Quite a while back, I did a post on Metacarta Labs’ Map Rectifier, an online application that let you upload a raster image, use online mapping services to georeference it, then export it in GeoTiff or WMS format. Map Warper is an extension of this online rectifier, with some similarities in the basic interface. But Map Warper adds some useful extra features:
- Your own personal account, where you can add and manage map images, including map descriptions and tags.
- The option to make your map private (Map Rectifier requires all maps to be public).
- You can crop the map to a user-specified polygon shape.
- You can preview the rectified image as an overlay on top of the reference image.
- In addition to GeoTiff and standard WMS export, you can also export it in KML format as an image overlay, or as a WMS link that can be used in the JOSM OpenStreetMap editor. You can also open a KML file directly in a Google Maps interface.
- On the downside, there’s no Google Maps layers to use for calibration/rectification, only OpenMaps data and Landsat data; the author indicates that he may add Google Satellite imagery at some point in the future, depending on the licensing limitations.
Maps are exported in geographic projection (lat/long), WGS84 datum, making them perfect for use with programs that generate raster maps for GPS units (e.g. TritonRMP Helper for Magellan Triton units, G-Raster for supported Garmin units).
The same map rectification engine is used by the New York Public Library’s Map Rectifier tool, a crowd-sourcing georeferencing tool. You can select from already georeferenced historic maps and export them in GeoTiff format, or select an uncalibrated map and georeference it yourself. Same export functions, although the KML image overlay in Google Earth is a network link, and the Google Maps view doesn’t work very well. Since the MapWarper application is open-source, you could use it to create your own crowd-sourcing website, like the NYPL site, to aid in georeferencing a large number of raster map images.