With last week's declaration by Twitter that it intends to start identifying places based on the coordinates of geo-coded Tweets, the location land rush is in full swing. A long list of companies including Twitter, Google , Foursquare , Gowalla , SimpleGeo , Loopt , and Citysearch are far along in creating separate databases of places mapped to their geo-coordinates. Mapping businesses, in particular, to the GPS locations near where people are checking in, Tweeting from or pegging a photo is the first step to be able to show them geo-targeted ads, which could help fuel local mobile online advertising in a major way. Here is the problem: These efforts at creating an underlying database of places are duplicative, and any competitive advantage any single company gets from being more comprehensive than the rest will be short-lived at best. It is time for an open database of places which all companies and developers can both contribute to and borrow from. But in order for such a database to be useful, the biggest and fastest-growing Geo companies need to contribute to it. I put this suggestion to Foursquare CEO Dennis Crowley the other night at a party, and he was enthusiastic about the idea.