
dem3d version 2.? requirements and features

The major features in this release have to do with displaying the raster data
in an SDTS Raster Profile DEM transfer. Desired functionality includes:

	*  Easily select a transfer to load. Initially, this will just involve a 
	   file selection dialog with perhaps a "*CATD.DDF" pattern. This specifies the
	   Catalog / Directory module in a transfer, and that module can be used to 
	   find all the other modules in the transfer. Later, this will be enhanced to 
	   display some data the user might find helpful when browsing several transfers
	   looking for a specific dataset or area. (Such as quad name, etc).

	*  Error detection and reporting for both native format and SDTS DEMs. dem3d
	   shouldn't crash if an improperly formated dataset of any type is selected for load.
	   We will probably need to modifiy the DEM library to help detect non native format
	   DEMs. sdts++ will hopefully have enough error detection to trap bad SDTS transfers.

	*  Load the the DEM data. Since USGS DEM transfers only contain one DEM, and dem3d
	   only displays one DEM, the user does not have to select from multiple DEMs. 
	   It's anticipated that additional file formats will be desired later (TIFF,
	   raw grayscale, etc) so the design should be flexible enough to accomodate them.

	*  Display the DEM data. Once loaded, the data should appear no different than native
	   format USGS DEM data. 

	*  The "DEM | Show Header Info..." menu option should be relabeled to be 
	   "DEM | Show Metadata...". Appropriate metadata for both native format and
	   sdts format DEMs should be displayed. Currently, it's up to the developer to
	   decide what should be shown, but at least the "interesting" metadata should be
	   included. Use the native format DEM contents as a guide as display as much of 
	   the correspoding SDTS metadata as possible.

	*  DRG overlay. Since version 1.x, we've experimented with the ability to drape a
	   7.5 minute DRG over a 7.5 minute DEM. We should move this out of the prototype
	   stage but there are a few problems to deal with:

	      * It should work for both 7.5 min native format and SDTS format DEMs.
		  * It should only allow the user to overlay a 7.5 min DRG on the exact same
		    7.5 min DEM. Other combinations should be detected and prevented.
		  * The design should allow for (eventually) arbitrary GeoTIFF images to be 
		    draped onto any DEM, so long as an appropriate projection has been selected
			for the DEM. This hints at the DEM Projection feature to be described later.
		  * Currently a downsampled texture is generated from the full size DRG. We should
		    look into the possibility of using the full size DRG for this purpose.

	*  DEM Projections. All but the 7.5 min DEMs are in geographic coordinates. We're 
	   eventually going to want the user to be able to select a projection and 
	   projection parameters and display the DEM in the projection system. This will
	   be critical if we are to ever have the ability to drape abitrary GeoTIFF images
	   on any DEM.

	*  Status bar information. I'd like for various data to be displayed in the status bar:

	      * DEM elevation under cursor. This can be done by reading the depth buffer and
		    "unprojecting" to find the (x,y,z) of the point in the DEM under the mouse
			cursor. This will involve some redesign of the scenegraph library to allow
			a ShapeNode's cumulative projection matrix to be determined.

		  * The (x,y) of the cursor. This information will be available from the
		    calculation of the elevation under the cursor. This information should be
			displayed in units appropriate to the projection selected for the DEM (meters,
			arc seconds, etc).

		  * The projection in use for the DEM. Currently, this would only be one of two:
		    "UTM" or "Geographic".

		  * Camera Position. Something like "Looking from alt: 76.5 deg, azim: 234.0 deg".
		    This is just an idea. May not be all that useful in practice.

	*  Other nice-to-have features. I'll just mention these briefly since they are at least
	   a major revision away:

	      * Different camera and light motion models. Especially for the camera... 
		    A "magic carpet" model that is like an ideal airplane.

		  * Ability to display multiple DEMs.

		  * An "elevation probe" for interactively determining elevations at user selected
		    points in the DEM. This may not be needed if the mouse cursor - depth buffer
			techique works as well as expected.

		  * Overlay of DLG data.

		  * User specified filters for the DEM data.

		  * 