Thank goodness for depthfinders. Without these electronic marvels, sportfishing as we enjoy it today would be impossible. "Fishfinders," as many refer to them, revolutionized the fishing scene during the 1960s. Some 30 years later, the manufacturers of these angling tools are still improving their products and making them easier to use.

Since the first recreational sonar units were introduced by the Lowrance family, dozens of companies have thrown their hat into this electronics business. To say the competition is stiff is like saying Christie Brinkley is sort of cute! Although the technology exists to produce high-quality and functional fishing sonar, not all the products in the marketplace live up to the advertisers' claims.

Next to a boat and outboard motor, sonar is the most important item an angler should invest in. I can't emphasize enough how important quality sonar equipment can be to angling success. A case in point: on my personal fishing boat I have a sonar unit mounted on the bow, a second (more powerful unit) on the console, a third in-dash flasher unit for high speed running, plus a GPS unit for navigation chores! Each of these units has a specific purpose.

Without getting into a complete discussion of how each unit is used, let me say that all four units are used frequently and have become invaluable for various fishing techniques and presentations.

Does every boat need this many fishing aids? No, but anglers are wise to invest in the highest quality sonar unit or units they can afford.

There are three different types of sonar commonly available to the recreational angler--liquid crystal readout, flashers, and video-type units. Of the three, liquid crystal readouts are hands down the most popular with anglers and available in the widest assortment of price ranges and feature levels.

A few anglers are diehard flasher users, and several companies manufacture excellent products for them. Still, unless you know how to interpret a flasher or are willing to learn how, a liquid crystal unit is easier for most anglers to use effectively.

Video units are excellent tools for studying structure and cover and for locating fish. Unfortunately, these units are bulky, expensive, and in many cases too fragile to be used in an open-bow fishing boat.

After sorting through the units available, most anglers end up purchasing a liquid crystal readout graph. The best liquid crystal units feature a display scene with 240 vertical pixels, or in other words a dot matrix picture that is high resolution and capable of showing structure, cover, and fish in visual detail.

Units that offer less than 125 vertical pixels can't offer enough detail to enable an angler to tell the difference between sunken brush and a school of fish near the bottom. Units with few vertical pixels are best used as simple depth finders. For some applications such as bass or pike fishing along shoreline cover, these units are adequate. For locating fish such as walleye or trout in deeper water, the higher-resolution units are mandatory.

A good liquid crystal unit should come equipped with a transducer that operates at or near 192 kHz. The sound waves created at 192 kHz are close together and capable of separating targets within a few inches of one another. This is critical if the sonar unit is to point out individual fish that are close to bottom or cover, or when several fish are schooled together.

Many sonar units will accept two or more transducer frequencies such as a wide-angle 50 kHz and 192 kHz by simply touching a key pad. This function is handy for those who fish downriggers and wish to monitor the location of hardware in relationship to fish or bottom.

Other handy sonar features include bottom or target zoom. By simply zooming in and taking a closer look at the bottom, interesting structure or fish marks, the resolution of liquid crystal units can be increased significantly. This is an invaluable feature that combines the high resolution of a paper graph with the convenience of a liquid crystal unit.

The ability to split the screen and view two functions at once is also an important feature. With split-screen viewing you can see both a 192 kHz and 50 kHz view at the same time, or view the sonar and GPS plotter at the same time, or display a narrow but detailed window of the water column on one side and a complete picture on the other. The combinations are almost endless.

No discussion of fishing sonar would be complete without some comments regarding global positioning systems. The navigational technology provided by satellite- based GPS signals are accurate, foolproof, and, for my money, an investment angler's can't afford to be without.

These products are available as stand-alone GPS units, GPS/sonar combinations, and hand-held versions. The advancements GPS has made in recent years is astonishing. Just as a single example: Eagle, a division of Lowrance Electronics, recently introduced a hand-held GPS unit called the Explorer. Smaller than a soda can and costing around $200, the Explorer offers state-of-the-art 12-channel satellite performance previously only available on units costing $1,000 or more.

This new product locks on in seconds, provides simultaneous communication with up to 12 satellites, an internal battery back-up, and the ability to store up to 750 position points, 250 waypoints, and up to 500 graphic event icon symbols.

Powered by 110-volt AC or 12-volt DC power, there's even a nickel metal hydride battery available that can be recharged up to 500 times. With these advancements in GPS technology and consumer-friendly pricing, the fishing and hunting public is taking a serious look at GPS.

In the 20-something years I've fished seriously, sonar has been a critical part of my angling success. I've always strived to use the best equipment, and I've never felt over-gunned. Regardless of the species you seek, take my advice and spend your sonar dollar wisely.