DSI Combo – Transducer Setup & Accuracy:

This is another very important step to understanding the difference between your conventional cone shaped sound wave and that of the high frequency beam shaped sound wave.

This will apply to Lowrance DSI, LSS-1 (DownScan) and Humminbird's new True DI systems coming in later this year (Africa). This will NOT apply to the current 'DI from SideImaging' such as the 797, 798, 997, 998, 1197 and 1198.

Take the 50° coverage angle of the 50kHz frequency for example, has the same coverage in all directions due to its cone shape. The high frequency beam however, will have a similar width (port to starboard), but less than a 10% coverage area from fore to aft.

This ultimately affects the accuracy of your waypoints created from sonar in a dramatic way. Due to the wide coverage area of the conventional cone shaped sound wave, there is nothing you can do to improve the accuracy of your waypoint other than changing to the much smaller <20° cone of the 200kHz frequency. But with the very narrow beam width (fore to aft) of the high frequency sonar, you can now take VERY accurate waypoints if done correctly.

http://www.fishtec.co.za/dsi%20006.jpg

So looking from the back the beams would appear to be identical:-

http://www.fishtec.co.za/dsi%20007.jpg


But looking from the side, they are very different:-

http://www.fishtec.co.za/dsi%20008.jpg http://www.fishtec.co.za/dsi%20009.jpg



Their respective coverage areas are also very different. Conventional cone in red and high frequency beam in orange.

http://www.fishtec.co.za/dsi%20010.jpg


Conventional Cone:- 50/83kHz
Pros
Great coverage area for finding fish
Installation angle (1° - 15°) of transducer not critical
Cons
Poor target separation and loss of bottom detail
Poor accuracy when marking a waypoint

High Frequency Beam:- 455/800kHz
Pros
Great target separation resulting in life like imagery
Great accuracy when marking a waypoint, especially when cross referenced
Cons
Poor coverage area, single suspended fish hard to detect as a result
Installation angle of transducer is critical

Marking a waypoint with DownScan:-

While idling along you might come across an object in lets say 20ft of water. You would immediately use your TrackBack feature placing your cursor above the object and save waypoint. But how accurate is this waypoint? You might approach this waypoint from a different angle the next time you visit this object and to your horror, it is gone!

In this image you can see that we passed over the object (red dot) and it was in our coverage area so we would have seen it on our display. Taking a waypoint here would give you a very accurate position on the ‘X’ axis (your course / heading), but on the ‘Y’ axis you could be out by up to nearly 10ft (Y1 – Y2) at a depth of only just over 20ft. Imagine miss casting to a lily pad by 10ft while casing to the shoreline. Would that bass leave the lily pad and come swim all the way over to your miss cast? So why would a bass’s behaviour be any different as you go deeper?

http://www.fishtec.co.za/dsi%20011.jpg


The way to rectify this possible waypoint position error is to make a big wide turn and approach you initial path from right angles. This will put you directly on the course of your first ‘X’ axis. As you go over the object it will be DIRECTLY beneath the transducer. So when you mark your waypoint now, you would have corrected your initial ‘Y” axis error to within a very small margin. Remember to delete your first waypoint from your first pass so you don’t get confused later. I also recommend using the 800kHz beam because it has an even smaller coverage area of the 455kHz.

http://www.fishtec.co.za/dsi%20012.jpg


Transducer angle:-

The angle of the transducer while installing now starts to play a major role in the accuracy of waypoints taken from the sonar.

If your transducer was angled too low, or heel heavy, you will effectively be looking forward the whole time. At a depth of only 20ft, you could be out by up to 5ft. This might not seem too bad right now, but what happens when you approach it from the opposite direction? The object to waypoint position would now be out by 10ft, and that now is a big problem.

http://www.fishtec.co.za/dsi%20013.jpg


This becomes even more of a problem when marking a drop off. Because when you create your first waypoint (001) as you reach the top of your drop off, all will seem fine.

http://www.fishtec.co.za/dsi%20014.jpg


But as you turn around and mark the drop off from the opposite direction, it will appear that the drop off has changed angles dramatically. Continued zig zagging will give the drop off the appearance of a jagged edge like that of a saw blade. But in fact, it was a dead straight ledge.

http://www.fishtec.co.za/dsi%20015.jpg


The easiest way to rectify this problem is to mark a known object from one direction, take a waypoint, then approach it from the opposite direction and mark it again. Keep changing the angle of the transducer by very small increments and do a bi-directional passes until the waypoints taken from opposite directions match up perfectly.

Hint:
Remember to compensate for distance between GPS receiver and transducer location when not using an external GPS puck mounted just above your transducer.