Using upper and lower limits
  |  First Published: May 2004

WHEN bottom bouncing for a feed or to collect bait, many of the decisions we make are based on our trust in the images we see on our sounder screen. But how much information are we missing because the sounder doesn’t have the ability to show it in its normal mode?


Take a typical screen reading at any depth. You will see the bottom drawing about one third of the way up from the bottom of the screen. All those pixels below the bottom drawing are wasted once we have ascertained bottom hardness using the greyline feature. You can shift that bottom drawing to the bottom of the screen by manipulating the range settings, but there’s an even more precise way to use all the pixels on the screen to your advantage.

Lowrance units have a feature in their menu called ‘upper and lower limits’. The upper limit of the segment of the water column that you wish to investigate is set, and then the lower limit. Provided there is at least 10ft (3.07m) between those two settings, you’ll be able to view it on the screen using all of the pixels. This is a different function from ‘zoom’, which can be activated when you’re using the upper and lower limits function to further enhance pixel usage.

To give an example, imagine being in water 240m deep using a sounder with 240 vertical pixels. A fish one metre thick would activate only 1 pixel, and would appear only if the bottom line was at the bottom of the screen. Usually it would 1/3rd of the way up the screen, in which case many of the pixels are wasted and even that large fish would not show.

Using upper and lower limits, even at that depth, we can look at the bottom and the 10ft (3.07m) of water above it and get a true picture of what is down there. This increment can be expanded to any size you wish and can be used anywhere in the water column.

The images here show coral trout sitting amongst wreckage of an old barge off Townsville. Much of the pixel count of the screen is wasted if you are fishing the bottom.



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