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 Specifications jor Flood Hazard Mapping Parlllers [April 2003]

A.3.1

Horizontal Accuracy Criteria

[February 2002J

Several accuracy standards are still in use today for the mapping industry. The following is a
description and comparison of the different standards used and which apply.

The NMAS (Bureau of the Budget, 1947) states:

Horizontal accuracy: For maps on publication scales larger than 1 :20,000, not more than 10% of the
points tesled shall be in error by more than 1/30'h of an inch. measured on the publication scale.

This I/30-inch standard for large-scale maps is called the Circular Map Accuracy Standard
(CMAS). The NMAS became obsolete for digital mapping products because computers can easily
change the scale of a map, and maps do not become more accurate just because the computer
enables users to "zoom in" on the map to display it at a larger scale.

To prevent abuse of digital mapping data, the mapping industry operated during much of the 1990s
under ASPRS 1990 standards. ASPRS 1990 standards established limiting RMSEs for three
classes of maps (Class I, Class 2, Class 3), along with typical map scales associated with the
limiting errors. Three times the "limiting RMSE" was essentially a 100-percent confidence level
standard.

In 1998, the FGDC published the NSSDA, which superseded both the NMAS and the ASPRS 1990
standards for digital mapping products. NSSDA implemented 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. Radial RMSE (RMSEr) calculations were
established, and radial accuracy (Accuracy,) at the 95-percent confidence level was established as
1.7308 x RMSEr. Accuracy, is defined as "the radius ofa circle of uncertainty, such that the true or
theoretical location of the point falls within that circle 95-percent of the time." NSSDA specifies
horizontal errors at the 95-percent confidence level, whereas the NMAS specified horizontal errors
at the 90-percent confidence level, and ASPRS 1990 specified horizontal errors at nearly the 100-
percent confidence level. When assuming all horizontal errors have a normal distribution, the
NSSDA/NMAS conversion factor is as follows:

Accuracy, = CMAS x 1.1406.

With NSSDA, RMSEr is defined in terms of feet or meters at ground scale rather than in inches or
millimeters at the target map scale. The RMSEr of a DFlRM panel is the cumulative result of all
errors, including those introduced by Mapping Partners in performing ground surveys, aerial
triangulation, map compilation. and digitization activities. The RMSEr and Accuracy, values
shown in Table A-I are the maximum permissible values established by NSSDA for base maps
compiled at 1"=500' and 1"=1000' underNMAS. Table A-I serves as a "crosswalk" between the
NMAS, NSSDA, and ASPRS 1990 horizontal accuracy standards. Note that RMSEr = square root
of (RMSE/ + RMSE/).

A-f>

Section A.3