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I've been salivating over the si units for quite a while now but there's one thing I haven't figured out. How is it that they show shadows of structure and fish? They use sonar, not vision so obviously the shadows aren't light generated. Is the shadow an actual sonar shadow or are they software generated to give better perspective for the user?
The shadows are where the fish or trees have blocked the sound waves from hitting the bottom further out. Just like shadows you see with a flashlight.
The one important thing to remember is that it can’t show every fish in the water. Keep in mind that objects directly behind other objects will not show up. This is the reason the technology generates the shadow effect. If you see a piece of structure but no fish, this doesn’t mean there aren’t fish.
I've wondered about that myself. If the shadows are where the sonar won't reach, being sent from the transducer and received from the tranducer, how can you see the shadows? To see the shadows of where the sonar is not reaching, would you not have to be "looking" from another point of view than the tranducer?
It must be another view generated by the software?
I have no idea, I just get curious at times.
talltimber,
That's what I don't understand either. Looking at some of the images I've seen on the web, it doesn't seem possible that the shadows being shown could be sonar shadows since sometimes they appear to be on the near side of the structure. It seems it would be possible for the software to "paint" a shadow on the bottom since it knows where the structure is relative to the bottom below. The most obvious answer is that it is a sonar shadow and it very well may be but that doesn't seem to jive with some of the pics. Like you said, if the shadow is a blind spot from the transducers viewpoint, you'd have to be looking from a different perspective to see it. Keep pondering I guess.
Like this image from the Lowrance site. I swear those shadows look like they're on the near side of the pilings.
http://mediabase.edbasa.com/erez4/ca...8bef1719ab.jpg
The shadows look normal to me.Quote:
Originally Posted by TN Hillbilly
Sometimes the shadows will tell you more about the object than the picture itself.
http://i173.photobucket.com/albums/w...Treecondos.jpg
This is what they really are.
http://i173.photobucket.com/albums/w...7/000_0138.jpg
Sometimes fish make good shadows even when the boat isn't moving.
bob baldwin
bryan, texas
997c SI
1197c SI
AS GRHA GPS
Can’t speak for any other brand but they are sonar shadows on the ‘Birds.
Sonar shows basically two things: how long it took the sonar signal to go out and bounce back from some object (bottom, fish, structure…) and the relative strength of that signal when it returns. This gets graphed on the display as depth or distance for us humans to understand and either a shade of color or multiple colors to represent the relative signal strength. Sonar shadows are showing that for that distance from the transducer, there was no sonar signal returned since there was something blocking it. Since the display is showing distance from the transducer; we are in effect looking at the sonar returns from a ‘heads down’ perspective.
Does that make sense?
My opinion, but I believe the boat would have to be moving somewhat to get a full image based on the SI narrow beam.Quote:
Originally Posted by bob baldwin
As long as the fish is moving U will get full image as he/she swims through he si beam especially if the fish is swimming parallel to si beam.OOPS I meant to say fish is swimming parallel to the boat not the si beam, this way the si beam would pick up the whole side & length of the fish sorry:(Quote:
Originally Posted by Cane Pole
Quote:
Originally Posted by jaxsprat
Ima dummy.
Ain't no dummies when it comes to sidescan, just some roads/trails we haven't walked yet.;)Quote:
Originally Posted by Cane Pole
I'm used to observing strait lines and arches.:) Even when trolling slow (.2-.4 mph) I have never seen a shadow that well defined. Usually the screen objects are just long lines of macaroni noodles at these slower speeds.Quote:
Originally Posted by jaxsprat
I am gonna pay more 'tention. However,with the gps showing a donut at the slower speeds, I have a tendency to ignore the screen.
Learn somethin' every day.
Right now, there are not to many fish to look at with the si sitting on my desktop.:eek::eek:
I've taken lots of these while fishing at anchor for channel cat.
bob baldwin
bryan, texas
997c SI
1197c SI
AS GRHA GPS
I have never see this type shot. But, I have seen lots of that spaghetti in the background.
I will start paying more attention for sure.
Quote:
Originally Posted by bob baldwin
I would say it definetly has to help that you are only in 6 feet of water and only looking out 25ft to each side. I wouldnt think it would show up nearly that nice if the si was looking out further and you were in deeper water. That is very impressive especially because I am a catfisherman myself. Was that shot taken in a river, lake, what?? Nice screen shots!!!
Correct - you need the fish pretty close to get these images. I took these in Lake Somerville, Texas behind Deer Island. I missed recording the best ever when a carp type fish went by and I could make out it's sucker mouth. In this shot, the boat is rocking slightly making the gps think it was moving. The puck is about 8 feet above the water.
bob baldwin
bryan, texas
997c SI
1197c SI
AS GRHA GPS
OK, I think maybe I understand how this works now. The sonar data is used to render a pic of the objects detected to show things from a side perspective. The sonar beam doesn't come directly from the side, it comes at an angle from the surface out toward the bottom. The data received is then processed to create a view showing a rendered view from the side. That's what allows the user to see shadows of returns. The returned data is used to draw a pic of the bottom, and anywhere there isn't a bottom return is filled in with a shadow. Anyone call bull to that?
See if this makes any sense to you...
Attachment 37706
Makes perfect sense to me!
Great illustration
One thing the may be a bit confusing for some - the image is not a snapshot. It's made up of many thin slices pieced together over time. Here is an image that may help - think of the scans being pulsed rapidly to build an image.
bob baldwin
bryan, texas
997c SI
1197c SI
AS GRHA GPS
Yes, makes sense. Thanks for the graphic. What was throwing me off is the fact that what appears in the images to be the foreground is actually the background.
Exactly...the only actual "real time" sonar is the 1 inch slice of water directly under the tranducer....Quote:
Originally Posted by TN Hillbilly
Everything else shown on the screen is sonar "history" produced by stacking those 1 inch slices on top of each other.....to form an image....
Just like in 2D sonar except that with the Si beams being so thin front-to-rear they allow for a picture-like image to be formed on the display. Sort of like an underwater version of an MRI or is it a CAT scan?