# What's the difference between [geodesy] and [geodesic]?

Apparently we have both and . The former has about 37 questions and the latter a mere 4.

Is there an actual difference between the two concept? They lie so far beyond my area of expertise, that I can't even begin to judge that.

I'd be glad if someone knowledgeable post some answer, and perhaps we can add them as synonyms, or merge them.

• From what Google has told me, "geodesy" is, loosely speaking, a study of the shape/size/position of objects on the Earth, while a "geodesic" is the shortest curve along a surface between two given points. I have a suspicion that few — if any — of our geodesy questions should be so tagged. – user642796 Oct 4 '13 at 12:30
• Am I the only one that feel like calling the tag geodesic, rather then either geodesics or geodesic-spaces seems odd? – Tobias Kildetoft Oct 4 '13 at 12:32
• @Tobias: I thought that simplicial-stuff was weird, but I was told it's quite normal. Since then, whenever something has to do (even remotely) with geometry, I am ambivalent towards it, like a cat towards global warming. – Asaf Karagila Oct 4 '13 at 12:34
• @ArthurFischer For all we know, it could be established mathematical lingo for "studying geodesics and geodesic spaces". That could easily be buried among the overwhelming everyday use of the term. It should also be a strong indication that geodesics (keeping in line with the plural convention for tags on MSE) seems to be the tag we should pursue -- perhaps with geodesy as synonym. – Lord_Farin Oct 4 '13 at 12:41
• @Lord_Farin: Perhaps enlightening is that a search on the arXiv for papers in the mathematics section whose abstract contains "geodesy" gave five hits. All but one used the term in a manner very similar to what I gave above. – user642796 Oct 4 '13 at 12:52
• @Lord_Farin: But I am perfectly willing to admit that I could be absolutely incorrect and just not in the know. That is why I posted a comment as opposed to an answer. – user642796 Oct 4 '13 at 12:53
• @ArthurFischer Good numerical evidence you bring in. It seems that we agree, however: "geodesy" is an ambiguous, quirky term for the subject at best (if it is one at all), so we'd better stick with geodesics. – Lord_Farin Oct 4 '13 at 12:57
• @Lord_Farin: I would second that. – user642796 Oct 4 '13 at 12:58
• It wouldn't be the first time people on this site have taken an established name for a existing technical field and used it as a tag for whatever they feel it sounds like it means... :( – user856 Oct 4 '13 at 18:17
• Geodesy is basically geographical surveying and measurement, often at a large scale and including longitude and latitude issues. About 5 of the questions tagged with geodesy have that tag correctly and the others should be tagged with geodesic. I will change the wrong ones. – Henry Oct 5 '13 at 0:40
• @Henry: That's a 30+ number of questions. Please try to limit yourself to a few edits a day (or at least a few edits every several long hours). – Asaf Karagila Oct 5 '13 at 0:42
• @Henry: Edits like that bump the questions, and it causes a huge flood on the main page. Pacing makes it... less bad. I'd also recommend posting a link to this meta thread (either on the post, or in the edit comment), but that's less critical. Perhaps if the author used the wrong tag more than once... – Asaf Karagila Oct 5 '13 at 0:45

There is a substantial difference between the two: Geodesy is basically geographical surveying and measurement, often at a large scale and including longitude and latitude issues, while a Geodesic is about extending some properties of straight lines to curved and other spaces.

So they should be kept distinct, even if they overlap when considering the shortest path on a sphere, spheroid or oblate ellipsoid of revolution.

Most of those previously tagged with geodesy should have been tagged with geodesic, so I have changed them. There are now $7$ tagged with geodesy and $34$ with geodesic, almost reversing the previous position.

• Perhaps you can also add tag wikis and excerpts to point out this difference, so it'll be easier to track the correctness of the tagging in the future? – Asaf Karagila Oct 8 '13 at 0:06
• @Asaf Karagila: I had already done that, but it might be worth somebody else improving them. Geodesy: The measurement and representation of the Earth, its shape and gravitational field, distances on the Earth, and coordinate systems for locating points on it including longitude and latitude. Geodesic: A geodesic is a generalisation of the notion of a straight line to curved spaces, and can be sometimes be thought of as the locally shortest or extremal path between points. – Henry Oct 8 '13 at 7:43

In metric geometry (in particular, in Riemannian geometry), a geodesic is a curve that gives you one possible shortest path between two points. It is a straight line on a plane, an arc on a sphere, a piecewise broken straight line when walking on a polygon or in Manhattan (with the metric d=|x_1-x_2| + |y_1-y_2|), etc.

I have never heard the term geodesy, even though I wrote a thesis on the subject of geodesics.

• I know what a geodesic is. – Asaf Karagila Oct 10 '13 at 21:03