Cars

52 Number 43, Fall 2002 cartographic perspectives Finally, a discussion of aquafication would not be complete without mentioning the portrayal of ocean floors and lake bottoms with shaded relief and continuous tone depth tints (Figures 1 and 12). Popularized by the ocean floor maps published by National Geographic, this style of bathymetric depiction is being used increasingly on NPS maps to show connectivity between underwater and terrestrial environments. Texture substitution The textures found on aerial photographs contain a wealth of rich visual cues for differentiating land cover. Forest canopies often appear hum- mocky, rocky areas can be rough and fractured, and agricultural fields appear as a patchwork of mottled tones. However, except for some water features, transferring these textures directly to a map is not feasible be- cause of the inherent flaws in aerial photographs discussed earlier. A technique known as texture substitution can be used to partially cir- cumvent this problem. Here's how it works. Using an aerial photograph or satellite image (registered to a base map), land cover is carefully delin- eated as Photoshop selections with pixel-level precision. These selections are then transferred to the base map, where they are filled with generic land cover textures cloned from the aerial photograph, or even from other photographs (Figure 9). The result is a reconstituted final product that selectively brings the best traits of aerial photographs—their often eye-catching beauty, realism, rich textures, and tiny details that readers find so fascinating—to the familiar and readable format of a map. Maps created with texture substitution appear more like a picture of a detailed physical model than an aerial photograph. On aerial photographs tex- tures begin coalescing at scales smaller than 1:50,000 (Imhof, 1982). This problem can be solved by substituting large-scale textures onto a small- scale map. The results of this procedure look surprisingly plausible and natural. The selection of appropriate generic textures to substitute is a critical consideration for preventing a map from turning into a grossly inaccu- rate caricature. Substituted textures must accurately characterize the land cover being depicted, with just enough variability to look authentic. How- ever—and this is extremely important—the variability must also be small and inconsequential in nature so it doesn't mislead readers with false information. Substituted textures should be completely avoided on very large-scale maps, because at these scales the generalized textures would be a blatantly incorrect replacement for actual detail. Substituted textures look most natural when used inside complex land "Maps created with texture substitution appear more like a picture of a detailed physical model than an aerial photograph." "The selection of appropriate generic textures to substitute is a critical consideration for preventing a map from turning into a grossly inaccurate caricature." Figure 8. Kohala, Hawaii: (left) The Pacific Ocean depicted as flat blue. (right) Waves, breaking surf, and a leeward sun glint suggest prevailing northeast trade winds without having to use labels. Clouds are optional.

• Cover