Like old maps? Have a little time on your hands? Maybe you want to participate in the New York Public Library's Map Rectifier project. NYPL put its collection of historic maps online, and is asking the public to help align all the old maps against a more precise contemporary map. The rectification process not only allows everyone to compare then and now, but it also helps resolve inaccuracies in the old maps. The contemporary map NYPL is using for this project is Open Street Map, which is sort of Wikipedia for cartography. By tagging "control points," specific coordinates that are constant for both maps, the historic one is brought into alignment with Open Street Map. At the NYPL site, in addition to actually doing the rectifying, members of the public can browse all the historic maps and also view the rectified ones in Google Earth. A video tutorial on the NYPL website, using an 1860 map of Central Park as an example, makes the process seem easy enough for anyone with basic computer skills. This project is an interesting example of crowdsourcing, and a great way to get to know a city better.