METHODS OF SAMPLING TlKE BENTHOS
229
A. Position-Jndifig
Radio-direction finding, radar and various commercial radio-position finding techniqucs havc done much to increase the accuracy of
position-finding a t sea. Accurate positioning is particularly valuable
for any surveys of the sea bed, as it allows tho possibility of returning
to the same place for repetitivc sampling. The Decca Navigator system
. is widely used in European watcrs, and its coverage is being extended
to other parts of the world where shipping density is high. Its accuracy
is often better than 100 m, and the range is some 240 miles from the
master transmitting station. Various chains have been established so
that there is now complete coverage over the North Sea and around
Britain. Farther from land, and on less frequented routes, however, i t
is still necessary to rely on less accurate methods.
When planning lines of stations it is now quite usual to use Decca
co-ordinate lines rather than lines of latitude and longitude (e.g.
Cabioch, 1961). This simplifies navigation and the wide parabolic
sweep of the lines may often be just. as satisfactory as straight lines.
13. Echo-sounding
The echo-soundcr is constantly in iisc, not only for measuring
depth, but in detecting shoals of fish, and it. is often possible to learn
something of the naturc of the bottom sediments from the trace. The
subject is covered in mctny works, and general accounts are givcn by
Barnes (1959b) and Cushing (1963).
C. Asdic
Asdic, in which the sound beam used in detection is turned towards
R horizontal direction, is mainly used for detection of shoals of fish in
mid-water. It has also been used for mapping bottom features and
deposits (Stride, 1959, 1961) and an example of thc use of this technique
is the very detailed survey of a herring spawning ground off the
Scottish coast described by 8tubt)s and Lawrie (1962). In this survey
the following instruments were employed : dredge, grab, camera,
television, echo-sounder and asdic, and the ships’ positions were
recorded continuously on R chart by the Decca Track Plotter. A comparison of the facility with which different types of gear coufd he used
is given.
Because of the great width scanned by Asdic, it is much more
suitable for surveys of the sea floor over ti wide area than i H thc echosounder.
229
A. Position-Jndifig
Radio-direction finding, radar and various commercial radio-position finding techniqucs havc done much to increase the accuracy of
position-finding a t sea. Accurate positioning is particularly valuable
for any surveys of the sea bed, as it allows tho possibility of returning
to the same place for repetitivc sampling. The Decca Navigator system
. is widely used in European watcrs, and its coverage is being extended
to other parts of the world where shipping density is high. Its accuracy
is often better than 100 m, and the range is some 240 miles from the
master transmitting station. Various chains have been established so
that there is now complete coverage over the North Sea and around
Britain. Farther from land, and on less frequented routes, however, i t
is still necessary to rely on less accurate methods.
When planning lines of stations it is now quite usual to use Decca
co-ordinate lines rather than lines of latitude and longitude (e.g.
Cabioch, 1961). This simplifies navigation and the wide parabolic
sweep of the lines may often be just. as satisfactory as straight lines.
13. Echo-sounding
The echo-soundcr is constantly in iisc, not only for measuring
depth, but in detecting shoals of fish, and it. is often possible to learn
something of the naturc of the bottom sediments from the trace. The
subject is covered in mctny works, and general accounts are givcn by
Barnes (1959b) and Cushing (1963).
C. Asdic
Asdic, in which the sound beam used in detection is turned towards
R horizontal direction, is mainly used for detection of shoals of fish in
mid-water. It has also been used for mapping bottom features and
deposits (Stride, 1959, 1961) and an example of thc use of this technique
is the very detailed survey of a herring spawning ground off the
Scottish coast described by 8tubt)s and Lawrie (1962). In this survey
the following instruments were employed : dredge, grab, camera,
television, echo-sounder and asdic, and the ships’ positions were
recorded continuously on R chart by the Decca Track Plotter. A comparison of the facility with which different types of gear coufd he used
is given.
Because of the great width scanned by Asdic, it is much more
suitable for surveys of the sea floor over ti wide area than i H thc echosounder.