1/12/2010: Unfortunately, the latest versions of the TatukGIS viewer have removed the “Export To Image” function, which was the most valuable function of this program. So I can’t really recommend it anymore; you’d be better off with a free GIS program like qGIS, MapWindow or MicroDEM, which do offer raster combination and export functions, albeit without the same degree of control as the older version of TatukGIS Viewer. If you look around online, you might still be able to find the older version somewhere.
The USGS Seamless Server data gateway I posted about yesterday has a filesize limit per download of 250 MB uncompressed (the zipped, compressed size of the downloaded files is usually much smaller than the uncompressed size). If raster map data you’ve selected for download has a combined size greater than 250 MB, the server breaks it up into smaller segments, each downloadable as a file. If you import all of these separate files together into a GIS program, it will automatically position all the pieces together in the right place. But you might find it more convenient to combine all of the individual pieces together into a single map file with the same resolution, and still georeferenced.
Free GIS programs tend to export the image re-sampled at a different resolution, usually one lower than the original. The TatukGIS Viewer is a big, and useful, exception. If you open the multiple map segments in the Viewer, you can export and save them as a combined, georeferenced image with the same resolution as the original images. Use the Add Layer command to import all the map segments, then select File => Export To Image from the menu. You can choose from JPG, PNG, BMP and TIF for the export image format. For the first three, the Viewer will create a world file that other GIS programs can use to georeference the image; for TIF files, the coordinate information will be embedded into the exported image in GeoTIFF format and also exported as a world file. In both cases, only the coordinate data is exported; the datum and exact nature of the coordinates (e.g. lat/long or UTM or Lambert Conformal Conic or …) is not preserved, and you’ll have to keep track of it some other way.Selecting TIF for the export type, and entering “test” for the filename, you’ll get the following export dialog:
From the Format dropdown, you can choose the color depth of the exported TIF, 24-bit (16.7 million colors), 8-bit (256), 4-bit (16) or 1-bit (black and white). Since maps usually have a limited color palette, you can often save filespace by saving them with a smaller color depth. The dropdown also offers the option of saving the TIFF file with LZW compression, which will make the file smaller, but may result in file compatibility problems with other programs.
For extent, Full saves all the data loaded, while Visible saves only that currently visible on screen; if you’ve zoomed in on one section of the map and only want to save that, you’d use Visible. This is especially useful for MRSID and ECW images that span large geographical areas, where you only need to use a small section of the total image. Under resolution, “best quality” saves it in the original resolution, “document” and “web” save it in a lower resolution, and “custom” lets you set the output size and resolution to the desired level (even upsizing the map if desired). It does add a small label to the lower left-hand corner of the image indicating that the image was created using the TatukGIS Viewer, but it’s unobtrusive.
The TatukGIS Viewer opens a large number of different image formats, including advanced compressed formats like MRSID, ECW, JPEG2000 (with or without world files); PNG, GIF, JPG, BMP, and standard TIF files georeferenced with world files; and, of course, GeoTIFF image files. You can use it to combine or subset any grouping of these kind of image files, and then export and save them in GeoTIFF format. It also handles vector data as well – more about that in the next post.
Edited 3/29 to correct uncompressed filesize from 100 MB to 250 MB.