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
Reason for this Standard: The purpose of this standard is to provide a single, uniform statistical methodology for evaluating the positional accuracy of points on maps and in digital geospatial data. Its use by Minnesota state agencies and local governments will improve understanding of the quality of publicly-funded data resources. Improved knowledge about accuracy helps minimize data redundancy and improves sharing of public data resources.
The standard 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 for specific uses. No minimum conformance level or accuracy threshold is mandated in this standard.
The NSSDA is one in a 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 geospatial 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 standard.
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 geospatial 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 and longitude) or vertical data (elevation).
To comply with the NSSDA, a data steward conducts a statistical test using the following steps:
- Determine if the test involves horizontal accuracy, vertical accuracy or both.
- Select a set of test points from the data set being evaluated.
- Select an independent data set of higher accuracy that corresponds to the data being tested.
- Collect measurements from identical points from each of those two sources
- Calculate a positional accuracy statistic using either the horizontal or vertical accuracy statistic worksheet.
- Prepare an accuracy statement in a standardized report form.
- 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
Applicability and Exclusions
Who cares about this standard?
This standard is important to 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 geospatial data to better define their appropriate use. This standard identifies a methodology for testing the geographic accuracy of a geospatial database and a format for reporting results of that test. It is only applicable for geospatial data - data representing geographic location - used in geographic information systems and mapping applications.
Use of the methodology defined in this standard is mandatory when a state agency has determined that an investment in a specific geospatial database warrants an explicit measure of its positional accuracy, and when a requisite independent data set of higher accuracy is available. The standard is particularly beneficial when those data are to be used for decision-support purposes or to express accuracy requirements in professional/technical contracts.
Use of the methodology in this standard is recommended when state agencies, local governments or private sector firms design new geospatial databases.
What constitutes compliance?
When an agency determines that an investment in geospatial data warrants the documented evidence of its positional accuracy, that agency shall use this standard to determine the accuracy measure. That agency shall then report the results of accuracy testing 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 geospatial data set.
The National Standard for Spatial Data Accuracy (Federal Geographic Data Committee Standard 007.3-1998) can be obtained at: http://www.fgdc.gov/standards/projects/FGDC-standards-projects/accuracy/part3/chapter3.
Find more information about the full set of Geospatial Positioning Accuracy Standards at:http://www.fgdc.gov/standards/projects/FGDC-standards-projects/accuracy.
The former Minnesota Governor's Council on Geographic Information 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(1999; PDF; 33 pages; 920K) may be obtained by pointing your web browser to:http://www.mngeo.state.mn.us/pdf/1999/lmic/nssda_o.pdf.
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. All the mathematics needed to calculate vertical and horizontal accuracy statistics are made easy with Excel worksheets that can also be downloaded from the link above.
Further information about this standard may be obtained from the Minnesota Geospatial Information Office (MnGeo).
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