Share Electronics fact I just learned on side scanning

**SeaRay** · 08-09-2016, 01:40 PM

I am adding my post just so I can follow this. I am still trying to learn my HDS7 side scan.

**rnvinc** · 08-09-2016, 02:27 PM

It is important to realize that the selected SI Range is not the actual amount of bottom coverage to the left and right (horizontal distance) ...

And while the water depth (23' is this scenario) is figured into the equation (of 40' SI range in this scenario) ... The 40' is Slant Range to the edge of the display (as Gene explained using a 40' long string and arcing this string as one end of the string is attached to the xducer) ...

Slant range is figured using the Pythagorean Theorem ...YUK!! Geometry !!!

In the OP example of 23' water depth with a SI Range setting of 40' ...

Slant Range vs Horizontal Range and Calculating Waypoints

Rickie

**crp4570** · 08-09-2016, 02:45 PM

Thx for the formula. I understand better when given a math formula.

**crp4570** · 08-09-2016, 02:48 PM

So when I stop the side scan and mark a waypoint, it accurately log its location. I.e. In the above example, it will mark the 32.7 mark when the 40 foot mark is displayed.

**rnvinc** · 08-09-2016, 03:19 PM

Originally Posted by crp4570

So when I stop the side scan and mark a waypoint, it accurately log its location. I.e. In the above example, it will mark the 32.7 mark when the 40 foot mark is displayed.

The unit automatically calculates the slant range to any specific pixel you place a waypoint on ...and assigns the calculated waypoint latitude and longitude coordinates as "summed" in that calculation ... (Is "summed"'a word?) ...

So if you place a waypoint at the edge of the display (with 40' SI Range) ... And then calculate the distance difference between that waypoint's assigned longitude and the boat's present assigned longitude = the difference should sum 32.7' ... (as meaning 32.7' horizontal distance to a point on the water's surface directly above that "thing" you placed the waypoint on) ...

Remember a waypoint's coordinates do not take into account water depth (other than using the water depth in its calculation of slant range to any horizontal pixel on the SI image display) ...

See Sonar Quiz #8 here ...
http://www.crappie.com/crappie/side-...onar-quiz-8-a/

Rickie

**Boa3** · 08-09-2016, 03:33 PM

You guys didn't know you were Mathathletes did you

**deltabound** · 08-09-2016, 03:35 PM

So if you turn off the contour lines on a hb unit so the beneath the boat view isn't showing will you get a true 40ft range or will it still be the slant measurement?

Sent from my iPhone using Tapatalk

**scrat** · 08-09-2016, 03:49 PM

So does this mean that Side imaging range show out farther to the sides when used in shallower water as it is not having to look downward as much.

**rnvinc** · 08-09-2016, 03:52 PM

Originally Posted by deltabound

So if you turn off the contour lines on a hb unit so the beneath the boat view isn't showing will you get a true 40ft range or will it still be the slant measurement?
Sent from my iPhone using Tapatalk

Well slant range ...sorta ...

The 40' (using the same 40' SI Range) with the HB "Contour On" setting eliminates the water column from the displayed image ... but the 40' is still slant range from the xducer to the edge of the display ...

So you're still only seeing 32.7' of bottom coverage (horizontal distance) ... the 32.7' is simply "stretched out" to fill the display without the water column ...

I will explain why the engineers designed it this way in a following post (because it's a lengthy explanation) ...

Rickie

**rnvinc** · 08-09-2016, 04:16 PM

Originally Posted by rnvinc

Well slant range ...sorta ...

The 40' (using the same 40' SI Range) with the HB "Contour On" setting eliminates the water column from the displayed image ... but the 40' is still slant range from the xducer to the edge of the display ...

So you're still only seeing 32.7' of bottom coverage (horizontal distance) ... the 32.7' is simply "stretched out" to fill the display without the water column ...

I will explain why the engineers designed it this way in a following post (because it's a lengthy explanation) ...

Rickie

The SI image is a very complex engineering feat ...

This is because the underwater world is 3 dimensional ... and the unit display is only 2 dimensional ...

Engineering the vertical SI scroll data was not that difficult ... It was merely stacking the SI ping slice data one horizontal row of pixels onto the previous horizontal row of pixels ...(remember 1 ping slice data = 1 horizontal pixel row if the Chart speed is scrolling at the same rate as the xducer is pinging)..

The challenge was how to display a 3 dimensional bottom detail onto a 2 dimensional display ...

This is where the slant range calculation design was incorporated ... Basically take into account water depth and the SI Range (slant range) to each target echo and plot echo data into the corresponding pixel in that calculation ... (This is what we see in the HB Contour On setting with no water column) ...

Now the real challenge ... How to display a 3 dimensional water column in this same SI image .. and make it discernible ...

The engineers decided to take the vertical pixel(s) where the bottom detail starts on each side and "move" it outward to make room in the 2 dimensional display for the water column ...

So this created another challenge ... What happens to the bottom detail when the edge of the bottom detail is "moved" outward ?? ...The display is still the same size ... So the existing displayed bottom detail has to be manipulated somehow to make room in the display for the water column ...

So the engineers decided to "compress" the existing bottom detail outward to make room for the water column ...

Imagine 2 accordion style doors opening at the center ... The folds (in the accordion doors) nearest the center are compressed more than the folds at the outer edges ...

Likewise in the SI image = the data closest to the water column is compressed more than the data farther away from the water column ...

You can see in the following image how the exact same data gets compressed in the SI image as compared to the "Contour On" SI data ...

This is why it is important to understand that range to targets in the SI image should be used as no more than estimations of distance to that target ...

Not only is there slant range calculations in the SI image ...there is also horizontal compression calculations in the SI image ..

Trust that your unit will calculate your SI marked Waypoints to get you in the ballpark ...

Then go verify that waypoint with 2d ... Or hang a lure in it and "re-mark" the waypoint ...

Rickie

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