Direct Data Visualization
Many methods of environmental data visualization require mapping (interpolation and/or extrapolation) of sparse measured data onto some type of grid. Whenever this is done, the visualization includes assumptions and uncertainties introduced by both the gridding and interpolation processes. For these reasons, it is crucial to incorporate direct visualization of the data as a part of the entire process. It becomes the operator's responsibility to ensure that the gridding and interpolation methods accurately represent the underlying data.
A common means for directly visualizing environmental data is to use glyphs. A "glyph" refers to a graphical object that is used as a symbol to represent an object or some measured data. For the purposes of this paper, glyphs will be positioned properly in space and may be colored and/or sized according to some data value. For a graphics display, the simplest of all glyphs would be a single pixel. A pixel is a dot that is drawn on the computer screen or rendered to a raster image. The issue of pixel size often creates confusion. Pixels (by definition) do not have a specific size. Their apparent size depends on the display (or printer) characteristics. On a computer screen, the displayed size of a pixel can be determined by dividing the screen width in inches or millimeters by the screen resolution in pixels. For example, a 19" computer monitor has a screen width of about 14.5 inches. If the "Desktop Area" is set to 1280 by 1024, the width of a pixel would be approximately 0.011 inches (~0.29 mm). If the "Desktop Area" were reduced, the apparent size of a pixel would increase.
There are virtually no limits to the type of glyph objects that may be used. Glyphs can be simple geometric objects (e.g. triangles, spheres, and cubes) or they can be representations of real-world objects like people, trees or animals.
For two-dimensional displays we generally use glyph objects which are two-dimensional (having no depth or z-coordinate information). Figure 1.4 is an example of such a display.
Figure 0.3 Two-Dimensional Glyphs
It is once we move to the three-dimensional world that glyphs become much more interesting. In Figure 1.5, cubes (hexahedron elements) are positioned, sized and colored to represent chemical measurements made in soil at a railroad yard in Sacramento, California. Axes were added to provide coordinate references and this picture was rendered with perspective effects turned on. This results in a visualization where parallel lines do not remain parallel and objects in the foreground appear larger than those in the background.
Figure 0.4 Three-Dimensional Cubic Glyphs
When representations of the borings are added, the figure becomes much more useful. Figure 1.6 shows the sample represented by colored spheres and tubes represent the borings. The tubes are colored alternating dark and light gray where the color changes on ten-foot intervals. This provides a reference to allow the viewer to quickly determine the approximate depth of the samples. The borings are also labeled with their designation. These last two figures both represent the same data, however it is clear which one provides the most useful information.
Figure 0.5 Three-Dimensional Glyphs with Boring Tubes
Glyphs can also be used to represent vector data. The most commonly encountered vector data represents ground water flow velocity. In this case, the glyph is not only colored and sized according to the magnitude of the velocity vector, but the glyph can also be oriented to point in the vector's direction. For this type of application, an assymetric glyph (as opposed to a sphere or cube) is used. Figure 1.7 uses a glyph that is referred to as "jet". It is an elongated tetrahedron that points in the direction of the vector. The data represented in this figure is predicted velocities output from a MODFLOW simulation to predict the groundwater flow field resulting from the dewatering of a gold mine pit.
Figure 0.6 Three-Dimensional Glyphs Representing Vector Data
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