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if the frequencies with which tactical gaps* of specific kinds will be en- countered are known. Since there was adequate evidence to demonstrate that different geographic regions exhibit different numbers and kinds of tactical gaps ,it was essential that a consistent and objective procedure be developed which, when used, would define the tactical gap characteristics of any geo- graphic region to which it was applied. 7. In view of these considerations, two of the major objectives were: a. To develop a widely applicable procedure for the acquisition of reliable data on the characteristics of a militarily sig- nificant tactical gap (hereafter called "gap"). The proce- dures which were to be developed were to be as general as possible so that only a minimum of modification would be required to make them applicable to any region of the world. b. To develop a procedure for presenting the acquired gap data in a form in which they could be readily used in operation research models dealing with the prediction of performance of various tactical gap crossing systems (bridging, rafting, fording, swimming) and their related engineering activities (grading, mat laying, surfacing). 8. At the time of initiation of the project, there were no performance prediction models for the various gap-crossing systems. Lacking the identi- fication of significant terrain factors that would have been provided by the models, it was thus necessary to study the various gap-crossing systems in considerable detail. From this examination, all terrain attributes that could be hypothesized to affect tactical gap-crossing materiel and activi- ties were derived. Table 1 is a matrix illustrating the relations among terrain factors and gap-crossing systems. 9. Even before data collection can begin, there are always two nagging sets of problems that must be resolved. First, since the significant factors are distributed in both time and space, the method of presenting the data for use or analysis must be established. In most instances, this is some form of map. The fact that maps are to be used raises the second question, namely, classification. Because classification is an unavoidable concomitant of any mapping process, consideration of the nature of an ideal classification system is relevant and, indeed, essential. There is one overwhelming requirement. The class intervals should be such that any point selected between class limits would introduce only an acceptably small error in prediction. That is, if all other factor values are held constant, the difference in prediction that would result from accepting a point taken near the lower end of the class range and -.:......'-,-,-...--..~-.-------.....-.-....--- *A tactical gap is a linear terrain feature (such as a ditch, canal, or river) which tends to impede the cross-country movement of personnel or vehicles. 15