Danny,

I'd use the closest to the transducer - if only for safety. Can you post the SS and DS images just for interest ?

You appear to be shooting fish in a barrel ?

T

I've seen this a few times (perhaps not so many "arches"). Thick weed (kelp etc) can show like this on the normal sonar, especially at anchor or going really slow. In RM I will switch to the down scan image and compare. The weed shows in more detail and you can often see the real seabed better.

I agree with Trevor - safer to draw the bottom at the minimum depth although not an issue in your example.

Meggs

Have you the same in DI?
It is what I use when not sure!

To me it looks like a relatively soft bottom (mud?) with some hard stuff underneath. The hard stuff is confusing the bottom-lock a little, and picking up the hard stuff occasionally. I think the consistency of the upper "bottom" is too good to be something like a weed layer...

This is the same spot in downscan. Depth is about 23 meters in a Swedish freshwater lake. So Matt is right, there is no vegetation layer involved, but probably sediment. The lake is situated in a region with a lot of lakes with a peaty soil and scattered boulders.

Which explains the reason why the depthfinder is having difficulties to determine depth on some places and the isolated strong returns, seemingly below bottom. (see screenshot 2D sonar)
This is only my opinion.

But this makes it really difficult to build accurate depthmaps.

Danny,

I take my depths from the DS channel for exactly this reason - most of my work is in peaty lakes where the bottom is fluid, fine sediment, often like jelly. I've also found that the 800hz channel is less reliable than the 455hz channel in this kind of environment. Be aware that the SS/DS can actually penetrate this type of bottom. I frequently image archaeolgy that is totally buried under up to 1ft of sediment, and this can give some very strange indications on what is visibly a featureless bottom.

T

Just to be sure I didn't miss something. Reefmaster isn't having yet a quick bottomline redraw tool like we used to have in Drdepth?

Not yet Danny, but we are planning on a complete bottom detection feature which will enable RM to redraw the bottom for a whole track, which should really help in this sort of situation.
Also it will be possible to use depths from any channel for mapping, and in your example I would guess that DS depths are OK - in which case these could be used.

Matt wrote: Really looking forward to that feature, fortunately I am still able to use Dr. Depth to do this and import into RM as csv. But re-drawing the bottom is a huge time consumer and a pain...

robm wrote:
Oh yes... 15-20 minutes each track! :-(

Matt wrote:
When will the bottom detection feature be available?

Hi!
In this attachment you can see my first mapping attempt, the area near my harbour.I am showing you two small ditches I found, but I suspect that they might be made by sonar logs with high speed and therefore different transducer tilt perhaps...Is my fear real? In that case what it can be done to correct it?I am pretty sure that I have overlapped this area many times, are the latter logs correcting a map area or not?