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A hypermedia database is essentially an implementation of a semantic network of concepts and relations. Concepts can consist of text, images, and graphics or combinations thereof. Relations consist of associative links that provide the capability to rapidly move from one concept to a related concept. The typically sequential process of reading as is done with conventional textual material is replaced by a multidimensional, interactive process involving text and images. Semantic nets are a model of memory. They lend themselves to graphic display, and their meaning tends to be intuitively clear. Hypermedia databases have been shown to be object oriented in nature [Campbell and Goodman, 1988]. They can handle more complex structured or interconnected data items consisting of text, graphics and images. A hypermedia database supports structured as well as arbitrary links among pieces of information and as such can be very large. In addition, the computer can exploit the pattern of links in a hypermedia database and give the user different perspectives on the database. For instance, the user may express an interest in "causes", and the system could organize information so as to emphasize the causal links.

Anaglyphic Images

In the process of terrain characterization shape is of primary consideration. Spectral information can aid in classifying surface material and conditions but shape is the primary factor in landform identification. As such, terrain analysis depends heavily on stereoscopic imagery to determine landform characteristics and to make decisions on factors such as trafficability. This imagery is in the form of left and right stereo pairs that are typically examined using a pair of lenses held to the eyes that aid in collimating the eyes to the image pair. Anaglyphs provide an alternative method of presenting two stereo views of an image to the left and right eye. This is done by encoding the views into a single image using different colors, and then viewing through complementary colored glasses (typically red/blue). Since the color channels of the image are used to encode left and right views this is ideally suited for airphoto stereo imagery which is typically gray scale and thus contains no pre-existing color information. In addition, this technique is most suitable for the HTML environment in that it does not require special hardware and is machine independent. The Desert Guide portion of the HTD contains a large number of such 3D images in the form of anaglyphs.

Synthetic Terrain Animations

Because landform shape is so important in terrain analysis other techniques were investigated for conveying three dimensional terrain information in the softcopy, computerized domain. These techniques were constrained by the requirement that they be readily useable in the cross platform HTML environment. 

One such technique was the use of USGS Digital Elevation Model (DEM) data in the production of fractal textured synthetic surfaces. Animations were generated that fly-over the synthetic terrain. This technique presented an improvement, in that many perspectives and scales are smoothly presented providing a greater overview of the terrain shape. Experimentation with various alluvial fans demonstrated that the lower resolution of this data will significantly limit the ability to represent smaller individual landforms or the details of larger landforms. Animations were generated that fly-over the fractal terrain. While the details of the fans in the Silurian Valley could not be easily seen, the animations effectively conveyed the sense of slope variation from the mountain tops, down the fan to the valley floor. Although details of individual landforms may be limited, these animations are very effective in demonstrating landforms relative to one another. Four virtual fly-over animations were generated that illustrate Bajadas in the Silurian valley area of California. These were added to the HTD and are interactively selectable from flight path designations on a LANDSAT image of the area. A stereo (analglyphic) animation of a Bajada was developed and incorporated as an animated GIF sequence.

Panoramic Imagery

Another "virtual" approach to landform shape representation in an HTML environment was utilized. Actual panoramic imagery was used to generate interactive virtual "walk-arounds" in space. With various photographic sequences it may be possible to provide the user with an interactive walk through of landform surfaces. The user actually points and moves about in the imagery.

Video Sequences

A video sequence demonstrating an armored vehicle stuck in a Sabkhah was digitized and added to the hypermedia database. This audio/video sequence very effectively demonstrates the potential hazards of this type of terrain. In the future, video of terrain fly-overs taken from helicopters can also be added to illustrate various major landforms.

JavaScript Display of Spectral Datasets

In the process of terrain characterization, spectral information can aid in classifying surface material and conditions. The Topographic Engineering Center (TEC) has a considerable amount of spectral reflectance data pertaining to desert areas. Some of this data has been implemented in the HTD using an algorithm developed in JavaScript. Not only is a spectral reflectance plot displayed, but it is integrated with the HTML of the spectral data sheet. This allows the ASCII datasets to have its metadata directly associated with it, and thus be self documenting. Links can be made from this mix of JavaScript and HTML to other topics (e.g. landform summary descriptions and pattern indicators) as well as to imagery of the actual surface material represented in the sample.

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