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. . November 6, 1975 Ronald I. Blewitt Map tolerances The FIA guideline for flood elevation profiles is :1:0.5 foot for the 10-,50-, lOa-and 500-year floods studied in detail. The Corps of Enginp.ers at Poztland, Oregon, in their instructions for the Lane, Washington and Polk County studies in Oregon issued the follo"ling instruction with regard to map compilation. "The 20, 50 and 100-year floodplain outlines shall be derived from aerial photos, field control, and river profiles by photogrammetric methods. The photogrammetrist shall striv'= to compute floodplain outlines within one foot of trw= ground elevation." Please note that two separate items are addressed (1) toler- ance of the floodwater surface elevations in the case of the FIA standard, and (2) variation of the flood outline which is por~ayed on maps from water surface elevations (flood elevation profiles). The latter must necessarily have a greater tolerance than the former. The Colorado Water conservation Board's Model Floodplain Regulation specifies in section 3.2 that the lowest floor of any building designed for human occupany or the storage of property--- shall be not less than one foot above the maximum water elevation of the computed intermediate regional (lOO-year) flood. Let us call this one foot requirement "freeboard". Assuming that the computed floodwater elevation at a given stream location is the maximum tolerance (-0.5 foot) lower than it should be, then the actual remaining freeboard is only 0.5 foot at that location. Assuming further that in no event should the lowest floor elevation be lower than the 100-year flood elevation (computed with zero tolerance) then as little as 0.5 foot tolerance remains for establishing the floodwater elevation on the ground. Establishment of the floodwater elevation for a given point from a base map withou1: contours is subject to errors inherent in the map, in scaling distance from the map, and in establishing the point location and its elevation on the ground by field surveys. National map accuracy standards require that 90% of all planimetric features which are well defined on the photograph be plotted so that their position on finished maps shall be accurate to within at least one-fortieth (1/40) of an inch of their true coordinate position, and none misplaced more than 1/20 of an inch.