Identification_Information:
  Citation:
    Citation_Information:
      Originator: Owens, Thomas, Author;
      Originator: McConville, David, Author;
      Originator: National Biological Service,Environmental Management Technical Center, Author;
      Publication_Date: 1996
      Title: Geospatial Application: Estimating the Spatial Accuracy of Coordinates Collected Using the Global Positioning System
      Geospatial_Data_Presentation_Form: Technical report
      Publication_Information:
        Publication_Place: Onalaska, Wisc.
        Publisher: National Biological Service,Environmental Management Technical Center
      Other_Citation_Details:
        Extent of Item: 13p. + Appendix
        Date as Writen: April 1996
        Illustrative Matter: Diagrams
        Illustrative Matter: Tables
        Series Title, Subtitle and Numbering as it appears on item: Long Term Resource Monitoring Program Technical Report 96-T002
        Notes: (Data entered via BMDB Web Input Form, #B608)

  Description:
    Abstract:
      Evaluating the accuracy of spatial data is important to determine
      the appropriate use of these data. However, a good method has not
      been documented to measure locational accuracy. The Global
      Positioning System (GPS) reduces the difficulty of measuring the
      location of objects and enables non-surveyors to determine their
      location with relative ease. This study applied a straight-forward,
      repeatable, and statistically sound method of estimating the
      horizontal accuracy of GPS-derived location data. We concentrated on
      the spatial accuracy of points because points represent simple
      locations and not cartographic abstractions such as lines or
      polygons. When GPS coordinates are taken at surveyed locations, the
      quantity of interest is the difference from the surveyed (assumed
      true) coordinates. This difference in coordinates is a bivariate
      quantity and the probability distribution function (PDF) can be
      described by an ellipse with the center at and . An ellipse is an
      appropriate shape for a PDF; it has two dimensions but is not
      rectangular because the joint probability of points occurring in the
      corners is very small, and it is generally not circular because X
      and Y are not necessarily the same. There are three ellipses of
      interest: the standard ellipse, the confidence ellipse, and the
      tolerance ellipse. The standard ellipse is a descriptive tool used
      to visualize the ellipse's shape and orientation. It contains about
      400f the sample, is not dependent on the sample size, and cannot
      be used for statistical inference. The other two ellipses have
      identical shapes and orientation but different major and minor axes.
      The confidence ellipse is an estimate of accuracy; the sample mean
      is or is not significantly different from the survey locations at a
      given . The tolerance ellipse is an estimate of precision; a given
      percentage of the population sampled is enclosed in the tolerance
      ellipse at a given . Thirty-six locations were measured and compared
      to surveyed locations. The average offset was -1.13 m in the
      northing (Y) direction and 0.18 m in the easting (X) direction.
      Hotelling's one-sample test determined that H0 (no significant
      departure from the survey locations exists) was rejected at the 0.05
      level, which indicates there was a systematic error in the sample in
      the south and east directions. Ninety-five percent of the population
      sampled (at the 0.05 level) was contained in an ellipse that was
      centered on 0.18, -1.13, and had a major axis of 7.49 m, and a minor
      axis of 5.12 m with an angle of 87.74o. Thus, if an additional point
      were taken, we are 95