Kaartpassen
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Transcript of Kaartpassen
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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
