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 for Flood Hazard Mapping Partner" [April 2003}

computer can take 10-foot contours, for example, and regenerate 5-, 2- or I-foot contours for
higher resolution display purposes.

Under ASPRS 1990 standards, which were used during much of the 1990s, the mapping industry
operated with vertical RMSEs equal to one-third of the contour interval for ASPRS Class I maps,
two-thirds of the colltour interval for ASPRS Class 2 maps, and full contour interval for ASPRS
Class 3 maps. Again, three times the limiting RMSEz was a standard at almost the 100-percent
confidence level (99.75-percent).

In 1998, NSSDA also superseded both the NMAS and ASPRS 90 vertical accuracy standards for
digital mapping products. Vertical RMSE (RMSEz) cll\culations were established, and vertical
accuracy (Accuracyz) at the 95-percent confidence level was established as 1.9600 x RMSEz when
errors follow a normal distribution. Accuracyz is defined as "the linear uncertainty value, sucb that
the true or theoretical location of the point falls within:!: of that linear uncertainty value 95-percent
of the time." Again, note that NSSDA specifies vertical errors at the 95-percent confidence level
whereas NMAS specified vertical errors at the 90-percel1t confidence level, and the ASPRS 1990
specified vertical errors at nearly the 100-percent confidence level. When assuming all vertical
errors have a normal distribution, the NSSDA/NMAS COI1version factor is as follows:

Accuracy,-=VMASx 1.1916

Contrary to NMAS, NSSDA does not specifically state that the apparent vertical error mllY be
decreased by assumiI1g a horizontal displacement within the permissible horizontal error.

With NSSDA, RMSEz is defined in tenns of feet or meters at ground scale rather than in terrns of
the contour interval of the published map. The RMSEz and Accuracyz values shown in Table A-2
are the maximum permissible values established by the NSSDA for digital elevation data
equivalent to maps compiled with 2-foot and 4-foot contours compiled under NMAS. Table A-2
serves as a "crosswalk" between the NMAS, NSSDA and ASPRS 1990 vertical acc\lfacy
standards. This comparison is only statistically valid, however, when vertical errors have a normal
distribution.

Thw.>, when FEMA $~c..ne,;; 1-foot cootmn: mterval m'l-??\I\g, foy eX'Ml\?\e, fu..,;; ..';; fue ,;;we a,;;
specifying that digital topographic data, regardless of wl1ether the data are digital contours, mass
points and breaklines, Triangulated Irregular NetworkS (TINs), or Digital Elevation Models
(OEMs), should have vertical RMSEz of 0.6 foot (18.5 centimeters) or Accuracyz of 1.2 feet (37
centimeters) at the 95-percent confidence level, for consistency with the NSSDA.

A-8

Section A.3