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Monument -- A known station, defining horizontal-and/or vertical position, established by any entity other than the U.S. Coast and Geodetic Survey (National Geodetic Survey). Monumentation n A network of surveying monuments, tied to one,another, providing precise information on elevations and positions in a given region, thereby facilitating the determination of elevatipns and positions at places between the monuments. National Datum -- The vertical and horizontal data that have been defined by the National Geodetic Survey; Mean Sea Level (MSL) for vertical control and state plane coordinates within each state for horizontal control. National Geodetic Survey (NGS) n The federal agency within the U.S. Depart- (llent of Commerce which is responsibie for establishing standards tor verti- cal and horizontal data and for establishing surveying criteria. (Formerly the U.S. Coast and Geodetic Survey) Nonstructural Measures -- Designed to address floodplain problems by minimizing or avoiding conflicts between development and occupation of land and its propensity to be flooded; pursuing a policy of preventing such problems before development occurs.(also Preventive) Numbered A Zone -- A zone on a Flood Insurance Hate Map that delineates in a detailed fashion the 1 OO-year floodplain with precise engineering infor- mation showing detailed water surface elevation and floodplain boundary information. Order of Control-- A classification of surveying based on the accuracy of the ground control used and on the accuracy of the survey results; categories established by the National Geodetic Survey are First Order, Second Order Classes I and II, and Third Order Classes I and II. Orthophotographic Map .- A composite of aerial photographs of an area that have been corrected for the tilt of the airplane and that have, through a scan- ning process, been adjusted to assure true scale throughout the composite. Overhead View -- (see Plan View) Overlayn A zone district which is in addition to an existing, underlying zone district and whose restrictions are in addition to the restrictions of the underlying zone district. Perennial Stream or (Stream Reach) -- A stream (or stream reach) which has measurable surface discharge more than 80 percent of the time. Discharge is at times in part or in total the result of springflow or ground-water seepage because the streambed is lower than surrounding ground-water levels. Physiographic -- Pertaining to the physical features of a place or region. Piping -- The state of developing cylindrical cavities through which water may pass. Planimetric Features -- Man-made features shown in the horizontal plane on a map. Plan View-- A two-dimensional representation of a stream channel and flood- plain which shows the area as viewed from above without showing vertical features (also Overhead View). Preventive n (see Non-Structuran. Probability Distribution Analysis n A mathematical analysis to determine the likelihood of occurrence of various volumes of streamflow expressed as a ratio of the number of times given volumes of streamflow can be expected to occur in a certain time period over the number of years in that time period. Probable Maximum Flood (PMF) n Theoretically the greatest flood physically pOSSible in a drainage basin. Profile View n A two-dimensional representation of a stream channel and floodplain which cuts vertically through the center of the channel in the direction of flow. (also Side View) Rain Gauge -- A gauge for measuring the intensity and the amount of rainfall. Rating Curve n An algebraic curve showing the relationship between the volume of water flowing past a given point on a stream and the depth of flow at that point. 56