Re: [udig-devel] Add images

["Kenton Williams" <Kenton_Williams@xxxxxxxxxxxxxx>]

If a world file does not exist, you can create one, but only if you know the following (values included as an example:

20.154 <the dimension of a pixel in map units in the x direction>
0.000 <rotation term for row>
0.000 <rotation term for column>
-20.154 <the dimension of a pixel in map units in the y direction>
424178 <the x coordinate of the center of pixel 1,1 (upper-left pixel)>
4313415 <the y coordinate of the center of pixel 1,1 (upper-left pixel)>

So, a world file using the values above would look like this if you opened it in a text editor:

20.154
0.000
0.000
-20.154
424178
4313415

These world files are just text files with a special extension.  For TIFF files, the world file extension is .tfw, for JPEG, it's jpw.  If you know the items that belong in the world file, just open up Notepad (in Windows) and make the file there,
saving it with the proper extension.  If, for example, your TIFF file is named somefile.tif, the world file would need to be named somefile.tfw.  If you have a JPEG named aerialview.jpg, your world file would be named aerialview.jpw.

See this website as a reference:  http://www.nps.gov/gis/av3_online/documents/section7/docs/section7_pg3.html

This method only gets you a coordinate reference, and does not have the ability to specify the name of the projection in use, nor can you specify the datum in use either.  If you don't have the information to go into the world file, you can use a
program called Hypercube to georeference (warp) the image - see http://www.tec.army.mil/Hypercube/ for the free download (Mac or PC).  You must read the instructions carefully to get the warping utility to work and you will need something that has a
coordinate reference system already to use as a target.  I have used shapefiles before, but it didn't work very well - Hypercube converted it to a very low resolution raster for target use.  If you can find and download a digital orthophoto
quadrangle (DOQ) for your area of interest, use that for target coordinate reference as long as the coordinate system of the DOQ matches that of your other data.  If not, something will have to be reprojected.

This may sound like an overly complicated concept, and it's certainly not a simple one.  With some patience and perseverence, you can do it.  Being able to georeference imagery for use in GIS is a very important skill that you can add to your resume.

Good Luck!

Kenton W.