Thread: A Questioning Mind

You will only see the dark water column...

35' SI Range means the unit will only display return echoes from within that 35' distance away from the xducer...

In your scenario of 35' SI Range...If you took a 35' rope and attached one end to the xducer...and then swing that rope in a semi-circle from 35' horizontal right to 35' horizontal left...every echo within that semi- circle would show on the display...(within the upper reaches of the selected SI beam frequency)...

Every echo outside of that semi-circle would not be shown on the display...

Setting the HB SI Contour mode to "On" would show a blank screen with no echoes anywhere because you told the unit not to show echoes closer than 35' Range with the Contour Mode setting...in addition to setting the SI Range setting at 35' told the unit not to display echoes farther than 35' Range...



Rickie

Thanks, Rickie....... so if you had range set ~5ft over depth, would you have a 10' width of down imaging in contour mode?

Originally Posted by cache22

Thanks, Rickie....... so if you had range set ~5ft over depth, would you have a 10' width of down imaging in contour mode?

As the Range setting in SI is attributed to "Slant Range" instead of "horizontal distance"...the amount of bottom showing will be less than 5' each side ...(in your scenario of 5' SI Range greater than depth)...

Robert explains "Slant Range" here....
Slant Range vs Horizontal Range and Calculating Waypoints

But your basic thought is still correct....(if I'm understanding your scenario correctly)...

*You can set the SI Range greater than the depth (5' SI Range in this instance)...
*This will show a "less than 5' section" of the bottom detail on each side of the "dark water column"...

= Then setting "Contour Mode" to "On" will eliminate the "dark water column"....and, in effect, stretch (horizontally only- not vertically) ....the remaining data on the screen towards the center line...(probably into something unrecognizable because the vertical data did not change)...

(Kinda like buttoning a shirt that's too small...will "stretch" the material towards the center...but the shirt didn't get any longer)...

Rickie

Does this mean that contour mode distorts (stretches) the side image when range is set wider?

Originally Posted by cache22

Does this mean that contour mode distorts (stretches) the side image when range is set wider?

In effect, Contour Mode is going to "stretch" the bottom detail showing on the display ...to a degree depending on the ratio of "dark water column to SI Range setting" ...showing on the display...

If the dark water column takes up a lot of the display...removing that dark water column (with Contour Mode On) will "stretch" the remaining bottom detail a lot more than if there were less "dark water column" and more bottom detail showing...

Here are 2 "Contour Mode On" examples..(extreme examples for clarity)...

*Water depth of 10' - SI Range 100'....the "Dark water column to SI Range ratio" is small = less horizontal bottom detail distortion...(with Contour Mode On)...
*Water depth of 90' - SI Range 100'....the "Dark water column to SI Range ratio" is large = more horizontal bottom detail distortion..(with Contour Mode On)...

This is because "Contour Mode On" is telling the unit to display the same amount of bottom detail currently showing on the display ...and stretch it horizontally over more of the screen real estate...

Now, importantly, that's the way it works in my head....I haven't actually tested this live on the water to see if my understanding is correct...

Rickie

Rickie,in roberts pic,when he change the contour mode,i counted the lines on right side and counted 20 to dark spot and 8 on other side of dark spot,with contour other way ,only 4 lines after dark spot,it looks like it start in middle of each side and then stretch out leaving the middle of each side as center of the stretch?

This is the depiction in Robert's Slant Range tutorial...(is this the image you refer to Gene?)...



And here's a partial clip from Robert's tutorial...

The records below show a portion of sonar image before and after correction for slant range and speed distortions. Note that the feature closer to the sonar is changed more by the correction process than the features further away.

I think the important thing to take from this is that the SI image (Contour Mode On or Contour Mode Off) may not be a "true" representation of actual linear distances across the lake bottom....

This is because the xducer is always above the bottom (dictated by depth)...and because the unit may be applying "correction factors" (proprietary that only the brand knows)...

This is why waypoints marked on the SI image should always be verified (and remarked) with inspection from the "down pointing" 2d cones ...

Rickie

Starting to make sense to me ...... thanks, Guys? Since it is rather difficult to pick up fish returns in contour mode, I think I shall leave it off.