at a given location. In the case of wind, speed and direction can be markedly affected by natural topographical features, such as hills and valleys, and by the presence of towns or even isolated buildings. In regard to storm surge, the depth and extent of the inunda- tion at a given coastal location is influenced by the shape of the coastline, the depth of offshore water and, of course, by any defences such as sea-walls. As regards earthquakes, local ground conditions can markedly affect the severity of ground motion. The spatial interactions of these factors determine the loss-producing potential of the storm or earth- quake. 25 8.2 Method of approximation In order to approximate the risk, a means was needed for obtaining a quantitative specifica- tion of the geographical arrays of various populations and properties at risk in the United States. A computerized grid system was constructed which was based upon a one-tenth of a degree latitude by one-tenth of a degree longitude unit grid. About eighty-five thousand units are needed to represent the three million square mile area of the forty-eight continguous states. Each grid area contains about 100 square l&meters at the latitude of northern Florida. Information on population, their vulnerabilities to loss, and the effect of local influences on the severity of an event can be assigned to each grid. For general impact assessment purposes, a detailed measure of the geographical distribution of the two hundred and twenty million persons and fifty million single-family dwellings in the United States has been obtained by allocation of numbers of persons and properties to the appropriate grid unit addressed in the computerized data bank. This national grid system is currently being used to assess the casualty and damage potentials of the various effects associated 
**Analysis**
The text appears to be discussing the method of approximating the risk of natural disasters, such as wind, storm surge, and earthquakes, in the United States. It mentions the construction of a computerized grid system to assign population, vulnerability to loss, and local influences to each grid unit. The text also discusses the assessment of casualty and damage potentials of various effects associated with these disasters. This suggests that the text is related to the field of disaster risk reduction and management.

**Category**
Natural resources and environment, Science and technology