BOARD01965

• Board Meeting Date: 3/22/2005
• Description: Flood Section - Presentation of the CWCB's Draft Rules and Regulations for Regulatory Floodplains in Colorado
• Board Meetings - Doc Type: Memo

GUidelines and Specificationsfor Flood Hazard Mapping Parlners [April2003j

A.2

Industry Geospatial Standards

[February 2002J

In 1998, the Federal Geographic Data Committee (FGDC) published Geospatial Positioning
Accuracy Standards, which replaced both the United States National Map Accuracy Standards
(NMAS) published by the Office of Management and Budget in 1947 (Office of Management and
Budget, 1947) and the American Society for Photogrammetry and Remote Sensing (ASPRS)
ASPRS Accuracy Standards for Large-Scale Maps (ASPRS, 1990). Designed specifically for
digital spatial data products, this new FGDC standard has three parts:

. Part I, Reporting Methodology (FGDC-STD-007.l-1998)

. Part 2, Standards for Geodetic Networks (FGDC-STD-007.2-1998); and

. Part 3, National Standard for Spatial Data Accuracy (FGDC-STD-007.3-1998)

FGDC-STD-007.1-1998 provides a common methodology for reporting the accuracy of horizontal
and vertical coordinate values of digital geospatial products. Specifically, the reporting standard in
the horizontal component (Accuracy,) is the radius of a circle of uncertainty, such that the true or
theoretical location of the point falls within that circle 95 percent of the time. The reporting
standard in the vertical component (Accuracyz) is a linear uncertainty value, such that the true or
theoretical location of the point falls within plus or minus of that linear uncertainty value 95
percent of the time. It also defines the meanings of "local accuracy" and "network accuracy" and
other terms used in the FGDC standard. Part I of the Geospatial Positioning Accuracy Standards is
available online at: www [ecic gnv/<tann3rn</nnclIn1f'nt</<tann3rn</ch3ptf'rl pdf.

FGDC-STD-007.2-1998 provides a common methodology for determining and reporting the
accuracy of horizontal and vertical coordinate values for geodetic control points represented by
survey monuments, such as brass disks and rod marks. It provides a means to directly compare the
accuracy of coordinate values obtained by one method (e.g., a classicalline-of-sight traverse) with
the accuracy of coordinate values obtained by another method (e.g., a Global Positioning System
[GPS] geodetic network survey) for the same point. It explains how "network accuracy" is
achieved by properly connecting survey and mapping data to control points in the National Spatial
Reference System (NSRS). Part 2 of the Geospatial Positioning Accuracy Standards is available
on the FGDC website at www [gck gov/<t3nd3rn</doclIment</<t3nd3rn</ch3pter? pdf.

FGDC-STD-007.3-1998 implements a statistical and testing methodology for estimating the
positional accuracy of points on maps and in digital geospatial data, with respect to georeferenced
ground positions of higher accuracy. If errors have a normal distribution and if systematic errors
have been eliminated as best as possible, the National Standard for Spatial Data Accuracy
(NSSDA) uses root-mean-square error (RMSE) to estimate positional accuracy of x, y and z
coordinates (RMSEx, RMSEy and RMSEz respectively). FGDC-STD-007.3-l998 defines RMSE
as the square root of the average of the set of squared differences between dataset coordinate values
and coordinate values from an independent source of higher accuracy for identical points and it
defines (horizontal) radial accuracy in terms of RMSE,. computed as a function of RMSEx and
RMSEy. FGDC-STD-007.3-l998 provides NSSDA testing guidelines, it relates Accuracy, and
Accuracyz (horizontal and vertical accuracies at the 95-percent confidence level) to RMSEr and

A-3

Section A.2