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Lowrance Sonar Tutorial- Part II


technical article from Lowrance

Back to Sonar Tutorial part I

Fish Arches
One of the most common questions that we receive is "How do I get fish arches to show on my screen?" It's really pretty simple to do, but it does require attention to detail, not only in the way you make the adjustments to the unit, but to the whole sonar installation.

It also helps to see the Why Fish Arch section below. This explains how arches are created on your sonar's screen.

Screen Resolution
The number of vertical pixels that the screen is capable of showing is called Screen Resolution. The more vertical pixels on a sonar's screen, the easier it will be for it to show fish arches. This plays an important role in a sonar unit's capability to show fish arches. The chart below lists the pixel sizes and area they represent down to 50 feet for two different screens.

PIXEL HEIGHT PIXEL HEIGHT
100 VERTICAL PIXEL SCREEN 240 VERTICAL PIXEL SCREEN
RANGE PIXEL HEIGHT RANGE PIXEL HEIGHT
0-10 feet 1.2 inches 0-10 feet 0.5 inches
0-20 feet 2.4 inches 0-20 feet 1.0 inches
0-30 feet 3.6 inches 0-30 feet 1.5 inches
0-40 feet 4.8 inches 0-40 feet 2.0 inches
0-50 feet 6.0 inches 0-50 feet 2.5 inches


As you can see, one pixel represents a larger volume of water with the unit in the 0 - 100 foot range than it does with the unit in the 0 - 10 foot range. For example, if a sonar has 100 pixels vertically, with a range of 0 - 100 feet, each pixel is equal to a depth of 12 inches. A fish would have to be pretty large to show up as an arch at this range. However, if you zoom the range to a 30-foot zoom (for example from 80 to 110 feet), each pixel is now equal to 3.6 inches. Now the same fish will probably be seen as an arch on the screen due to the zoom effect. The size of the arch depends on the size of the fish - a small fish will show as a small arch, a larger fish will make a larger arch, and so on. Using a sonar unit with a small number of vertical pixels in very shallow water, a fish directly off the bottom will appear as a straight line separate from the bottom. This is because of the limited number of dots at that depth. If you are in deep water (where the fish signal is displayed over a larger distance of boat travel), zooming the display into a 20 or 30 foot window around the bottom shows fish arches near the bottom or structure. This is because you have reduced the pixel size in a larger cone.


               
100 pixels 240 pixels

On the right above is a section of a 240 vertical pixel screen. On the left is a simulated version of the same screen with only 100 vertical pixels. As you can see, the screen on the right has much better definition than does the one on the left. You can see fish arches much easier on the 240 pixel screen.

Chart Speed
The scrolling or chart speed can also affect the type of arch displayed on the screen. The faster the chart speed, the more pixels are turned on as the fish passes through the cone. This will help display a better fish arch. (However, the chart speed can be turned up too high. This stretches the arch out. Experiment with the chart speed until you find the setting that works best for you.)

Transducer Installation
If you still don't get good fish arches on the screen, it could be the transducer's mounting is incorrect. If the transducer is mounted on the transom, adjust it until its face is pointing straight down when the boat is in the water. If it is angled, the arch won't appear on the screen properly. If the arch slopes up but not down, then the front of the transducer is too high and needs to be lowered. If only the back half of the arch is printed, the nose of the transducer is angled too low and needs to be raised.

Fish Arch Review

1. Sensitivity
Automatic operation with Advanced Signal Processing (ASP™) turned on should give you the proper sensitivity settings but, if necessary, the sensitivity may be increased.

2. Target Depth
The depth of the fish can determine if the fish will arch on the screen. If the fish is in shallow water, the fish is not in the cone angle very long, making it difficult to show an arch. Typically, the deeper the fish, the easier it is to show an arch.

3. Boat Speed
The boat's engine should be in gear at an idle or just above. Experiment with your boat to find the best throttle location for good arches. Usually, a slow trolling speed works best.

4. Chart Speed
Use at least 3/4 chart speed or higher.

5. Zoom Size
If you see markings that are possible fish, but they do not arch, zoom in on them. Using the zoom function lets you effectively increase the screen's resolution.

Final Notes on Fish Arches
Very small fish probably will not arch at all. Because of water conditions such as heavy surface clutter or thermoclines, the sensitivity sometimes cannot be turned up enough to get fish arches. For the best results, turn the sensitivity up as high as possible without getting too much noise on the screen. In medium to deep water, this method should work to display fish arches.

A school of fish will appear as many different formations or shapes, depending on how much of the school is within the transducer's cone. In shallow water, several fish close together appear like blocks that have been stacked in no apparent order. In deep water, each fish will arch according to its size.

Why Fish Arch
The reason fish show as an arch is because of the relationship between the fish and the cone angle of the transducer as the boat passes over the fish. As the leading edge of the cone strikes the fish, a display pixel is turned on. As the boat passes over the fish, the distance to the fish decreases. This turns each pixel on at a shallower depth on the display. When the center of the cone is directly over the fish, the first half of the arch is formed. This is also the shortest distance to the fish. Since the fish is closer to the boat, the signal is stronger and the arch is thicker. As the boat moves away from the fish, the distance increases and the pixels appear at progressively deeper depths until the cone passes the fish.

If the fish doesn't pass directly through the center of the cone, the arch won't be as well defined. Since the fish isn't in the cone very long, there aren't as many echoes to display, and the ones that do show are weaker. This is one of the reasons it's difficult to show fish arches in shallow water. The cone angle is too narrow for the signal to arch.

Remember, there must be movement between the boat and the fish to develop an arch. Usually, this means trolling at a slow speed with the main engine. If you are anchored or stopped, fish signals won't arch. Instead, they'll show as horizontal lines as they swim in and out of the cone.

Actual On-The-Water Chart Recordings
The following chart records are from a Lowrance X-85 liquid crystal graph sonar. It has 3000 watts of transmitter power, a 240 x 240 pixel screen and operates at 192 kHz.

X-85 Sample 1

This shows a split-screen view of the water beneath the boat. The range on the right side of the screen is 0 - 60 feet. On the left, the screen has a 30-foot "zoom" rangeof 9 to 39 feet. Since the unit is in the automatic mode, (shown by the word "auto" at the top center of the screen) it picked the ranges to keep the bottom signal on the screen at all times. The water depth is 35.9 feet.

The unit was used with an HS-WSBK "Skimmer®" transducer mounted on the transom. The sensitivity level was adjusted to 93% or higher. Chart speed was one step below maximum.

A. Surface Clutter
The markings at the top of the screen can extend many feet below the surface. This is called Surface Clutter. It's caused by many things, including air bubbles created by wind and wave action or boat wakes, bait fish, plankton and algae. Many times larger fish will be seen feeding on the bait fish and other food near the surface.

B. GRAYLINE®
GRAYLINE® is used to outline the bottom contour which might otherwise be hidden beneath trees and brush. It can also give clues to the composition of the bottom. A hard bottom returns a very strong signal, causing a wide gray line. A soft, muddy or weedy bottom returns a weaker signal which is shown with a narrow gray line. The bottom on this screen is hard, composed mainly of rock.

C. Structure
Generally, the term "structure" is used to identify trees, brush, and other objects rising from the bottom that aren't part of the actual bottom. On this screen, "C" is probably a tree rising from the bottom. This record was taken from a man-made lake. Trees were left standing in several areas when the lake was built, creating natural habitats for many game fish.

D. Fish Arches
The X-85 has a significant advantage over many competitive units in that it can show individual fish with the characteristic arched mark on the screen. (See Why Fish Arch for more information.) On this screen, there are several large fish holding just off the bottom at "D," while smaller fish are hanging in the middle of the screen and near the structure.

E. Other Elements
The large, partial arch shown at "E" is not a fish. We were trolling near the entrance to a cove that had hundreds of tires banded together with wire cables. Other cables anchored the tires to the bottom. The large arch at "E" was created when we passed over one of the large cables that anchored the tires.

X-85 Sample 2

This shows a full-screen zoom view of the water beneath the boat. The range is 8 - 38 feet, which gives a 30-foot zoom. Since the unit is in the automatic mode, (shown by the word "auto" at the top center of the screen) it picked the ranges to keep the bottom signal on the screen at all times. The water depth is 34.7 feet.

The unit was used with an HS-WSBK "Skimmer®" transducer mounted on the transom. The sensitivity level was adjusted to 93% or higher. Chart speed was one step below maximum.

A and B. Fish Arches
The X-85 has a significant advantage over many competitive units in that it can show individual fish with the characteristic arched mark on the screen. (See Why Fish Arch for more information.) On this screen, there are several large fish holding just off the bottom at "B", while an even larger fish "A" is hanging directly above them.

C. Structure
Generally, the term "structure" is used to identify trees, brush, and other objects rising from the bottom that aren't part of the actual bottom. On this screen, "C" is probably a large tree or trees rising from the bottom. This record was taken from a man-made lake. Trees were left standing in several areas when the lake was built, creating natural habitats for many game fish.

D. Surface Clutter
Surface Clutter "D" at the top of the screen extends below 12 feet in places. Small fish can be seen beneath the surface clutter. They are probably feeding.



Back to Sonar Tutorial part I



Original article from Lowrance

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