geography

If you need to locate a user based on their IP address, what services are available (free and not free services are fine)? P.S. I understand that some users use proxies etc., that means the result is not 100% accurate. That's ok. Most geolocation services allow you to download a database full of IP Address to city or country maps. Some also provide web service apis for free (limited number of requests) or a paid subscription. MaxMind has one such service that you can use which is free to determine the user's location. Their minFraud service allows 500 free queries per day. A sample for their

This is a followup to this question . I seem to be stuck on this. Basically, I need to be able to convert back and forth to referring to coordinates either in the standard degree system OR by measuring a distance north from the south pole along the international date line, and then a distance east starting from that point on the date line. To do this (as well as some more general distance-measuring stuff), I have one method for determining the distance between two lat/lon points, and another method that takes a lat/lon point, a heading and a distance, and returns the lat/lon point at the end

I have a set of latitudes and longitudes of locations. How to find distance from one location in the set to another? Is there a formula ? The Haversine formula assumes a spherical earth. However, the shape of the earh is more complex. An oblate spheroid model will give better results. If such accuracy is needed, you should better use Vincenty inverse formula . See http://en.wikipedia.org/wiki/Vincenty's_formulae for details. Using it, you can get a 0.5mm accuracy for the spheroid model. There is no perfect formula , since the real shape of the earth is too complex to be expressed by a formula.