Laserfiche WebLink
<br />Guidelines and Specifications/or Flood Hazard Mapping Partners [April 20113] <br /> <br />A.3 <br /> <br />Accuracy Guidelines <br /> <br />[February 2002] <br /> <br />For decades, when topographic maps were produced photogrammetrically and printed in hardcopy <br />fonn, users became accustomed to specifYing map requirements in terms of the published map <br />scale and contour interval. Users generally understood the meaning of those terms, and <br />photogrammetrists had well-established procedures for determining the flying height, forward <br />overlap and side lap, aerial triangulation, and map compilation procedures necessary to consistently <br />produce a map to the required map scale and contour interval, in conformance with the National <br />Map Accuracy Standard (NMAS). <br /> <br />With the new NSSDA designed for digital geospatial data, the defining parameters are much more <br />complex. What once had been a simple standard printed on one page (i.e., the NMAS) has become <br />a complex standard (i.e., the NSSDA) that is nearly one inch thick. The more complex standard <br />was necessary to accommodate the variables and options available with digital geospatial data and <br />the increased complexity of digital data that are subject to use in many different ways. <br /> <br />FEMA has chosen to define its digital topographic data accuracy requirements in tenns of map <br />scale and contour interval equivalent to NMAS terminology, but also cross-referenced to radial <br />root-mean-square-error (RMSEr) and/or AccuracYr (to define horizontal accuracy at the 95-percent <br />confidence level), and RMSEz and/or Accuracyz (to define vertical accuracy at the 95-percent <br />confidence level) as defined by the NSSDA, assuming the errors have a normal distribution. The <br />NSSDA does not cover what to do when errors do not follow a normal distribution, as is often the <br />case with LIDAR datasets. In Chapter 12, "DEM Quality Assessment" of Digital Elevation Model <br />Technology and Applications: The DEM Users Manual (ASPRS, 2001), the ASPRS proposes a 5- <br />step approach that includes the use of the 95th percentile error as the next best method for <br />estimating Accuracyz when it is detennined (through statistical "outliers," discussed in Section <br />A.8.3.1) that checkpoint errors do not follow a normal distribution. <br /> <br />As shown in Tables A-I and A-2, these NMAS and NSSDA terms are also cross-referenced to <br />ASPRS, 1990 Class I, Class 2, and Class 3 standards, which are terms still used by the industry. <br />To simplifY the terminology, FEMA has reduced the complex requirements to two standard choices <br />for digital elevation data, expressed as equivalent contour intervals: <br /> <br />. <br /> <br />Two-foot equivalent contour interval for flat terrain (Accuracyz = 1.2 foot at the 95-percent <br />confidence level). This means that 95 percent of the elevations in the dataset will have an <br />error with respect to true ground elevation that is equal to or smaller than 1.2 feet. <br /> <br />. <br /> <br />Four-foot equivalent contour interval for rolling to hilly terrain (Accuracyz = 2.4 ft at the <br />95-percent confidence level.) This means that 95 percent of the elevations in the dataset <br />will have an error with respect to true ground elevation that is equal to or smaller than 2.4 <br />feet. <br /> <br />The FEMA Lead for a Flood Map Project--ilsually, the Regional Project Officer (RPO) or the <br />Project Officer (PO) at FEMA Headquarters-may select non-standard alternatives when valid and <br />compelling reasons for specifYing other accuracy standards exist. <br /> <br />A-5 <br /> <br />Section A.3 <br />