Wednesday, September 07, 2005

Digital Raster Graphics

Among the limitations of the shareware/demo version of OziExplorer is the inability to import map images in formats that provide georeferencing information like tfw (tif), jgw (jpeg), and GeoTiff file formats.

If you go to the USGS website at http://topomaps.usgs.gov/drg you will find out about DRG's:

"A digital raster graphic (DRG) is a scanned image of a U.S. Geological Survey (USGS) standard series topographic map, including all map collar information. The image inside the map neatline is georeferenced to the surface of the earth and fit to the Universal Transverse Mercator projection. The horizontal positional accuracy and datum of the DRG matches the accuracy and datum of the source map. The map is scanned at a minimum resolution of 250 dots per inch."

If you are looking for images from CA you can download DRG's, which in fact translates to a .tiff file and the corresponding .tfw georeference at the California Spatial Information Library.

There select the MapSurfer Interface. Zoom to the area of interest, click the red information button, drag a rectangle in the area of the map you are looking for and wait for a list of options to be displayed at the bottom of the window.

Now click the link for the maps you want to download. You will find among them 1:250K, 1:100K and 1:24K Series on NAD 27 and NAD 83 datum in untrimmed or trimmed files. Download both the .tif and .tfw extensions.

In OziExplorer Trial (not the Shareware/Demo version), select File | Import Map... Single DRG File. Point to the directory where you saved both the .tif and the .tfw files. Remember which NAD datum you picked and select the correponding option in the import combo box.

After this you will have a .map file in the formatted supported by OziExplorer opened for use. The colors from the resulting image appear washed out, showing the extra work the Topo! guys had to get better results from the original maps. It won't have street names either. But you do have a much higher resolution and several more zoom levels.

Here you will find some more information regarding DRG's files like what exactly is this trimming business:

"The standard DRG images include map collar information as displayed on a regular USGS paper map. Trimmed DRG images have been clipped to the neatline of the quad, allowing for easier image mosaicking and edgematching."