Wm Randolph Franklin and Clark Ray.
Higher isn't necessarily better: visibility algorithms and experiments.
In Thomas C. Waugh and Richard G. Healey, editors, Advances in GIS Research: Sixth International Symposium on Spatial Data Handling, 751–770. Edinburgh, 5–9 Sept 1994. The International Geographical Union's Commission on Geographical Information Systems and The Association for Geographic Information, Taylor & Francis.
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