Would it be possible to use the GPS on the Nokia N810 to pick up an external GPS transmitter? Like something you would put on your car or your dog so you could see its location at all times.

I was about to answer "No", but re-reading your question I suspect that what you meant to ask was
"Would it be possible to use the navigation software on the Nokia N810 to receive and use data from a remote GPS receiver".

If so, then yes, in principle at least. But you would need a way of transmitting the data. The GPS receiver on the dog may be combined with a radio transmitter of some sort, e.g. VHF or something. Or even a GSM phone (they use those on sheep). So, what you then need on your N810 is a way of receiving the data. If it's via VHF or some other kind of radio then you would need a receiver, if the dog GPS unit sends data via a GSM phone (which could be via GPRS or even the occasional SMS) then you would need a piece of software to retrieve the data from wherever it gets stored. And finally a little piece of software to feed NMEA data to gpsd, the GPS daemon on your N810.

I have some good news ..........and some bad news.

The good news:
I have already used Maemo-Mapper on my N800 to track our cat.
I duct taped my blue tooth GPS receiver to our cat and loaded the VE (Virtual Earth) map and tracked it. See screen shot:



The purple square denotes the POI (point of interest) which is my house. The blue dot is the location of my GPS receiver. You can select "track" and the blue dot will leave a blue trail of where the cat has been.

The bad news:
Bluetooth has a range of about 30 feet.

If you try this with one of your pets, I would use that blue painters tape instead of duct tape (poor kitty).

How's the cat?

You could do it with two N810s if they are both on the net.

But maybe the cat wouldn't be happy with an N810 attached by duct tape.

You tell me!

lol.. I didn't mention the GPS BT option in my reply.. presumably a dog or a car would be out of BT range most of the time. Or maybe not, my nephew nearly panicked after two hours of looking everywhere for his new puppy. In the end he found the creature sleeping under a blanket.

Use your N810 with MaemoMapper to either connect to a KISS TNC and 2m radio or to the APRS-IS internet feed.

Equip your cat with one of the all-in-one APRS trackers (www.byonics.com) with radio, GPS, tracker, and battery.

Your N810 will update with the location of the second device as it beacons it's position, in real time.

Just need your Ham license.

Ah, yeah, that will work nicely too. I'm seeing cars of friends in maemo mapper that way.