Introductionto Oceanography Name Lab Exercise#1 BATHYMETRY

Definitions:

Bathymetry:The measuringand chartingof the topographyof the sea floor. Contour:A lineon a chartor map that connectspoints of equalelevation or depth relativeto a givenbase (or datum)level. Datum level: The zero with referenceto which the altitudesof land surfaces and depthsof the sea aredetermined. Fathom:A unit of length,equal to six feet, usedmainly in measuringthe depthof the sea. Knot:The unit of speedused at sea. lt is equivalentto one nauticalmile per hour. lsobath:A line on a chartthat connectspoints of equaldepth. lsobathinterval: The verticaldistance between adjacent isobaths. Scale:A calibratedline or barthat shows the relationshipbetween units of lengthused on a particularmap and the actualunits they represent. The scalemay alsobe a fractionalone: for example,a scaleof 1:500,O00means that one unit on the map (centimeters,millimeters, inches,or any otherlinear unit) represents 500,000 of the sameunits on the earth. Sounding:a measurementof water depth

Show all your calculations.

1. In the chart providedon page l0ir(froma,U.S. Coast & GeodeticSurvey Chart of an areaoff southernCalifornia) only the majorsoundings have been provided. Examinethe chart and, without drawingin any contourlines, describe the general appearanceof the sea floor.

2. Afteryou have answeredthe questionabove, contour the soundingson the chart usinga 1OO-meterisobath interval.

103 3. Comparethe contouredbathymetry with your originaldescription based on the soundingsalone. ls the validityof your impressiongreater with or without the contourlines? Why?

4. On pagetOE, use the graphto constructa cross-section,or configuration,of San Pedrochannel, which extendsfrom pointA to point B. To do so, lay the top horizontalline A-B of the uppergrid alongthe A-B lineof the chart.Whenever a soundingoccurs along or very nearthat line,make a tick markon the graphat the samehorizontal distance from A or B, but downwardso that it is oppositethe appropriatedepth as notedon the verticalaxis. Thus, your first markfor the 20- metersounding near A will be at 20 meterson the verticalscale. When you have completedthe A-B transect,connect the ticks with a smoothline to producea simplifiedcross-section, or profile,of the channel.

Notethat this profileis verticallyexaggerated: that is, the verticalscale is much largerthan the horizontalscale. Vertical exaggeration (V" E ,, ) is arithmetically expressedas: V.E. : VerticalScale (Fractional HorizontalScale (Fractional)

Usingthis simpleequation, determine the verticalexaggeration of your profileand write it in the space providedat the bottom of the graph.

5. On the lower grid of the Cross-sectionSheet, pg. 100,the horizontaland vertical scalesare equal, so your profilewill haveno verticalexaggeration. Draw this configurationon the lower grid usingthe depthdata from the upper,vertically exaggerated,grid.

6. Brieflydescribe and comparethe exaggeratedand the true shapeof the topographyof the San PedroChannel. Which of these[profiles would be most usefulto an oceanographerinterested in the bathymetryof the channel?Why?

r04 SAN PEDROCHANNEL All soundingsin meters

Conlour interval 100 melers 5ro

Scale, nautical flrites. ..).., .:),, I ,.ti;t: San Pedro ?50.000 't2,-:

North 5l 1{t \ . 19$, n '..,. ' .:ailii

Palos Verdes

.: t.::,1

45

nz.i 495 25iil

550

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SanlaCatatina lsland

105 Oneof the most accuratedevices for measuringand chartingsubmarine topographyis a PrecisionDepth Recorder (PDR). lt is similarto a fathometerin constructionand placementaboard ship except that the timingof soundpulses is preciselycontrolled by use of a tuningfork within the instrument.Some of these instrumentshave timing accuraciesof betterthan one part in 30OO.Such precision is not requiredfor routinenavigation, but is mandatoryfor accuratescientific work.

The figureon page tsf is part of a PDRprofile of the RedondoSubmarine Canyon. Verticallines, such as the two shown in the figureare markedevery 15 minutesas well as wheneverthere is a changeon the ship'scourse. On this recordingthe navigatorin the bridgefixed the ship'sposition at 1200 hoursand 12O9 hoursso that the oceanographercould accurately locate the axisof the submarinecanyon (notedat 12O3hours) on a marinechart. Also shown arethe ship'scourse (C 175' ) and its speedpf 7 knots (S7K).

The regularlyspaced horizontal lines are at 1O-fathom(60-foot) intervals, and they indicatethat the north edgeof the canyonis at a depthof about330 feet. This portionof the canyonwas recordedon the 0-200 fathom scale;however, as the vesselproceeds further offshore where the canyonprogressively deepens, the scale must be changedto 2OO-400fathoms, 400-600 fathoms,and so on, to accommodatethe greaterdepths.

The followingquestions are concernedwith the precisiondepth recorder traces of the RedondoSubmarine Canyon on the next page.

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

8. What is the width of the canyon,in nauticalmiles, betweenthe notationfor 12OOhours and that for 12O9hours?

106

Crosssections

A B

100

200

400 g o (D soo g E 600 o

700

800

900

0510152A253035 1000 Horizontaldlstanc€ (kilomeiers)

HorlzontalScale 1 :250,000 Vertlcal Scale 1 :1O,000 V€rtical exaggeration X

A roo0g 2oooE 3000 ; 4oooB 0 05r0ls2a2s3035 5oo0 Horizontaldistance (kilometers)

Scale t :25O,000 No Yerticalexaggerailon

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