Hi all,

I was looking at the output from a lowrance Elite 7ti and comparing the traditional downlooking sonar results with the down-scan sonar results. The traditional sonar return is always deeper than the down-scan. The down-scan has a nice hard return, but if I correct the depths points to occur on the bottom return then it is consistently above the traditional sonar return.

I played with scales and colors (brightness and contrast) but it the pattern stays consistent. Which bottom return would be better?

Here is a picture of the issue with traditional sonar on top and down-scan on the bottom. It is attached if the link does not work.

https://goo.gl/photos/qFAFv5QuT5NrM1Fb9



Thanks,
Jim

Interesting. I have noticed the same using my HDS-units. I thought it was related to the very different coneangle and shape of the cone, but your data suggest there is something else going on.

Hi Jim,

Mayby the issue comes with the selected NMEA sentences. I think it could be a possibility the sentence "DPT" (Depth of Water) is selected for one sonar and the NMEA sentence "DBT" (Depth below transducer) for the other sonar.

Sam

Hi!

It is normal... I always use the downscan to modify the depth data because the downscan is the most exact scan.

Thanks everyone for your comments.

Sam, The "depth below surface" and "depth below transducer" is an interesting comment. I don't think I imported it with any depth offset for the transducer as I usually correct that in a single pass in post processing in a database after exporting the data. I will re-import with different settings and see if it goes away.

Heiko, Do you have any information (published preferable) to support that the downscan depth is better than the traditional sonar? I would agree in concept, but would really like some way to justify my selection.

Thanks again,
Jim

Heiko wrote:
This is somewhere between a modified thruth and plain wrong. It would depend on the ducer used, depth and the gradient of the bottom structure. Which is why you should use both a narrow coneangle 2D-ducer and DownScan and compare them, for the best end-result.