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<br />Guidelines and Specijicationsjor Flood Hazard Mapping Partners [Apri/2003] <br /> <br />Table A-I. Comparison of Horizontal Accuracy Standards <br /> <br />NMAS NMAS NSSDA NSSDA ASPRS ] 990 <br />Map Scale CMAS AccuracYr RMSE, Class 11213 <br /> 90% confidence level 95% confidence level Limiting RMSE, <br />1"=500- 16.7 feet 19.0 feet 1 1.0 feet 7.1 feet (Class I) <br /> 14.1 feet (Class 2) <br /> 21.2 feet (Class 3) <br />1"= 1,000' 33.3 feet 38.0 feet 22.0 feet 14.1 feet (Class I) <br /> 28.3 feet (Class 2) <br /> 42.4 reet (Class 3) <br />]" = 2.000' 40.0 feet 45.6 feet 26.3 feet 28.3 feet (Class I) <br /> 56.5 feet (Class 2) <br /> 84.9 feet (Class 3) <br /> <br />Thus, when FEMA specifies a base map at I" = 500', for example, this is the same as FEMA <br />specifying that a digital base map should have a horizontal (radial) RMSE,. of II feet or Accuracy, <br />of 19 feet at the 95-percent confidence level, for consistency with the new NSSDA. <br /> <br />When a base map is compiled at 1"=1,000' and is published at a hardcopy map scale of 1"=500', <br />the horizontal accuracy remains that of the 1"=1,000' map scale. Therefore, such a 1"=500' map <br />would be compiled to meet 38-foot horizontal accuracy at 95-percent confidence level, rather than <br />19-foot horizontal accuracy at 95-percent confidence level as is normally expected of maps <br />published at a scale of 1"=500'. This is an example where "zooming in" on a map image does not <br />make the map any more accurate. <br /> <br />A.3.2 <br /> <br />Vertical Accuracy Criteria <br /> <br />[February 2002J <br /> <br />Several standards for computing vertical accuracy also are in use by the mapping industry. These <br />are discussed and compared below. <br /> <br />The NMAS (Bureau of the Budget, 1947) states: <br /> <br />Vertical accuracy, as applied to contour maps on all publication scales, shall be such <br />that not more than 10 percent of the elevations tested shall be in error more than <br />one-half the contour interval. In checking elevations taken from the map, the <br />apparent vertical error may be decreased by assuming a horizontal displacement <br />within the permissible horizontal error for a map of that scale. <br /> <br />This one-half contour interval under NMAS is called the Vertical Map Accuracy Standard <br />(YMAS). NMAS became obsolete for digital mapping products because computers can easily <br />change the scale and contour interval of a map, and maps are not more accurate because the <br /> <br />A-7 <br /> <br />Section A.3 <br />