Free Geography Tools

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.

Someday, working with latitude/longitude in degrees-minutes-seconds will be a bad memory, and everyone will use either decimal degrees or rational metric units like UTM exclusively. But until that happy day, if you need a desktop utility to easily convert from degrees-minutes-seconds to decimal degrees or degrees-minutes (and the opposite direction as well), you may find it useful to have the Degree/Minute/Second Converter handy. It’s a small download at 307 kB with a full installer, but make note of which Start Menu folder it puts the shortcuts in (default is Accessories). Operation is dead simple:

Enter the degrees, minutes and seconds separated by spaces, set the number of decimal places with the Precision option, select the result type you want, then click Convert. It converts in the opposite sense directly, i.e. enter a decimal degree and have it converted to degrees-minutes or degrees-minutes-seconds. In this latter mode, checking Min/Sec Blank Fill removes the zero from the front of single digit minutes and seconds (e.g. 1 instead of 01 minute, 4.2 instead of 04.2 seconds). Can’t quite figure out what Degree Zero Fill does, though ….

The State Plane Coordinate System (SPCS) is a projected coordinate system that achieves greater accuracy than UTM by dividing states up into sub-zones based on county, and then uses a local projection in that sub-zone (typically Transverse Mercator for zones that are oriented more north-south, and Lambert Conformal Conic for those that run more east-west). Keeping track of which county goes with which zone for every state can be a pain. The State Plane Zone Locator is a free utility that makes looking up the state plane system for any county easy. Download the utility from this site (look for the “Download It!” link), and install it. Oddly enough, the install doesn’t put any shortcuts into the Start Menu or on the Desktop, but the default install directory for the program is “C:\Program Files\MentorSoftwareCO\CountyLookup\”, and the executable is “CountyLookup.exe”‘. Run the program, and use the drop-down selections to choose the state and county you’re interested in:

The State Plane system ID info (NAD83 only) will show up at the bottom, in this example AZ83-C (AZ Central, NAD83). You also get the state and county Federal Information Processing Standards (FIPS) codes.