The nautical chart is an image of a part
of the earth in two dimensions. This reproduction is a two dimensional
image of a part of the earth, which is of course 3 dimensional. This
results in various distortions, but as long as two requirements are met we
can use this image for navigational purposes. Firstly, the angles between
three objects in the chart should be the same as the angles between the
real objects which they represent. Secondly, a straight course should
appear as a straight line in the chart.
To fulfil these demands our chart ought to have both parallels & meridians
which are straight and parallel. As such the meridians & parallels will be
perpendicular to each other.
A well known method to create such a chart is called the Mercator
Projection after Gerard Kremer (Mercator),
a Flemish scholar who studied in 's Hertogenbosch and Leuven. In 1569 he
invented the projection which made him famous.
His chart was designed for sailors and constructed by wrapping a cylinder
around the planet so that it touches the equator. On this cylinder the
surface of the earth is projected and finally the cylinder is cut open to
yield our chart. But where the meridians converge on the globe they run
parallel in the projection (see chart below), indicating the distortion.
Look, for example, at a high parallel. The length of such a parallel on
the globe is much smaller than the equator. Yet, on the chart they have
exactly the same length creating a distortion which gets bigger nearer to
the poles. The figure below shows us the construction of the mercator
projection. From this it is clear that only the vertical scales should be
used for measuring distances.
scale depicted on the right demonstrates the distortion. While the two
little grey markers have the same size, the upper one measures only 0.71
degrees. So, distances (in miles or in minutes) should not only be read on
the vertical scale, but also at approximately the same height.
The horizontal scale is only valid for one
latitude in the chart and can therefore only be used for the coordinates
(a point, but not a line). If you divide the surface of the earth in eight
pieces, and lift one out and project it, you end up with the figure below.
The result is that both A-A' and B-B' are now as long as the bottom of the
chart and are 'too long'.
But there are of course other
projections in use by sailors. An important one is the
Stereographic projection which is constructed
by projecting on a flat plane instead of a cylinder. On this chart
parallels appear as slightly curved and also the meridians converge at
high latitudes. So, strictly speaking, a straight course will not appear
as a straight line in the chart, but the parallels remain perpendicular to
the meridians. Most often, distortions are scarcely noticed when this
projection is used to chart a small area. Like the mercator projection,
the vertical scale represents a meridian and should be used for measuring
Another projection is the Gnomeric projection
on which the meridians are again converging. But most importantly, the
parallels are arcs of a circle while great circles appear as straight
lines. On a sphere the shortest route between A and B is not a straight
line but an arc (part of a great circle). Though this is also true when
you -for example- cross a little bay, we use for simplification a
Loxodrome (a handy straight line on your
mercator chart which does not reflect your shortest route). On a Gnomeric
chart this same loxodrome is an arc, while your shortest route (a great
circle) ends up as a straight line. Hence, the gnomeric projection is
particularly useful when sailing great circles (like when you dabble in
circumnavigation) and is clearly beyond the scope of a coastal navigation
Organization of the
The publisher responsible for the information in the chart.
"British Admiralty Charts" or "Imray Charts". Check their
The Title gives a description of the area covered by the
chart. For example: "The Mediterranean Sea".
Different chart types of the same area can be distinguished
by the chart's number.
Most likely the Mercator projection as described above.
Charts covering small areas can be constructed by
For example: 1:193000. But since the chart is distorted this
holds only for one specific latitude in the Mercator chart.
The scale gives an indication of how detailed the chart is.
The definition of the relationship between the ellipsoid
adopted as the model of the Earth's shape, and the Earth
itself. Though there are hundreds of datums in use, most are
only locally valid.
The WGS-84 datum
is global in scope and positions obtained by satellite
navigation systems are usually referred to this datum.
Therefore a correction needs to be applied to a WGS-84 GPS
position to agree with charts using other horizontal datums.
For example to correct WGS-84 to the European datum, add
0.06'N, 0.04'E to the WGS-84 position indicated by the GPS.
Fortunately, most GPS receivers may be set to display
positions in several other datums besides WGS-84.
Chart Sounding Datum:
The tidal datum to which soundings and drying heights on a
chart are referred. Often shortened to 'chart datum' when it
is clear that reference is not being made to a horizontal
datum. Chart Sounding Datums are also used as reference for
heights (lighthouses, mountains, bridges). Multiple datums
can be used in one chart: L.A.T. for soundings and M.L. for
Soundings & Height
Units: Soundings and Heights can
be stated in -for example- meters, feet or fathoms. Nowadays
even most British charts use the metric system.
Natural scale at for example 40° 15',0 latitude where the
horizontal scale can be used for measuring distances and
where the chart scale is true.
Most charts neither have the precision nor the resolution to
fully use the (differential) GPS positioning potential.
Moreover, still plenty of charts result from surveys done in
the 19th century.
Also, GPS data often requires a correction for a local
horizontal chart datum before it can be used in the chart.
Corrections & Edition:
The chart is for example a 1996 edition but is - when
properly corrected - still valid in 2000. Corrections are
published several times and should be mentioned in the
bottom left corner of the chart.