UPPER and lower limits function is one that most sounders have on their menu keyboard. The reason it is used less frequently than it should is that the ‘zoom’ feature is usually easier to access. The zoom feature does not give a true representation of the bottom and other objects detected by the sonar beam. As the zoom is magnified, what we are looking at on the screen is compressed in from the sides and elongated upwards. This in effect turns a mediocre bottom structure such as a round bommie into a sharp pinnacle therefore perhaps leading us to make a decision to fish a piece of bottom that may not be as productive as one elsewhere.
The upper and lower limits feature on the other hand will allow us to take a horizontal slice out of that water column and expand it in its entirety, over the full screen. This in effect gives us a clearer picture by having a small section of water amplified over a larger pixel screen. Objects such as fish, weed and structure that could not be seen previously due to the small size of their screen image compared with the rest of the screen, are often more visible when this function is utilised.
Various brand sounders have differing rates of upper and lower limits. Some Lowrance and Humminbird units offer this function in increments of five metres. In effect this allows one to bottom fish in 100 metres of water and view just the bottom and the five metres of water column above it. This expansion of the picture is a boon to those looking for specific bottom structure and to see fish in that habitat, even from a distant surface.
This function may be used to view a level in the water above the bottom such as that occupied by a thermocline. Lookout bass, this is deadly technology!
Having set our sensitivity high so that we can see those thermoclines easily, the top part of our screen blacks out and appears cluttered. By utilising the upper limits on the unit, we can effectively remove that section while leaving the bottom reading, making a screen easy to read and to detect fish with.
Screen shot 1 (above) shows what the bottom and bait schools appear like when the unit is set an auto depth range. The range is from 0 metres (the surface) to 300m. In screen shot 2 (below), the top 100m and bottom 75m of the water column have been excluded. This dedicates greater numbers of pixels to the interesting region between 120m and 220m. The separation of the bait schools from the bottom and the greater definition of the bottom itself are valuable fishing tools.Reads: 670