Introduct ion to Oceanography NameLab Exercise #1

BATHYMETRY

Defini t ions:

Bathymetry: The measur ing and chart ing of the topography of the sea f loor.Contour: A l ine on a chart or map that connects points of equal e levat ion or

depth relat ive to a given base (or datum) level .Datum level: The zero with reference to which the altitudes of land surfaces

and depths of the sea are determined.Fathom: A uni t of length, equal to s ix feet , used mainly in measur ing the

depth of the sea.Knot: The uni t of speed used at sea. l t is equivalent to one naut ical mi le per

hour .lsobath: A l ine on a chart that connects points of equal depth.lsobath interval : The vert ical d istance between adjacent isobaths.Scale: A cal ibrated l ine or bar that shows the relat ionship between uni ts of

length used on a part icular map and the actual uni ts they represent.The scale may also be a f ract ional one: for example, a scale of1:500,O00 means that one uni t on the map (cent imeters, mi l l imeters,inches, or any other l inear uni t ) represents 500,000 of the same uni tson the earth.

Sounding: a measurement of water depth

Show all your calculations.

1. In the chart provided on page l0 i r ( f rom a,U.S. Coast & Geodet ic Survey Chart ofan area of f southern Cal i fornia) only the major soundings have been provided.Examine the chart and, wi thout drawing in any contour l ines, descr ibe the generalappearance of the sea f loor.

2. Af ter you have answered the quest ion above, contour the soundings on thechart using a 1OO-meter isobath interval .

103

3. Compare the contoured bathymetry wi th your or ig inal descr ipt ion based on thesoundings alone. ls the val id i ty of your impression greater wi th or wi thout thecontour l ines? Why?

4. On page tOE, use the graph to construct a cross-sect ion, or conf igurat ion, of SanPedro channel , which extends from point A to point B. To do so, lay the tophor izontal l ine A-B of the upper gr id along the A-B l ine of the chart . Whenever asounding occurs along or very near that l ine, make a t ick mark on the graph at thesame hor izontal d istance from A or B, but downward so that i t is opposi te theappropr iate depth as noted on the vert ical axis. Thus, your f i rst mark for the 20-meter sounding near A wi l l be at 20 meters on the vert ical scale. When you havecompleted the A-B transect, connect the t icks wi th a smooth l ine to produce asimpl i f ied cross-sect ion, or prof i le, of the channel .

Note that th is prof i le is vert ical ly exaggerated: that is , the vert ical scale is muchlarger than the hor izontal scale. Vert ical exaggerat ion (V" E , , ) is ar i thmet ical lyexpressed as:

V.E. : Vert ical Scale (Fract ionalHor izontal Scale (Fract ional)

Using this s imple equat ion, determine the vert ical exaggerat ion of your prof i le andwrite it in the space provided at the bottom of the graph.

5. On the lower gr id of the Cross-sect ion Sheet, pg. 100, the hor izontal and vert icalscales are equal , so your prof i le wi l l have no vert ical exaggerat ion. Draw thisconf igurat ion on the lower gr id using the depth data f rom the upper, vert ical lyexaggerated ,grid.

6. Br ief ly descr ibe and compare the exaggerated and the true shape of thetopography of the San Pedro Channel. Which of these [prof i les would be mostuseful to an oceanographer interested in the bathymetry of the channel? Why?

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SAN PEDRO CHANNELAl l soundings in meters

San Pedro

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Sanla Catatinalsland

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Conlour interval 100 melers5 r o

Scale, nautical flrites.I

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North

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One of the most accurate devices for measur ing and chart ing submarinetopography is a Precis ion Depth Recorder (PDR). l t is s imi lar to a fathometer inconstruct ion and placement aboard ship except that the t iming of sound pulses isprecisely control led by use of a tuning fork wi th in the instrument. Some of theseinstruments have t iming accuracies of bet ter than one part in 30OO. Such precis ionis not required for rout ine navigat ion, but is mandatory for accurate scient i f ic work.

The f igure on page tsf is part of a PDR prof i le of the Redondo Submarine Canyon.Vert ical l ines, such as the two shown in the f igure are marked every 1 5 minutes aswel l as whenever there is a change on the ship 's course. On this recording thenavigator in the br idge f ixed the ship 's posi t ion at 1200 hours and 1 2O9 hours sothat the oceanographer could accurately locate the axis of the submarine canyon(noted at 12O3 hours) on a marine chart . Also shown are the ship 's course (C 175') and i ts speed pf 7 knots (S7K).

The regular ly spaced hor izontal l ines are at 1O-fathom (60-foot) intervals, and theyindicate that the north edge of the canyon is at a depth of about 330 feet. Thisport ion of the canyon was recorded on the 0-200 fathom scale; however, as thevessel proceeds further of fshore where the canyon progressively deepens, the scalemust be changed to 2OO-400 fathoms, 400-600 fathoms, and so on, toaccommodate the greater depths.

The fol lowing quest ions are concerned with the precis ion depth recorder t races ofthe Redondo Submarine Canyon on the next page.

7. What is the depth of the canyon in fathoms ?

8. What is the width of the canyon, in nautical miles, between the notation for12OO hours and that for 12O9 hours?

106

BA

Cross sections

0 5 1 0 1 5 2 A 2 5 3 0 3 5

Horizontal dlstanc€ (kilomeiers)

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700

800

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Horizontal distance (kilometers)

Scale t :25O,000No Yertical exaggerailon

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