Hi John, Well there would be some level of accuracy question. The more accurate the polygon the larger the database. Also if the lat/lon data was provided by GPS, there is the question of signal availability. Basically it just seems dumb that I need to tell my GPS what time zone it's in. LOL Best Regards Larry ----- Original Message ----- From: John Haxby To: tz@lecserver.nci.nih.gov Sent: Thursday, February 03, 2011 1:54 PM Subject: Re: lat/lon to time offset database On 3 Feb 2011, at 18:33, Larry Ober wrote: I am looking at building a small embedded system (No OS) that will require determining the local time from the lat/lon of a location. So basically I need lat/lon to time offset from "GMT". Does that work? I'm sure I can find places in Spain that are that are close to Lisbon than they are Madrid but being in Spain the timezone should be Europe/Madrid. Also, when you're on going through the channel tunnel do you go from being closer to Paris than you are to London only when you've left Calais? Or before you get to Dover? Or have I got hold of the wrong end of the stick? jch