# What is a delta in lat/lon called?

What is the delta between two lat/lon pairs called? Delta degrees? Arcs?

Is it some quantity of distance but what is its unit of measurement?

And related: Can we use pythagoras to calculate this distance?

eg. `d = (0, 3) and (4, 0) = 5?`

You can't use the simple Pythagorean theorem as that one's for planes whereas the distances you are talking about now are on a curve. For that, you'll have to use spherical trigonometry. From Wikipedia:

Let be the geographical latitude and longitude of two points (a base "standpoint" and the destination "forepoint"), respectively, and their absolute differences; then , the central angle between them, is given by the spherical law of cosines: The distance d, i.e. the arc length, for a sphere of radius r and given in radians, is then Note that using r = 6,371.009 metres is appropriate for calculating great-circle distances between points on the Earth's surface, in which case the result d will also be in metres.

It's called a great circle distance btw.

• Ah, and an arc would be the difference between two points on the same line of lat or lon? Nov 29 '12 at 14:09
• (1) The Wikipedia formula is usually not used in practice, due to numerical problems in computing the inverse cosine of small angles: see gis.stackexchange.com/questions/4906. (2) "Arc" is a vague term. In the context of this question, Ricky, you probably mean "geodesic." However, "lines" of latitude are not geodesics (except the Equator itself): that is to say, in traversing from point A to point B (both on a common latitude), you do not stay at a constant latitude. See Why is the 'straight line' path across a continent so curved?. Nov 29 '12 at 16:39

I've seen a Euler Pole plus an angular displacement used to describe "delta". This is used a lot in plate reconstruction, where these deltas are often referred to as "rotations", but I'm not sure if that is a formal definition. 