Extract
Modifications of Tanaka's Illuminated Contour Method.
Introduction
Contour lines are an important cartographic tool used to symbolize continuous surfaces. In representing topography, contour lines allow quantitative measurements of elevation, slope, and relief. One criticism of the contour method, however, is that it does not present an easily recognized visual image of topography, especially to the untrained eye. In 1950, Kitiro Tanaka promoted an innovative method to overcome this shortcoming. His technique assumed an illumination from the northwest (presuming a north-oriented map). He drew white contours to represent illuminated topography, and black contours to represent non-illuminated or shaded topography. All contours were drawn on a gray background to maximize visibility. He varied thickness of black and white contours based on the cosine of the angle [[Theta].sub.T] between two vectors (Figure 1). The first vector A was aspect of a surface, the azimuthal direction of maximum change in slope, and was given by a compass bearing on a horizontal plane. The second vector [I.sub.A] was the azimuthal direction of the illumination vector, also on a horizontal plane. [ILLUSTRATION OMITTED] Tanaka applied a form of analytical hill shading to the contours, as if each contour line were a step on a model surface capable of being lighted from a given illumination direction. The thickness and spacing of black and white contours on a gray background created tonal variations that resulted in a three-dimensional shading effect. Tanaka's method resulted in a strikingly three-dimensional visualization of the topography of a rugged volcanic region of Kyushu, Japan (Figure 2). In his 1950 article, Tanaka called his technique "the relief contour method," although it is normally referred to as the "illuminated contour" or the "Tanaka" method (Imhof 1982). It should be noted that Tanaka was not the first to utilize this method, as several older maps, the first as early as 1870, incorporated illuminated contours on a gray background (Imhof 1982). Tanaka, however, produced what has becom...See the full content of this document
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