There’s no really complete free substitute for a good commercial program like Global Mapper or FME for reprojecting raster images from one coordinate system to another. Not only do they support virtually every projection and image format, but they also have batch conversion capabilities for converting many images in a single step. But they’re expensive, and overkill if you only have a small number of basic conversions you want to do; for those basic needs, there are some simple and free alternatives.
MicroDEM – I’ve covered the use of MicroDEM before for converting graphic files with worldfiles into GeoTiffs with both georeferencing and coordinate system data embedded. But you can also use it to convert GeoTiffs in the NAD27 datum to the program’s native WGS84 projection using essentially the same process: open the NAD27 GeoTiff, zoom it to 1:1, and then export it as a GeoTiff in WGS84 (read the original post for more info). That’s all it does, change the datum – it maintains the projection of the original (i.e. if you start with UTM or geographic, the two projections it supports, that’s what you’ll wind up with). And it converts indexed color files to RGB, making the filesize lot larger. But for the common need to convert USGS topo maps from NAD27, like those freely available from Libremap or other sources, to WGS84, it may be enough for some people.
gdalwarp – Part of the GDAL library (included in FWTools), this command-line raster re-projection utility is extremely powerful, but its command-line nature makes it less than user-friendly, and learning the proper codes and syntax for converting from one coordinate system to another isn’t my idea of fun. If you have one conversion you do on a regular basis, setting up a batch file might make that process a bit easier, but a graphical user interface would be even better. Note to command-line fans: Yes, I’m a wimp.
MapWindow – I include MapWindow because, not coincidentally, it does include a graphic interface to gdalwarp that supports many projections/datums, and which also appears to have a limited batch processing capability. Access it using the GIS Tools => Image => Reproject Images menu function (make sure GIS Tools is enabled in the Plugins menu). The image file and coordinate system selections (source and target) are straightforward. The one problem is … I haven’t been able to get it to work on my system. After going through the whole process, I keep getting an error message saying that image export through GDAL isn’t supported. But I’m guessing this will be fixed reasonably soon, and as a bonus, MapWindow also comes with a really nice shapefile coordinate system converter.
But until MapWindow is functional, I’ve found another free program that re-projects georeferenced image files from one coordinate system to another, supports a fair number of datums and projections, and supports export in indexed color format. More on that in the next post.