fathometer n : depth finder for determining depth
of water or a submerged object by means of ultrasound waves [syn:
sonic
depth finder]
English
see
fathometer
Proper noun
- A trade
mark.
A fishfinder is a type of fathometer, both being
specialized types of
echo
sounding systems, a type of Active
SONAR. ('Sounding' is
the measurement of water depth, a historical nautical term of very
long usage.) The fishfinder uses active sonar to detect fish and
'the bottom' and displays them on a graphical display device,
generally a
LCD
or
CRT
screen. In contrast, the modern fathometer (from
fathom plus
meter, as in 'to measure') is
designed specifically to show depth, so may use only a digital
display (useless for fish finding) instead of a graphical display,
and frequently will have some means of making a permanent recording
of
soundings (which
are merely shown and subsequently electronically discarded in
common sporting fishfinder technology) and are always principally
instruments of navigation and safety. The distinction is in their
main purpose and hence in the features given the system. Both work
the same way, and use similar frequencies, and, display type
permitting, both can show fish and the bottom. Thus today, both
have merged, especially with the advent of computer interfaced
multipurpose fishfinders combining
GPS technology, digital
chart-plotting, perhaps
radar and electronic compass
displays in the same affordable sporting unit.
Operating theory
In a generalized sense, an electrical
impulse from a
transmitter is converted
into a
sound wave by
the
transducer,
called a
hydrophone,
and sent into the water. When the wave strikes something such as a
fish, it is reflected back and displays size, composition, and
shape of the object. The exact extent of what can be discerned
depends on the frequency and power of the pulse transmitted. The
signal is quickly
amplified and sent to the
display. Knowing that the speed of the wave in the water is 4921
ft/s (1500 m/s) in seawater, 4800 ft/s (1463 m/s) in freshwater
(typical values used by commercial fish finders), the distance to
the object that reflected the wave can be determined. The process
can be repeated up to 40 times per second and eventually results in
the bottom of the ocean being displayed versus time (the fathometer
function that eventually spawned the sporting use of fishfinding.)
Note: This discussion of the propagation of
sound in water is simplified,
speed of sound in water depends on the temperature, salinity and
ambient pressure (depth). This follows approximately this formula
(del Grosso, 1974): c=1448.6+4.618T-0.0523T^2+1.25*(S-35)+0.017D
where c = sound speed (m/s) T = temperature (degrees Celsius) S =
salinity (pro mille) D = depth This will give variations in speed
through the water column
General interpretation
The image above, at right, clearly
shows the bottom structure -- plants, sediments and hard bottom are
descernible on sonar plots of sufficiently high power and
appropriate frequency. Slightly more than halfway up from the
bottom to the left of the screen center and about a third away from
the left side, this image is also displaying a fish -- a light spot
just to the right of a 'glare' splash from the camera's flashbulb.
The X-axis of the image represents time, oldest (and behind the
soundhead) to the left, most recent bottom (and current location)
on the right; thus the fish is now well behind the transducer, and
the vessel is now passing over a dip in the ocean floor or has just
left it behind. This obviously depends on both the speed of the
vessel and how often the image is updated by the echo
sounder.
General history in sporting and fishing
Early sporting
Fathometers for recreational boating used a rotating light at the
edge of a circle which then flashed 'synchronized in time' with the
received echo (corresponding to depth); these also gave a small
flickering flash for echos off of fish. They did nothing to display
the trend of the bottom depth over time, nor anything about bottom
structure. They operated strictly in a 'snapshot mode', as do the
cheap digital fathometers of today. They were hardly ideal in a
wave tossed small craft or in bright light, but they were good for
holding the boat in the safe channel, assuming one could actually
see the light....
Commercial and naval units
Commercial and Naval Fathometers
of yesteryear used a
Strip
Chart Recorder where an advancing roll of paper was marked by a
stylus to make a permanent copy of the depth, usually with some
means of also recording time (Each mark or time 'tic' is
proportional to distance traveled) so that the strip charts could
be readily compared to navigation charts and maneuvering logs
(speed changes). Much of the world's ocean depths have been mapped
using such recording strips. Fathometers of this type usually
offered multiple (chart advance) speed settings, and sometimes,
multiple frequencies as well. (Deep Ocean -- Low Frequency carries
better, Shallows -- high frequency shows smaller structures (like
fish), submerged reefs,
wrecks, or other bottom composition features of interest.) At high
frequency settings, high chart speeds, such fathometers give a
picture of the bottom (and any intervening large or schooling fish)
relatable to navigation position data. Fathometers of this constant
recording type are still mandated for all large vessels (100+ tons
displacement) in restricted waters (i.e. generally, within 15 miles
of land).
Birth of the fishfinder
Eventually, CRTs were married with
a fathometer for commercial fishing and the fishfinder was born.
With the advent of large LCD arrays, the high power requirements of
a CRT gave way to the LCD in the early 1990's and fishfinding
fathometers reached the sporting markets at prices nearly anyone of
modest means can afford. Today, sporting fishfinders lack only the
permanent record of the big ship navigational fathometer, and that
is available in high end units that can use the ubiquitous computer
to store that record as well.
Fish symbols in the screen of the fishfinder
Most units
just attach the FISH symbol to every echo which is not connected to
the bottom or surface. With those you can not find fish lying in or
on top of the bottom. Others use a detection of a special
fingerprint in echosignals. With those you can find living fish and
fish that have recently died. With these units also in some cases
fish spawn will be shown as a large fish. Some kind of seafish will
be not marked as fish, but you will see an echo.