Mercator projection | Gnomonic Projection | Chart Projections | Merchant Navy Knowledge

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Mercator projection | Gnomonic Projection | Chart Projections | Merchant Navy Knowledge
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The surface of the Earth is a sphere and charts are flat surfaces. It is impossible to transfer the features on a sphere to a flat surface without distorting the features. In making navigation charts, the chart maker must flatten out the surface of the Earth to put it on a plane. The process is known as Chart Projection.

Terms we must know before learning the types of projections:
Projection - the extension of lines or planes to intersect a given surface; the transfer of a point from one surface to a corresponding position on another surface by graphical or analytical means.

Map Projection - a systematic drawing of lines on a plane surface to represent the parallels of latitude ant the meridians of longitude of the Earth or a section of the Earth. A map projection may be established by analytical computation or may be constructed geometrically.

Tangent - meeting a curve or surface in a single point if a sufficiently small interval is considered.

Plane - a surface of such nature that a strait line joining 2 of its points lies wholly in the surface (a flat surface).


The chart maker starts with a Developable Surface that is the area of the Earth that can be flattened to form the plane. There are different ways that the surface of the Earth can be flattened to make a chart. Each method to project the surface of the Earth onto a chart has its own advantages and disadvantages. The smaller the scale, the more noticeable the difference between the different chart projections. On the larger scale charts, like harbor charts, all the projections are practically identical.



Mercator Projection

Cylindrical Projection
A cylinder is placed around the Earth and is tangent to the equator
The planes of the meridians are extended & they intersect the cylinder in a number of vertical lines.
The parallel lines of projection (longitude lines) are equidistant from each other unlike the terrestrial meridians. On the Earth, the meridians converge with increased latitude.
On the earth, the parallels of latitude form circles whose diameter decreases with increasing latitude.
On the cylinder, the parallels are shown perpendicular to the projected meridians. The cylinder maintains equal diameter throughout so there is distortion as to the diameter of the latitude circles.

Most common projection used for navigation is the Mercator projection which is classified as a Cylindrical Projection.
The cylinder is tangent along the equator.
The meridians and parallels are expanded at the same ratio with increased latitude.
The ratio is know as Meridional Parts - The length of a meridian, expressed in minutes of arc at the equator as a unit, constitutes the number of Meridional Parts corresponding to that latitude. (Used to make Mercator Projections & Mercator Sailings.)
60 nm between each minute of Latitude will look like it is much farther as you get away form the equator. The Parallels are spread out as they get away from the equator but in the real world each degree is the same distance apart.






Lambert Conformal

First you must understand the basic conic projection.
Conic Projection -
Points on the surface of the Earth are transferred to a tangent cone.
When the axis of the cone coincides with the axis of the Earth, the parallels appear as arcs of circles and the meridians appear as strait or curved lines converging toward the nearest pole.
The point at which the cone is tangent, is know as the standard parallel.
A conic projection tangent to the equator is actually a cylindrical projection because the height of the vertex of the cone would be near infinity.
At the poles, the height of the cone is 0 so the cone becomes a plane.

The distance along any meridian between consecutive parallels is in correct relation to the distance on the earth
The scale is correct along any meridian and along the Standard Parallel.

Secant Conic or Conic Projection with 2 standard parallels -
Like the name says, there are 2 standard parallels that the cone is tangent to. This actually cuts into the earth.
The are between the standard parallels is compressed and the area beyond is expanded.
If the spacing of the parallels is altered, such that the distortion is the same along them as along the meridians, the projection becomes conformal. That is called a Lambert Conformal Projection.
Great Circles draw as strait lines - these charts are often used for aeronautical charts or used for polar navigation.



Gnomonic Chart Projection
A plane is placed tangent to the surface of the Earth.
The points are projected from the center of the Earth to the plane.
The projection is perspective - how the Earth looks from a certain point of view & is projected onto a plane to create an image on the chart.

Oblique Gnomonic-
A tangent plane is placed on the Earth. This projection is perspective from the