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Images, Snapshots and Videos => General Pictures and Screen Captures => Topic started by: Free2Fish on October 18, 2014, 11:50:12 AM

Title: Fish sizes on 999 HD SI
Post by: Free2Fish on October 18, 2014, 11:50:12 AM
I've only used this unit a couple of times and would like some help in interpreting this image. I think these objects are fish and when I measure the largest and compare it to the width of the signal I get about 4 feet long. As far as  know this lake only holds crappie, largemouth bass and walleyes...none of which would approach that length in this lake.

Can fish sizes be determined in this way or are these measurements non-linear in some way?

Cheers,
Harry[attachimg=1]
Title: Re: Fish sizes on 999 HD SI
Post by: rnvinc on October 18, 2014, 06:01:34 PM
Longer fish echoes can be fish moving with the boat direction...

Fish that happen to be moving the same direction the boat is moving will be "pinged" more successive times than an immobile fish or a fish moving in a different direction than the boat...creating a longer echo ...

Here is an example that clearly shows how some fish echoes look longer simply because the fish were moving with the boat direction...


(http://i477.photobucket.com/albums/rr134/rnvinc/HB/null_zpsfbc4ce68.jpg) (http://s477.photobucket.com/user/rnvinc/media/HB/null_zpsfbc4ce68.jpg.html)


Rickie
Title: Re: Fish sizes on 999 HD SI
Post by: Free2Fish on October 18, 2014, 06:20:31 PM
Thanks Rickie, good explanation. In water that shallow there's a good chance some of the closer fish were moving away as the boat went by.

Harry
Title: Re: Fish sizes on 999 HD SI
Post by: rnvinc on October 19, 2014, 01:51:08 AM
BTW ... there is some distortion factor that makes it difficult to accurately size things in the SI image ...

http://forums.sideimagingsoft.com/index.php?topic=946.0 (http://forums.sideimagingsoft.com/index.php?topic=946.0)

Humviewer's measuring tool takes some of this in account for fixed objects...

Fish echoes are more of a guess ...

Rickie

Title: Re: Fish sizes on 999 HD SI
Post by: Free2Fish on October 19, 2014, 12:06:26 PM
Thanks for the link, more good info.

Now that I think about it, there may be other factors that would influence target image sizes besides fish movement and the slant distortion and target compression listed in the tutorial.

Any skewing of the boat path would tend to expand/compress target size as would the angle of the fish to the sonar signal (but perhaps only in the extreme). For example, a large fish directly facing the signal might show a very small return image.

Harry
Title: Re: Fish sizes on 999 HD SI
Post by: rnvinc on October 19, 2014, 08:51:51 PM
Thanks for the link, more good info.

Now that I think about it, there may be other factors that would influence target image sizes besides fish movement and the slant distortion and target compression listed in the tutorial.

Any skewing of the boat path would tend to expand/compress target size as would the angle of the fish to the sonar signal (but perhaps only in the extreme). For example, a large fish directly facing the signal might show a very small return image.

Harry

A very good point...

Rickie
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