Formula for Earth radius at any given latitude.

The thread 'distance between lon and lat', at the end of last year, was
interesting. Those with an interest in the subject might want to look at
the Haversine Formula on the page linked below. I can't remember if this
was mentioned in the thread or not, the thread is so long it'll take ages
to check, and my useless newsgroup software won't let me search!

Haversine Formula:
http://www.movable-type.co.uk/scripts/gis-faq-5.1.html


My question is slightly different. The mean radius of the Earth is approx.
6,372 km, but what formula is used to calculate the radius of the Earth
for any given latitude?

I wish my trig was up to working this out but I'm just a humble computer
scientist with no degree in math.

Thank you all for any help in this.

R * cos(latitude)

As I understand your question.

Uffe Kousgaard wrote:

Thanks, but I think I explained my question badly; I'll re-phrase it.

For any specific latitude, what is the formula for calculating the radius
of the Earth at that latitude.

So if I wish to calculate the radius of the Earth at the latitude of
N56:29:35 (56.492956), what formula can I use?

Thanks.

6372 * cos(56.492956/180*pi) = 3518 km.

As you move toward one of the poles, this will finally become 0.

Or do you have Earth flattening in mind? If not, the answer can also be that
the radius doens't change.

Actually, your approach calculates the radius of a circle of latitude.
The radius of curvature of the Earth is a bit more complicated.

This issue is discussed in rather excruciating detail in the FAQ 5.1
article I mentioned in my previous post. (Look for the section labeled
5.1a which is just short of half way through the article.) For
convenience, and because that post could easily get separated from this
one, here it is again:

http://www.usenet-replayer.com/faq/comp.infosystems.gis.html

- Bob