【文章內(nèi)容簡(jiǎn)介】 0. Finally, the human eye?s S can be defined as the size of a retinal cell, and the typical eye has order 108 retinal cells, implying an L/S ratio of 10,000. Interestingly, then, the screen resolution that users find generally satisfactory corresponds approximately to the parameters of the human visual system。 it is somewhat larger, but the puter screen 7 typically fills only a part of the visual field. These examples suggest that L/S ratios of between 103 and 104 are found across a wide range of technologies and settings, including the human eye. Two alternative explanations immediately suggest themselves: the narrow range may be the result of technological and economic constraints, and thus may expand as technology advances and bees cheaper。 or it may be due to cognitive constraints, and thus is likely to persist despite technological change. This tension between technological, economic, and cognitive constraints is well illustrated by the case of paper maps, which evolved under what from today?s perspective were severe technological and economic constraints. For example, there are limits to the stability of paper and to the kinds of markings that can be made by handheld pens. The costs of printing drop dramatically with the number of copies printed, because of strong economies of scale in the printing process, so maps must satisfy many users to be economically feasible. Goodchild [2022]has elaborated on these arguments. At the same time, maps serve cognitive purposes, and must be designed to convey information as effectively as possible. Any aspect of map design and production can thus be given two alternative interpretations: one, that it results from technological and economic constraints, and the other, that it results from the satisfaction of cognitive objectives. If the former is true, then changes in technology may lead to changes in design and production。 but if the latter is true, changes in technology may have no impact. The persistent narrow range of L/S from paper maps to digital databases to the human eye suggests an interesting speculation: That cognitive, not technological or economic objectives, confine L/S to this range. From this perspective, L/S ratios of more than 104 have no additional cognitive value, while L/S ratios of less than 103 are 8 perceived as too coarse for most purposes. If this speculation is true, it leads to some useful and general conclusions about the design of geographic information handling systems. In the next section I illustrate this by examining the concept of Digital Earth. For simplicity, the discussion centers on the log to base 10 of the L/S ratio, denoted by log L/S, and the speculation that its effective range is between 3 and 4. This speculation also suggests a simple explanation for the fact that scale is used to refer both to L and to S in environmental science, without hopelessly confusing the listener. At first sight it seems counter~ntuitive that the same term should be used for two independent properties. But if the value of log L/S is effectively fixed, then spatial resolution and extent are strongly correlated: a coarse spatial resolution implies a large extent, and a detailed spatial resolution implies a small extent. If so, then the same term is able to satisfy both needs. THE VISION OF DIGITAL EARTH The term Digital Earth was coined in 1992 by . Vice President Al Gore [Gore, 19921, but it was in a speech written for delivery in 1998 that Gore fully elaborated the concept (~~Pl9980131 .html): “Imagine, for example, a young child going to a Digital Earth exhibit at a local museum. After donning a headmounted display, she sees Earth as it appears from space. Using a data glove, she zooms in, using higher and higher levels of resolution, to see continents, then regions, countries, cities, and finally individual houses, trees, and other natural and manmade objects. Having found an area of the pla she is interested in exploring, she takes the equivalent of a ?magic carpet ride? through a 3 D visualization of the terrain.” This vision of Digital Earth (DE) is a sophisticated graphics system, linked to a prehensive database containing representations of many classes of phenomena. It implies specialized hardware in the form of an immersive environment (a headmounted 9 display), with software capable of rendering the Earth?s surface at high speed, and from any perspective. Its spatial resolution ranges down to 1 m or finer. On the face of it, then, the vision suggests data requirements and bandwidths that are well beyond today?s capabilities. If each pixel of a 1 m resolution representation of the Earth?s surface was allocated an average of 1 byte then a total of 1 Pb of storage would be required。 storage of multiple themes could push this total much higher. In order to zoom smoothly down to 1 m it would be necessary to store the data in a consistent data structure that could be accessed at many levels of resolution. Many data types are not obviously renderable (eg, health, demographic, and economic data), suggesting a need for extensive research on visual representation. The bandwidth requirements of the vision are perhaps the most daunting problem. To send 1 Pb of data at 1 Mb per second would take roughly a human life time, and over 12,000 years at 56 Kbps. Such requirements dwarf those of speech and even fullmotion video. But these calculations assume that the DE user would want to see the entire Earth at Im resolution. The previous analysis of log L/S suggested that for cognitive (and possibly technological and economic