Author Topic: Fish sizes on 999 HD SI  (Read 2962 times)

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Offline Free2Fish

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Fish sizes on 999 HD SI
« on: October 18, 2014, 08:50:12 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


Offline rnvinc

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Re: Fish sizes on 999 HD SI
« Reply #1 on: October 18, 2014, 03:01:34 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...





Rickie
« Last Edit: October 18, 2014, 03:07:58 PM by rnvinc »

Offline Free2Fish

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Re: Fish sizes on 999 HD SI
« Reply #2 on: October 18, 2014, 03:20:31 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

Offline rnvinc

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Re: Fish sizes on 999 HD SI
« Reply #3 on: October 18, 2014, 10:51:08 PM »
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

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

Fish echoes are more of a guess ...

Rickie


Offline Free2Fish

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Re: Fish sizes on 999 HD SI
« Reply #4 on: October 19, 2014, 09:06:26 AM »
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

Offline rnvinc

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Re: Fish sizes on 999 HD SI
« Reply #5 on: October 19, 2014, 05:51:51 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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