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ChartChart fundamentalsfundamentals

Whats a nautical chart?

A special-purpose map or book or a specially compiled database from which such a

map or book is derived that is issued by or on the authority of a government,

authorised hydrographic office or other relevant government institutions, and is

designed to meet the requirements of marine navigation

Shows: water depth, shoreline, topographic features, aids to navigation (buoys &

lights), hazards to navigation (wrecks), other navigational information

Work area:

o Plots courses

o Ascertains positions

o Relationship of ship to surrounding area

Assists in avoiding dangers & arriving safely CONTINUOUSLY CORRECTED

Different forms

o Paper charts

Traditional

British Admiralty: > 3000 paper charts

Major activity: position plots at regular intervals

o ARCS

= Admiralty Raster Chart Services

Digitally scanning of paper chart

Displayed in ECDIS (= Electronic Charts Display Information Station)together with position derived from e.g. GPS

No intelligence

Same accuracy and reliability as paper chart

o ENCs

= Electronic Navigation Charts

New navigation methodology

Vector charts compiled from database

Intelligent: systems can be set up to give warning of danger

Only vector charts that may be used in place of paper charts

A little history of charts

Earliest maps: clay-maps from Babylon (3 500 years ago)

Ancient Greeks developed geographic science and cartography

650 BC: Mediterranean = centre of world

Twelfth century: charts created by mariners by plotting coastlines along constant

compass bearings = portolan charts (no curvature of earth; network of direction lines

(rhumbs); great deal of detail on coasts)

Fifteenth century: means of sailing out of sight of land => 2 problems: longitude +

projection on plane surface Sixteenth century: Mercator chart

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Geographical coordinates

Earth is 3-dimensional => other coordinates than x- and y-axes

Meridians (longitude + converge at poles) and parallels (latitude + parallel to eachother)

Great circles: all meridians + equator (divides earth in two exact halves)

Small circles: remaining parallels

Geodesy = branch of earth sciences or the scientific discipline that deal with the

measurement and representation of the earth

o Geoid:

Reference surface for heights/depths

Imaginary surface perpendicular to plumb line

Coincides on average with mean sea level

VERTICAL DATUM

o Ellipsoid

Reference surface for locations

HORIZONTAL DATUM

Chart projections

Introduction

Different types

Desirable properties of projection:

o True shape of physical features

o Correct angular relationship (= conformal / orthomorphic)

o Areas in correct relative proportions

o Constant scale values

o Great circles as straight lines

o Rhumb lines as straight lines

Some are mutually exclusive

Types of projections

Type of developable surface determines classification

Further classification depends on centre

Name indicates type and principal features

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Cylindrical projections

Mercator projection

Cylinder around earth, tangent along equator & planes of meridians extended

Lines of projection equidistant from each other (meridians are parallel)

Parallels perpendicular to meridians and of same diameter DISTORTION (bigger near to poles)

Meridional parts:

o At equator: degree of longitude = degree of latitude

o As distance increases: degrees of latitude the same, degrees of longitude

shorter

o On Mercator: degrees of longitude the same => increase length of meridians

o Distance must be increased by same amount the actual length of parallel has

been extended

o Expansion = secant of latitude => e . sec l = g

ORincreased meridian length = latitude x sec latitude

o New length of meridians is called meridional parts (lc) & expressed in minutes

Disadvantages Mercator projection

o Projection cannot include poles

o Great circle tracks = curved lines

o Small areas in correct shape but increased in size

Advantages Mercator projection

o Conformal, expansion same in all directions & angles are true

o Direction measured directly

o Distances measured directlyo Rhumb lines (= lines of constant heading) = straight lines

Transverse Mercator projection

Cylinder tangent along meridian

Tangent great circle is fictitious equator

Actual meridians and parallels appear as curved lines

Straight line same angle with fictitious meridians, not with terrestrial meridians

Used for representing small area in exact shape

Star charts

Oblique Mercator projection

Cylinder tangent along great circle other than equator or meridian Depict are near vicinity of great circle

Rectangular projection

Uniform spacing of parallel

Where distortion is not important

Star chart

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Conic projections

Simple conic projection

Single tangent cone

Meridians as straight lines converging toward nearer pole

Standard parallel (tangent to cone) as arc of circle with centre in apex of cone Other parallels concentric circles

Distance along meridian between consecutive parallels is in correct relation with

distance on earth

Circle represents pole

Scale is correct along any meridian and standard parallel

Other parallels too long

NOT CONFORMAL

Mapping area with large spread of longitude and narrow band of latitude

Lambert conformal projection

Secant cone intersecting at 2 standard parallels => increases useful latitude Area between standard parallels = compressed, beyond = expanded

Spacing of parallels altered => distortion same among parallels as meridians =>

CONFORMAL

Great circle = very approximately straight line

Aeronautical charts & polar region

Polyconic projection

Series of cones

Each parallel base of tangent cone

Scale correct along any parallel & central meridian

Other meridian: scale increases Parallels as non-concentric circles, meridians as curved lines converging to pole

Used in atlases

NOT CONFORMAL

Azimuthal projections

Points projected directly

Bearing of any point from point of tangency is correct

Simplest case: plane tangent at one pole, meridians straight lines, parallels concentric

circlesGnomonic projection

Plane tangent to earth

Points projected geometrically from centre

When oblique

o Meridians as straight lines converging to nearer pole

o Parallels except equator as curves

o Distance scale changes rapidly

o NOT CONFORMAL NOR EQUAL AREA

Distortion very great

Any great circle as straight line

Ocean passages!

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Stereographic projection

Tangent plane

Projection from point on surface opposite to point of tangency

Scale increases, but more slowly

Entire hemisphere without excessive distortion

Great circles through point of tangency as straight lines, others as circles or arcs of it

Polar region

Orthographic projection

Projection from infinity to tangent plane

NOT CONFORMAL NOR EQUAL AREA

Used in navigational astronomy

Useful illustrating and solving navigational triangle

Equator and parallels appear as straight lines (if plane is tangent at point on equator)

Meridians as ellipses, except through point of tangency

Azimuthal equidistant projection

An azimuthal equidistant projection is an azimuthal projection in which the distancescale along any great circle through the point of tangency is constant

If pole point of tangency => meridians as straight lines & parallels equally spaced

concentric circles

Other point => concentric circles represent distances from point, meridians & parallels

as curves

NOT CONFORMAL NOR EQUAL AREA

Entire earth can be shown

Used for star finder

Polar projections

Principal considerations

o Conformality (angles shown correctly)

o Great circle representation (great circles as straight lines, more useful at high

latitudes)

o Scale variation (constant scale)

o Meridian representation (straight meridians)

o Limits (small area)

Modified Lambert conformal projection

o

Parallel very near pole as higher standard parallelo Little stretching to complete circle of parallels

o Nearly conformal

o Great circle almost a straight line

o Scale distortion little if carried out about 25 or 30

Polar stereographic projection

o Conformal

o Straight line closely approximates a great circle

o Scale distortion not excessive but greater than modified Lambert conformal

projcetion

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Syntax

Mercator projection Daily navigation

Advantages Disadvantages

Conformal

Constant scale Rhumb lines as straight lines

Positions can be read

Great circle track as curved lines

No poles No equal area

Gnomonic projection Crossing oceans / passage planning

Advantages Disadvantages

Great circles as straight lines

Azimuth is the same

Not conformal

Not equal area

No constant scale

Rhumb lines as curved lines

Polar stereographic projection Polar navigation

Advantages Disadvantages Conformal

Great circles as straight lines

Keeps azimuth

Constant scale

Meridians as straight lines

Rhumb lines as curved lines

Distortion

Chart informationChart information

Chart scales

Scale is ratio of a given distance on the chart to the actual distance, which it represents

on the earth

Small-scale chart: large area, for route planning & offshore navigation

Large-scale chart: small area, used as vessel approaches land

British Admiralty classification

o Sailing charts

Smallest scale

Planning, fixing position at sea, plotting dead reckoning on longvoyage

< 1:600,000

Shoreline and topography generalized

Shown: offshore soundings, principal navigational lights, outer buoys

and landmarks visible at considerable distance

o General charts

Coastwise navigation outside of outlying reefs and shoals

Between 1:150,000 & 1:600,000

o Coastal charts

Inshore coastwise navigation, entering and leaving bays & harbours ofconsiderable width, navigating large inland waterways

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Between 1:50,000 & 1:150,000

o Harbour charts

Navigation and anchorage in harbours & small waterways

> 1:50,000

Factors relating to accuracy

Accuracy depends upon accuracy hydrographic surveys

Source notes given in the title refer to those surveys

Based upon very old surveys => caution

Number of soundings and their spacing indicates completeness of survey

NAVIGATOR SHOULD USE THE LARGEST SCALE CHART AVAILABLE FOR

THE AREA IN WHICH HE IS OPERATING, ESPECIALLY WHEN OPERATING

IN THE VICINITY OF HAZARDS

Colours

At least 4 colours, other colours may be used for buoys and indicating light sectors

o Pale gold: land areas on metric Mercator chart

o Darker gold: more urban areas

o Grey: land areas on fathom chart

o Black: most symbols, printed information (title block, chart number ) & all

borders

o Magenta: multi-use colour because shows well under red light, attracting

attention (routeing measures, safety zones, ice limits, compass roses, lights andlight ranges, radio reporting points, caution notes)

o Blue: water areas => how darker the blue, how shallower the water

o Green: drying heights

Soundings

Drying heights

Heights

Title block

Information in the chart

Chart symbols

Plotting and pilotingPlotting and piloting

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Introduction

Dead reckoning

Lines of position

The position fix

Chart principles

Running fix

The estimated position

Relative bearing

Danger bearing

Turn bearing

Double angle fix

Four-point fix

Special angle fix

Tides and currentsTides and currents

Origin of the tides

Chart datums

Tides and tidal predictions

Information from the chart

Information from tide tables

Information from tidal curves

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Tidal stream versus current

Tidal streams

Currents

Co-tidal / co-range charts