made the 75 foot shadows are suspended at 20 feet (based on the latest fish images shown on the DI image in the screen shot), the diagonal would be 77.6 feet... if my math is right. (Pythagorean Theorem)

[stosiak]

How about if you put your curser on the object and set waypoint. Then just hit Go TO, would that calculate the distance from the boat to the object and leave the math out.

[CV_X21]

My bad! 77.6 feet would be to the fish shadows. I calculate it would be 32 feet to the fish that made the shadows.

[BassCatKev]

How about if you put your curser on the object and set waypoint. Then just hit Go TO, would that calculate the distance from the boat to the object and leave the math out.

That's a great question but I don't think it would be any more exact. If you don't know what depth the fish is, the distance would have to be ballpark. In the example above, we had DI telling us a target was approx 20 feet below the boat. If we don't have some depth reference, then everything changes. Plus like Wayne said, we don't know the depth over there where you're looking at the fish/object.