Sounders and GPS

Sounders and GPS


Depth sounder technology has come a long way in recent years, and this will continue. There are dozens of models to choose from, and they are cheaper and more user friendly than ever, with even the entry level sounders having lots of features.

If you want to catch more fish, more regularly, then you will need a depth sounder. We use them mainly for their ability to locate fish and identify the structure of the bottom, but depth sounders are also an invaluable aid to navigation, especially if you do not have a GPS received.

Fishfinder sounder HDSWith a sounder, you can roam back and forth over likely spots to identify exactly where the fish are congregating on the bottom. Even if there are no fish evident, you can identify a potential fish holding site such as a reefy bottom, submerged tree stump, etc. With a good quality sounder, you will also be able to tell the composition of the bottom – whether the bottom is sand, mud, weed or rock, etc.

Good depth sounders can also display thermoclines. This is the border, or meeting point between a surface layer of warm water, and an underlying layer of cool water. Fish will often congregate close to the thermocline layer to feed on plankton, tiny baitfish and other organic matter than can be trapped or suspected above the denser, cooler water column underneath.

Depth sounders function by emitting a constant series of sound waves or “pulses” from the depth sounder, via a transducer. If the transducer is facing directly down underneath the boat, then these pulses will be directed down toward the sea bottom in a cone like share. The depth sounder then measures the distance between it and the sea bottom by measuring the time it takes fro the signal to strike the bottom and bounce back up to the transducer. This information is then converted into digital or graphic display of the bottom and contour. In addition to picking up bottom echoes, the sound pulses will reflect off other objects it encounters, including solitary fish, schools of bait fish, submerged trees, shipwrecks, etc.

As your boat moves through the water, the display shown on your depth sounder will constantly be updated, providing the angler with an ever changing view of the sea bottom or portion of the water column which is being viewed. On the sounder screen, the sound pulses or echoes will be displayed as a long unbroken line which will move up and down, depending on the water depth and contour.

A recent introduction to the market are depth sounders which use what is known as CHIRP technology to achieve a much better picture quality than you will find in traditional sounders with a single or dual frequency transducer. CHIRP, or “compressed high intensity radar pulse” technology basically involves the use of a special transducer which uses a modulated pulse so that a signal is sent out across a range of frequencies, as opposed to say a single 50 or 200 kHz frequency. The cone angle width also changes to match the specific frequency expanding and contracting to suit. When a CHIRP sounder pulse strikes an object at a particular frequency, the pulse returned to the display will be at the same frequency. Subsequent objects will be detected at a slightly different frequency and cone angle and this changing frequency will result in much better screen detail, and make it much easier to separate and identify small targets on the screen.

Depth sounder transducers can be mounted through the hull, inside what is termed a “wet box” inside the boat, or secured externally to the transom wall. The latter is the most common method for trailerboats. If mounted on the transom wall, a hazy picture or a screen with a lot of interference may be caused by turbulence or air bubbles running across the face of the transducer. To avoid this, move the transducer away from the outboard motor.


The “global positioning system” is a satellite based global navigation system developed by the US Military. It comprises a network of 24 satellites which orbit the Earth at approximately 20,000km above the Earth’s surface. GPS was initially developed for military applications, but is now widely available for civilian use, albeit downgraded. They are typically accurate to 10-15m.

May depth sounders also include a GPS inbuilt into a single multifunction unit.

I use a Lawrence HDS-12 Gen2 Touch.


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