CHAPTER 30HYDROGRAPHYANDHYDROGRAPHICREPORTS3000.Introductionfactors. Cartography is the final step in a long processwhichleadsfromrawdatatoausablechartforthemarinerBecause the nautical chart is so essential to safe navi-The mariner,in addition tobeing theprimary user ofgation, it is important for the mariner to understand thehydrographicdata, is also an important sourceofdata usedcapabilities and limitations ofboth digital and paper chartsin the production and correction of nautical charts.ThisPrevious chapters have dealt with horizontal and verticalchapter discusses the processes involved in producingadatums, chart projections, and other elements of carto-nautical chart, whether in digital or paper form, from thegraphic science.This chapter will explain some basicinitial planning of a hydrographic survey to thefinal print-concepts of hydrography and cartography which are impor-ing.With this information, the mariner can better evaluatetant to the navigator, both as a user and as a source of datathe information which comes to his attention and can for-Hydrography is the science of measurement and descrip-ward it in a form that will be most useful to chartingtion of all of the factors which affectnavigation, includingagencies,allowing themtoproduce more accurate anduse-depths,shorelines, tides, currents,magnetism,and otherful charts.BASICSOFHYDROGRAPHICSURVEYING3001.PlanningTheSurveyif it meets the expected accuracy standards, so the tidegauges can be linked to the vertical datum used for the sur-Thebasic documents used to produce nautical chartsvey.Horizontal control is reviewed to checkfor accuracyare hydrographic surveys.Much additional information isand discrepancies and to determine sites forlocal position-included, but the survey is central to the compilation of aing systems tobe used in the survey.chart. A survey begins long before actual data collectionLine spacing refersto the distance between tracks to bestarts. Some elements which must be decided are:runby the survey vessel. It is chosen to provide thebest cov-erageofthe areausingtheequipment available.Line spacing.Exactareaofthesurvey..Typeof survey(reconnaissance or standard)andis afunction of the depthofwater,thesound footprint ofthescale to meet standards of chartto be produced.collection equipment to beused, and the complexity of the:Scope of the survey (short or longterm)bottom.Oncelinespacingischosenthehydrographercan·Platforms available (ships, launches,aircraft, leasedcomputethetotal miles of surveytracktobe run and haveanvessels,cooperativeagreements)idea ofthetime required for the survey,factoring in the ex:Support work required (aerial or satellite photogra-pectedweather and other possible delays.The scale ofthephy,geodetics,tides)survey,orientation to the shorelines in the area,and the meth-.Limiting factors (budget, political or operational con-odof positioningdetermine line spacing.Planned tracks arestraints,positioning systemslimitations, logistics)laid out sothattherewill benogaps between sound linesandOnce these issues are decided, all information avail-sufficient overlaps between individual survey areas.able in the survey area is reviewed. This includes aerialLines with spacinggreater than the primary survey'sphotography,satellite data, topographic maps, existing nau-linespacing arerun atrightangles totheprimarysurveyde-tical charts,geodetic information,tidal information,andvelopment to verify data repeatability.These are calledanything else affecting the survey.The survey plannerscross check lines.thencompilesoundvelocityinformation,climatology,waOthertaskstobecompletedwiththesurveyincludebottomter claritydata, anypast surveydata,and informationfromsampling,seabedcoring,productionofsonarpicturesofthesealights lists,sailing directions,and notices to mariners.Tidalbed, gravity and magnetic measurements (on deep oceaninformation is thoroughly reviewed and tide gauge loca-tions chosen.Local vertical control data is reviewed to seesurveys),and sound velocity measurements in the water column411
411 CHAPTER 30 HYDROGRAPHY AND HYDROGRAPHIC REPORTS 3000. Introduction Because the nautical chart is so essential to safe navigation, it is important for the mariner to understand the capabilities and limitations of both digital and paper charts. Previous chapters have dealt with horizontal and vertical datums, chart projections, and other elements of cartographic science. This chapter will explain some basic concepts of hydrography and cartography which are important to the navigator, both as a user and as a source of data. Hydrography is the science of measurement and description of all of the factors which affect navigation, including depths, shorelines, tides, currents, magnetism, and other factors. Cartography is the final step in a long process which leads from raw data to a usable chart for the mariner. The mariner, in addition to being the primary user of hydrographic data, is also an important source of data used in the production and correction of nautical charts. This chapter discusses the processes involved in producing a nautical chart, whether in digital or paper form, from the initial planning of a hydrographic survey to the final printing. With this information, the mariner can better evaluate the information which comes to his attention and can forward it in a form that will be most useful to charting agencies, allowing them to produce more accurate and useful charts. BASICS OF HYDROGRAPHIC SURVEYING 3001. Planning The Survey The basic documents used to produce nautical charts are hydrographic surveys. Much additional information is included, but the survey is central to the compilation of a chart. A survey begins long before actual data collection starts. Some elements which must be decided are: • Exact area of the survey. • Type of survey (reconnaissance or standard) and scale to meet standards of chart to be produced. • Scope of the survey (short or long term). • Platforms available (ships, launches, aircraft, leased vessels, cooperative agreements). • Support work required (aerial or satellite photography, geodetics, tides). • Limiting factors (budget, political or operational constraints, positioning systems limitations, logistics). Once these issues are decided, all information available in the survey area is reviewed. This includes aerial photography, satellite data, topographic maps, existing nautical charts, geodetic information, tidal information, and anything else affecting the survey. The survey planners then compile sound velocity information, climatology, water clarity data, any past survey data, and information from lights lists, sailing directions, and notices to mariners. Tidal information is thoroughly reviewed and tide gauge locations chosen. Local vertical control data is reviewed to see if it meets the expected accuracy standards, so the tide gauges can be linked to the vertical datum used for the survey. Horizontal control is reviewed to check for accuracy and discrepancies and to determine sites for local positioning systems to be used in the survey. Line spacing refers to the distance between tracks to be run by the survey vessel. It is chosen to provide the best coverage of the area using the equipment available. Line spacing is a function of the depth of water, the sound footprint of the collection equipment to be used, and the complexity of the bottom. Once line spacing is chosen, the hydrographer can compute the total miles of survey track to be run and have an idea of the time required for the survey, factoring in the expected weather and other possible delays. The scale of the survey, orientation to the shorelines in the area, and the method of positioning determine line spacing. Planned tracks are laid out so that there will be no gaps between sound lines and sufficient overlaps between individual survey areas. Lines with spacing greater than the primary survey’s line spacing are run at right angles to the primary survey development to verify data repeatability. These are called cross check lines. Other tasks to be completed with the survey include bottom sampling, seabed coring, production of sonar pictures of the seabed, gravity and magnetic measurements (on deep ocean surveys), and sound velocity measurements in the water column
412HYDROGRAPHYANDHYDROGRAPHICREPORTS3002.Echo Sounders In Hydrographic Surveyingso that all soundings can be corrected for tideheight andthus reduced to the chosen vertical datum.Tide correctionseliminatetheeffect of thetides on thecharted watersandEchosounders were developed in the early1920s,andensure that the soundings portrayed on the chart are thecomputethedepth ofwaterbymeasuringthetime ittakesminimum availabletothemarineratthe soundingdatumfora pulse of sound to travel from the source to the sea bot-Observed,not predicted, tides are used to accountforbothtom and return. A device called a transducer convertsastronomically and meteorlogically induced water levelelectrical energy into sound energy and vice versa.Forbasicchanges during the survey.hydrographic surveying,thetransduceris mountedperma-nently in thebottom ofthe survey vessel, which then follows3003.Collecting SurveyDatathe planned trackline, generating soundings along the track.Themajordifferencebetweendifferenttypesof echoWhile sounding data is beingcollected alongtheplannedsounders is in the frequencies they use.Transducers can betracklines bythesurvey vessel(s),a variety of other relatedacclassified according to theirbeam width, frequency,and pow-tivities are taking place. A large-scale boat sheet is producederrating.Thesoundradiatesfromthetransducerinaconewith many thousands of individual soundings plotted. A com-withabout 50%actuallyreachingto sea bottom.Beamwidthplete navigation journal iskeptofthe survey vessel'sposition,isdetermined bythefrequencyofthepulseand the sizeofthecourse and speed.Side-scan sonarmay bedeployed to investi-transducer.In general, lowerfrequencies produce a widergate individual features and identify rocks, wrecks, and otherbeam,and ata givenfrequencya smallertransducer will prodangers. Time is the parameter which links the ship's positionduce awiderbeam.Lowerfrequencies alsopenetrate deeperwith the various echograms,sonograms,journals,and boatintothewaterbuthavelessresolutionindepthHigherfre-sheets that make up the hydrographic data package.quencieshavegreater resolution indepth,but less range,sothe choice is a trade-off. Higher frequencies also require a3004.ProcessingHydrographicDatasmallertransducer.Atypicallowfrequencytransduceroper-ates at 12kHz and a high frequency one at200kHzDuring processing, echogram data and navigationalTheformulafordepthdeterminedbyanecho sounderisdataarecombined withtidaldataand vessel/equipmentcor-V×T+K+DrD=rections to produce reduced soundings.This reduced dataiscombinedonaplotof thevessel'sactualtracktheboatwhere D is depth from the water surface,V is the aver-sheetdatatoproduceasmoothsheet.Acontouroverlayisage velocity of sound inthe water column,T is round-tripusuallymade totestthelogic of all thedata shown.All ano-timeforthepulse,Kisthe system indexconstant,andD,ismolous depths are rechecked in either the survey records orthedepthofthetransducerbelowthesurface(whichmaynotin thefield.Ifnecessary,sonardata arethen overlayed toan-be the same as vessel draft). V, Dr, and T can be only gener-alyze individual features as related to depths.It may takeallydetermined,and Kmust bedeterminedfrom periodicdozens of smooth sheetsto cover the areaofa complete sur-vey.The smooth sheets arethen ready for cartographers.calibration, In addition, T depends on the distinctiveness ofwho will choose representative soundings manually or usingthe echo, which may vary according to whether the sea bot-automated systemsfrom thousands shown,to produceatom ishard or soft.Vwill varyaccordingtothedensityofthenautical chart.Documentationoftheprocess is such thatanywater, which is determined by salinity,temperature, andpressure,and may vary both in terms of area and time.Inindividual sounding on any chart can betraced back to itsoriginal uncorrected value.See Figure 3004.practice, average sound velocity is usually measured on siteand the samevalue usedfor an entire survey unless variationsinwatermassareexpected.Suchvariations couldoccur,for3005.RecentDevelopments InHydrographic Surveyingexample,in areas ofmajor currents.While V is a vital factorindeepwatersurveys,itisnormalpracticetoreflecttheechoTheevolutionof echo sounders has followed the samesounder signal offa plate suspended under the ship at typicalpattern of technological innovation seen in otherareas.Indepthsfor the survey areas in shallowwaters.TheKparam-the 1940s lowfrequency/wide beam sounderswere devel-eter, or index constant, refers to electrical or mechanicalopedforshipstocoverlargeroceanareasinlesstimewithdelavsinthecircuitryandalsocontainsanyconstantcorrec-some loss of resolution.Boats used smaller sounders whichtion due to the change in sound velocity between the upperusuallyrequiredvisualmonitoringofthedepth.Later,nar-layers of water and the average used for thewholeproject.row beam soundersgave ship systemsbetterresolutionFurther.vesselspeedisfactoredinandcorrectionsarecom-using higher frequencies,but with a corresponding loss ofputed for settlement and squat, which affect transducerarea.These were then combined into dual-frequency sys-depth.Vessel roll,pitch,and heaveare also accountedfor.Fitems.All echo sounders,however,used a singletransducernally,theobservedtidaldataisrecordedinordertocorrectwhichlimited surveys to singlelines ofsoundings.Forboatthe soundings during processingequipment,automaticrecordingbecamestandardTides are accurately measured during the entire surveyThe last three decades have seen the development ofmulti-
412 HYDROGRAPHY AND HYDROGRAPHIC REPORTS 3002. Echo Sounders In Hydrographic Surveying Echo sounders were developed in the early 1920s, and compute the depth of water by measuring the time it takes for a pulse of sound to travel from the source to the sea bottom and return. A device called a transducer converts electrical energy into sound energy and vice versa. For basic hydrographic surveying, the transducer is mounted permanently in the bottom of the survey vessel, which then follows the planned trackline, generating soundings along the track. The major difference between different types of echo sounders is in the frequencies they use. Transducers can be classified according to their beam width, frequency, and power rating. The sound radiates from the transducer in a cone, with about 50% actually reaching to sea bottom. Beam width is determined by the frequency of the pulse and the size of the transducer. In general, lower frequencies produce a wider beam, and at a given frequency, a smaller transducer will produce a wider beam. Lower frequencies also penetrate deeper into the water, but have less resolution in depth. Higher frequencies have greater resolution in depth, but less range, so the choice is a trade-off. Higher frequencies also require a smaller transducer. A typical low frequency transducer operates at 12 kHz and a high frequency one at 200 kHz. The formula for depth determined by an echo sounder is: where D is depth from the water surface, V is the average velocity of sound in the water column, T is round-trip time for the pulse, K is the system index constant, and Dr is the depth of the transducer below the surface (which may not be the same as vessel draft). V, Dr, and T can be only generally determined, and K must be determined from periodic calibration. In addition, T depends on the distinctiveness of the echo, which may vary according to whether the sea bottom is hard or soft. V will vary according to the density of the water, which is determined by salinity, temperature, and pressure, and may vary both in terms of area and time. In practice, average sound velocity is usually measured on site and the same value used for an entire survey unless variations in water mass are expected. Such variations could occur, for example, in areas of major currents. While V is a vital factor in deep water surveys, it is normal practice to reflect the echo sounder signal off a plate suspended under the ship at typical depths for the survey areas in shallow waters. The K parameter, or index constant, refers to electrical or mechanical delays in the circuitry, and also contains any constant correction due to the change in sound velocity between the upper layers of water and the average used for the whole project. Further, vessel speed is factored in and corrections are computed for settlement and squat, which affect transducer depth. Vessel roll, pitch, and heave are also accounted for. Finally, the observed tidal data is recorded in order to correct the soundings during processing. Tides are accurately measured during the entire survey so that all soundings can be corrected for tide height and thus reduced to the chosen vertical datum. Tide corrections eliminate the effect of the tides on the charted waters and ensure that the soundings portrayed on the chart are the minimum available to the mariner at the sounding datum. Observed, not predicted, tides are used to account for both astronomically and meteorlogically induced water level changes during the survey. 3003. Collecting Survey Data While sounding data is being collected along the planned tracklines by the survey vessel(s), a variety of other related activities are taking place. A large-scale boat sheet is produced with many thousands of individual soundings plotted. A complete navigation journal is kept of the survey vessel’s position, course and speed. Side-scan sonar may be deployed to investigate individual features and identify rocks, wrecks, and other dangers. Time is the parameter which links the ship’s position with the various echograms, sonograms, journals, and boat sheets that make up the hydrographic data package. 3004. Processing Hydrographic Data During processing, echogram data and navigational data are combined with tidal data and vessel/equipment corrections to produce reduced soundings. This reduced data is combined on a plot of the vessel’s actual track the boat sheet data to produce a smooth sheet. A contour overlay is usually made to test the logic of all the data shown. All anomolous depths are rechecked in either the survey records or in the field. If necessary, sonar data are then overlayed to analyze individual features as related to depths. It may take dozens of smooth sheets to cover the area of a complete survey. The smooth sheets are then ready for cartographers, who will choose representative soundings manually or using automated systems from thousands shown, to produce a nautical chart. Documentation of the process is such that any individual sounding on any chart can be traced back to its original uncorrected value. See Figure 3004. 3005. Recent Developments In Hydrographic Surveying The evolution of echo sounders has followed the same pattern of technological innovation seen in other areas. In the 1940s low frequency/wide beam sounders were developed for ships to cover larger ocean areas in less time with some loss of resolution. Boats used smaller sounders which usually required visual monitoring of the depth. Later, narrow beam sounders gave ship systems better resolution using higher frequencies, but with a corresponding loss of area. These were then combined into dual-frequency systems. All echo sounders, however, used a single transducer, which limited surveys to single lines of soundings. For boat equipment, automatic recording became standard. The last three decades have seen the development of multiD V T × 2 -KDr = + +
413HYDROGRAPHYANDHYDROGRAPHICREPORTSANDNAVIGATIONeSOUNDINGBOATSHEETSONOGRAMSECHOGRAMSomJOURNALSCALECORRECTSOUNDINGSPLOTSIDE SCAN TRACKSPLOTPOSITIONSONTRACK SHEETON SONAR COVERAGESHEETENTER0000TIOALDATUMREDUCERSTRACKSHEETReducedsoundingsequals scaledFeature analysissoundings plusandevaluationvarious correctionsSOUNDING SHEET家查欢临TRACK SHEETPencil and inksoundingsANDEOCORRECTIONSREVIEWANDEDITFigure 3004. The process of hydrographic surveying
HYDROGRAPHY AND HYDROGRAPHIC REPORTS 413 Figure 3004. The process of hydrographic surveying
414HYDROGRAPHYANDHYDROGRAPHICREPORTSidepiempaeEW00O000800m50043294386440244824539457345824578457846244727476247744758475847784778479047974806482148304843487948888000m500mFigure3005.Swathversussingle-transducersurvevs
414 HYDROGRAPHY AND HYDROGRAPHIC REPORTS Figure 3005. Swath versus single-transducer surveys
415HYDROGRAPHYANDHYDROGRAPHICREPORTSple-transducer, multiple-frequency sounding systems which areofthissystem isfixed bythedistancebetweenthetwooutermostable to scana wide areaofseabed.Two general types are in use.transducers and is not dependent on waterdepthOpen waters are best surveyed using an array of transducersA recent development is Airborne Laser Hydrogra-spread out athwartships across the hull of the survey vessel.phy (ALH).An aircraft flies over the water,transmitting aTheymayalso be deployed from an array towed behind the ves-laserbeam.Part of thegenerated laserbeam is reflectedbysel at some depthto eliminate corrections for vessel heave,roll,the water's surface,which isnoted by detectors.The rest pen-and pitch. Typically,as many as 16 separate transducers are ar-etratestothe seabottom and is alsopartiallyreflected,this israyed,sweepingan arc of90Thearea coveredbythese swathalso detected,Water depth can be computed from the differ-surveysvstemsisthusafunctionofwaterdepth.Inshallowwaenceintimesofreceiptofthetworeflectedpulses.Twoter, track lines must be much closer together than in deep water.different wavelength beams can also beused, one which re-This is fine with hydrographers, because shallow waters needflectsoffthesurfaceofthewater,andonewhichpenetratesmorecloselyspaceddatatoprovideanaccurateportrayaloftheand isreflectedofftheseabottom.Theobviouslimitationofbottom oncharts.The second type ofmultiplebeam system usesthis system is water clarity.However, no other system cananarrayofvertical beamtransducersriggedoutonpolesabeamsurvey at 200 or so milesper hour while operating directlythe survey vessel with transducers spaced to give overlappingover shoals,rocks,reefs,and other hazardstoboats.Bothpolar andmany tropical waters are suitableforALH systems.coveragefor thegeneral waterdepth.This is anexcellent configurationforveryshallowwater,providingverydenselyspacedDepth readings up to40meters havebeen made,and at cersoundingsfromwhichanaccuratepictureofthebottomcanbetain times of the year, some 80% of the world's coastalmadeforharborand small craft charts.Thewidthoftheswathwaters are estimated to be clear enough for ALH.HYDROGRAPHICREPORTS3006.ChartAccuraciesagencies requiresactive participationbymariners indata col-lection and reporting Examples of the type of informationrequired are reports of obstructions, shoals or hazards to navi-Thechart results froma hydrographic survey canbenogation,sea ice,soundings,currents,geophysicalphenomenamore accuratethanthe surveythe survey's accuracy,inturn, issuch as magnetic disturbances and subsurfacevolcanic eruplimited by the positioning system used. For many older charts.thepositioning system controllingdata collection involved us-tions, and marine pollution In addition, detailed reports ofharborconditionsandfacilitiesinbothbusyandout-of-the-ingtwo sextants to measurehorizontal angles between signalsestablishedashore.Theaccuracyofthismethod,andtoalesserway ports and harbors helps charting agencies keep their products current.Theresponsibility for collecting hydrographicextenttheaccuracyofmodern,shorebasedelectronicpositioning methods, deteriorates rapidly with distance.This oftendata by U.S.Naval vessels is detailed in various directives anddeterminedthemaximum scalewhichcouldbeconsideredforinstructions.Civilianmariners.becausetheyoftentraveltoathe final chart. With the advent ofthe Global Positioning Sys-widerrangeof ports,alsohavean opportunityto contributesubstantialamountsofinformation.tem(GPS)andtheestablishmentofDifferentialGPSnetworksthemariner cannownavigatewithgreater accuracythancould3008.ResponsibilityForInformationthe hydrographic surveyor who collected the chart source dataTherefore,exercisecarenottotakeshoal areasorotherhazardscloseraboard than was past practice because they may notbeTheDefenseMappingAgency,theU.S.NavalOcean-ographic Office (NAVOCEANO), the U.S. Coast Guardexactly where charted. This is in addition to the caution theand theCoast andGeodeticSurvey(C&GS)aretheprimarymarinermustexercisetobesurethathisnavigationsystemandchart are on the same datum.The potential danger to themari-agencies which receive,process, and disseminatemarinener increaseswithdigital charts becausebyzooming in,hecaninformationintheU.S.increasethechart scalebeyondwhat canbesupportedbytheDMA provides charts and chart update (Notice to Mari-source data.Theconstant and automatic update of the vesselsners)and other nautical materials for the U.S.military servicesposition onthe chartdisplay cangivethe navigatorafalsesenseand for navigators in general in waters outside the U.S.NAVOCEANO conducts hydrographic and oceano-ofsecurity,causinghimtorelyontheaccuracyofachartwhenthesourcedatafrom whichthechart was compiledcannot supgraphic surveys of primarily foreign or internationalportthe scale ofthechartdisplayed.waters,and disseminates informationtonavalforces,governmentagencies,and civilians.The Coast and Geodetic Survey (C&GS) conducts hy-3007.NavigationalAndOceanographicInformationdrographic and oceanographic surveys and provides chartsMariners at sea,because of their professional skills andfor marine and air navigation in the coastal zones of thelocation,representa uniquedata collection capability unob-United States and itsterritoriestainable by any government agency.Provision of high qualityThe U.S. Coast Guard is charged with protecting safetynavigationalandoceanographic informationbygovernmentof life and propertyat sea,maintaining aids to navigation
HYDROGRAPHY AND HYDROGRAPHIC REPORTS 415 ple-transducer, multiple-frequency sounding systems which are able to scan a wide area of seabed. Two general types are in use. Open waters are best surveyed using an array of transducers spread out athwartships across the hull of the survey vessel. They may also be deployed from an array towed behind the vessel at some depth to eliminate corrections for vessel heave, roll, and pitch. Typically, as many as 16 separate transducers are arrayed, sweeping an arc of 90°. The area covered by these swath survey systems is thus a function of water depth. In shallow water, track lines must be much closer together than in deep water. This is fine with hydrographers, because shallow waters need more closely spaced data to provide an accurate portrayal of the bottom on charts. The second type of multiple beam system uses an array of vertical beam transducers rigged out on poles abeam the survey vessel with transducers spaced to give overlapping coverage for the general water depth. This is an excellent configuration for very shallow water, providing very densely spaced soundings from which an accurate picture of the bottom can be made for harbor and small craft charts. The width of the swath of this system is fixed by the distance between the two outermost transducers and is not dependent on water depth. A recent development is Airborne Laser Hydrography (ALH). An aircraft flies over the water, transmitting a laser beam. Part of the generated laser beam is reflected by the water’s surface, which is noted by detectors. The rest penetrates to the sea bottom and is also partially reflected; this is also detected. Water depth can be computed from the difference in times of receipt of the two reflected pulses. Two different wavelength beams can also be used, one which reflects off the surface of the water, and one which penetrates and is reflected off the sea bottom. The obvious limitation of this system is water clarity. However, no other system can survey at 200 or so miles per hour while operating directly over shoals, rocks, reefs, and other hazards to boats. Both polar and many tropical waters are suitable for ALH systems. Depth readings up to 40 meters have been made, and at certain times of the year, some 80% of the world’s coastal waters are estimated to be clear enough for ALH. HYDROGRAPHIC REPORTS 3006. Chart Accuracies The chart results from a hydrographic survey can be no more accurate than the survey; the survey’s accuracy, in turn, is limited by the positioning system used. For many older charts, the positioning system controlling data collection involved using two sextants to measure horizontal angles between signals established ashore. The accuracy of this method, and to a lesser extent the accuracy of modern, shore based electronic positioning methods, deteriorates rapidly with distance. This often determined the maximum scale which could be considered for the final chart. With the advent of the Global Positioning System (GPS) and the establishment of Differential GPS networks, the mariner can now navigate with greater accuracy than could the hydrographic surveyor who collected the chart source data. Therefore, exercise care not to take shoal areas or other hazards closer aboard than was past practice because they may not be exactly where charted. This is in addition to the caution the mariner must exercise to be sure that his navigation system and chart are on the same datum. The potential danger to the mariner increases with digital charts because by zooming in, he can increase the chart scale beyond what can be supported by the source data. The constant and automatic update of the vessels position on the chart display can give the navigator a false sense of security, causing him to rely on the accuracy of a chart when the source data from which the chart was compiled cannot support the scale of the chart displayed. 3007. Navigational And Oceanographic Information Mariners at sea, because of their professional skills and location, represent a unique data collection capability unobtainable by any government agency. Provision of high quality navigational and oceanographic information by government agencies requires active participation by mariners in data collection and reporting. Examples of the type of information required are reports of obstructions, shoals or hazards to navigation, sea ice, soundings, currents, geophysical phenomena such as magnetic disturbances and subsurface volcanic eruptions, and marine pollution. In addition, detailed reports of harbor conditions and facilities in both busy and out-of-theway ports and harbors helps charting agencies keep their products current. The responsibility for collecting hydrographic data by U.S. Naval vessels is detailed in various directives and instructions. Civilian mariners, because they often travel to a wider range of ports, also have an opportunity to contribute substantial amounts of information. 3008. Responsibility For Information The Defense Mapping Agency, the U.S. Naval Oceanographic Office (NAVOCEANO), the U.S. Coast Guard and the Coast and Geodetic Survey (C&GS) are the primary agencies which receive, process, and disseminate marine information in the U.S. DMA provides charts and chart update (Notice to Mariners) and other nautical materials for the U.S. military services and for navigators in general in waters outside the U.S. NAVOCEANO conducts hydrographic and oceanographic surveys of primarily foreign or international waters, and disseminates information to naval forces, government agencies, and civilians. The Coast and Geodetic Survey (C&GS) conducts hydrographic and oceanographic surveys and provides charts for marine and air navigation in the coastal zones of the United States and its territories. The U.S. Coast Guard is charged with protecting safety of life and property at sea, maintaining aids to navigation