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A Methodology for Measuring and Reporting Positional Accuracy in Spatial Data

Date Issued: Approved by the Minnesota Governor's Council on Geographic Information June 12, 2000

Applicability
Who cares about this standard?
All developers and users of existing or planned spatial data sets containing geographic information about Minnesota when those data are to be used in analyzing spatial relationships, supporting public policy decisions and/or producing maps.

When do they apply? When do they not apply?
This standard has been developed to provide detailed information about the positional accuracy of publicly-funded spatial data to better define their appropriate use. This standard identifies a methodology for testing the geographic accuracy of a spatial database and a format for reporting results of that test. It is only applicable for spatial data - data representing geographic location - used in geographic information systems and mapping applications. Use of the methodology referenced in this standard is mandatory when a state agency has determined that an investment in spatial data warrants the gathering of detailed information about the positional accuracy of those data. Use of this standard is recommended when state agencies, local governments or private sector firms design new spatial databases.

Purpose of this Standard:
The purpose of this standard is to provide a single, uniform statistical methodology for estimating the positional accuracy of points on maps and in digital spatial data. Its use by Minnesota state agencies and local governments will improve the understanding of the quality of publicly-funded data resources. Improved knowledge about accuracy helps minimize data redundancy and increase shareability of these public information resources. State of Minnesota IRM Standard 19 requires the implementation of the National Standard for Spatial Data Accuracy (NSSDA). The NSSDA is a reporting standard, referred to as a data usability standard by the Federal Geographic Data Committee. Data usability standards describe how to express the applicability of a data set, including data quality, assessment, accuracy and reporting or documentation standards. No minimum conformance level or accuracy threshold is mandated in this standard.

The NSSDA is part of a developing series of standards referred to as the Geospatial Positioning Accuracy Standards. This suite of five standards is intended to provide consistency in measuring and reporting the accuracy of point spatial data collected for different activities (e.g., geodetic surveying, topographic mapping, bathymetric mapping, facilities management mapping, cadastral surveying). The NSSDA is Part 3 of that series and is the only part of the series dealt with in Minnesota IRM Standard 19. Find more information about the full set of Geospatial Positioning Accuracy Standards at: http://www.fgdc.gov/standards/projects/FGDC-standards-projects/accuracy

Standard Requirements:
The NSSDA is a Federal Geographic Data Committee standard (FGDC-STD-007.3) created in 1998 for the purpose of measuring and reporting the accuracy of maps and spatial data which are produced by or for federal agencies. It uses root-mean-square-error (RMSE) to estimate positional accuracy. RMSE is 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. Methods are presented in this standard for testing either horizontal data (latitude & longitude) or vertical data (elevation).

To comply with the NSSDA, a data steward conducts a statistical test using the following steps:

  1. Determine if the test involves horizontal accuracy, vertical accuracy or both.
  2. Select a set of test points from the data set being evaluated.
  3. Select an independent data set of higher accuracy that corresponds to the data being tested.
  4. Collect measurements from identical points from each of those two sources
  5. Calculate a positional accuracy statistic using either the horizontal or vertical accuracy statistic worksheet.
  6. Prepare an accuracy statement in a standardized report form.
  7. Include that report in a comprehensive description of the data set metadata.

Accuracy is reported in ground distance at the 95% confidence level. This means that 95% of the positions in the dataset will have an error that is equal to or smaller than the reported accuracy. Accuracy reports are presented in a standardized phrase to be included in the data set's description documentation report (typically referred to as a metadata report). For example, a typical accuracy statement for tested horizontal data would take the form:

Tested ____ (meters/feet) horizontal accuracy at 95% confidence level

Compliance:
What constitutes compliance?
When an agency determines that an investment in spatial data warrants the gathering of detailed information about the positional accuracy of those data, that agency shall use the methodology described in the NSSDA to determine the accuracy measure. That agency shall then report the results of the accuracy testing process using the format prescribed in the standard. It is recommended that agencies integrate this standard into new system designs whenever possible.

How will compliance be measured?
Evidence of compliance will be determined based on reports of satisfactory presentation of accuracy information in metadata accompanying a tested spatial data set.

Obtaining a copy of the National Standard for Spatial Data Accuracy:

Copies of the NSSDA can be downloaded at: http://www.fgdc.gov/standards/projects/FGDC-standards-projects/accuracy/part3/chapter3, Portable Document File; PDF; 78K.

Maintenance:
The U.S. Department of the Interior, U.S. Geological Survey, National Mapping Division, maintains the National Standard for Spatial Data Accuracy (NSSDA) for the Federal Geographic Data Committee. Address questions concerning the NSSDA to: Chief, National Mapping Division, USGS, 516 National Center, Reston, VA 20192.


References and Sources of More Information:
The Minnesota Governor's Council on Geographic Information has developed an implementer's guide to the NSSDA. A copy of the Positional Accuracy Handbook; Using the NSSDA to measure and report geographic data quality may be obtained by pointing your web browser to: http://server.admin.state.mn.us/. The handbook describes how positional accuracy can be measured and reported for databases that contain geographic features like roads, rivers and property lines. Five practical examples illustrate the process using databases developed at Minnesota Departments of Transportation and Natural Resources, the city of Minneapolis, Washington County and Lawrence Mapping. Copies of the handbook may be downloaded in PDF format (33 pages; 920K) or an electronic order for a paper copy may be placed at this site. All the mathematics needed to calculate vertical and horizontal accuracy statistics are made easy with Excel worksheets that can also be downloaded from this site.

Further information about this standard may be obtained from Office of Geographic and Demographic Analysis phone: 651.201.2481