Difference in use between "geodetic" and "geodesic" terms

Question

I usually write questions and answers on https://gis.stackexchange.com.

English is not my first language, and every time I have to write any of these terms I come back into doubt about which of the two corresponds.

A geodesic, can be understood as the shortest path (contained in a curved surface) between two points. (https://en.wikipedia.org/wiki/Geodesic)

But I have the problem when I have to refer to something related to geodesy (https://en.wikipedia.org/wiki/Geodesy), e.g., "geodesic distance", "geodesic calculations", "geodesic math", "direct geodetic problem", "geodetic datum", "geodetic coordinates".

I don't know which of the two terms I should use.

What is specifically the use that should be given to the terms "geodetic" and "geodesic"?

Answer 1

I too frequent the GIS Stack Exchange and use English as my first language in Australia.

I was not sure of the answer so I used search terms of "adjective geodesy" to find this Dictionary.com definition for geodetic:

geodetic [jee-uh-det-ik]

adjective

1. pertaining to geodesy.
2. geodesic.

It looks like geodetic and geodesic mean the same thing and are equally valid to use. My preference is to use geodetic as the adjective of geodesy.

Answer 2

The definitions use here mostly come from the US DMA Glossary of mapping, charting and geodetic terms.

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There should be only one adjective, geodesic, from geodesy.

Geodesy: (1) The science concerned with determining the size and shape of the Earth. (2) The science that locates positions on the Earth and determines the Earth's gravity field. The definition can be extended to other planetary bodies. (3) The branch of surveying in which the curvature of the Earth must be taken into account when determining directions and distances.

The French term ligne géodésique (geodesic line), the equivalent of a straight line on a curved surface, in particular on Earth ellipsoid, was introduced by Laplace in Traité de mecanique céleste and analyzed by Biot in Analyse du Traité de mécanique céleste, de P.-S. Laplace (1800):

La ligne géodésique est très propre à nous éclairer sur la véritable figure de la terre.

However ligne géodésique was translated both into geodesic line and geodetic line by different authors, including Hutton who proposed both translations when explaining Laplace idea:

Hutton in a A Course of Mathematics for the Use of Academies, as Well as Private Tuition (1822):

A line traced in the manner we have now been describing, or deduced from trigonometrical measures, by the means we have indicated, is called a geodetic or geodesic line.

The two terms are synonymous, in the DMA glossary a geodetic line is actually a geodesic line:

• Geodetic line: See line, geodetic
• Line, geodetic: A geodesic on an ellipsoid

But as of today some usages are commonly accepted for the two adjectives:

• Geodesic: Pertaining to the geodesic line, e.g. geodesic curvature.

• Geodetic: Taking into account Earth spheroid shape.

  • Geodetic survey: A survey in which the figure and size of the earth is considered.

  • Geodetic coordinates: The quantities of latitude, and longitude, which define the position of a point on the surface of the earth with respect to the reference spheroid. Also called geographic coordinates.

  • World Geodetic System WGS 84, the system based on WGS84 ellipsoid and used for GNSS coordinates.

  • Geodetic datum, geodetic equator, geodetic azimuth, etc.

Thus geodetic is used in relation with Earth shape and geodesy, while geodesic is used in relation with a geodesic line, and has a mathematical sense.

Answer 3

My understanding is: geodesic approximates the surface of the earth by being based on an ellipsoid geodetic does the same but includes a correction for altitude.

So these equate to the same thing if you are measuring the distance between ships at sea but if you measuring the distance between cities, or a ship and the top of Mt Everest they differ.