From Social Patterns
A user wants to annotate a person, place or thing, with a geographic tag. Usually in the form of lat/long, the information is often translated into an address that can be placed on a map.
Use this pattern when you want to place objects (people, places or things) on a map.
If the item is a photo and there is exif data which includes lat/long, associate that location string with the object.
Automatically place the item on a map.
If the item has an address associated with it, like a business, automatically place the item on a map.
Allow the user to associate an address with an object.
Consider allowing users to drag and drop items (photos, listings, friends) onto a map and associating a location with the item.
Allow the user to refine the location by offering an address form. Allow full address, city/state combo or even zip/postal code. Fields should be optional.
Allow users to enter specific latitude and longitude numbers indicating a point on a map, as an alternative to picking a spot on a map.
Use the emerging format for geo data:
The first item is the tag “geotagged” allowing all items to be searched from a common tag. The other tags are geo:lat and geo:long which was established by Geobloggers and has been popularized by sites like flickr, Panoramio and delicious.
Clearly indicate how the information will be used.
Offering geo-tagging capabilities to assets, particulary photos, allows them to not only exist in time but also in a context related to the real world. Placing images on a map give people a sense that this image really exists and if I went there I might see this too. Geo-tagging also announces to a person's network that they were there, at this spot.