What is the difference between the positional accuracy reports and the quantitative positional accuracy assessments?

The Horizontal and Vertical Positional Accuracy Reports are complete descriptions of the accuracy of the positional information in the data set, including the identification of tests used, testing methodology, results obtained, etc.

In developing the metadata standard, reviewers asked that a summary method of providing results also be permitted. While this summary method would not relieve producers of the obligation to provide a thorough report, it would allow users, especially those using a data catalog, to quickly exclude those data sets that obviously would not serve a user's needs. The Horizontal and Vertical Quantitative Positional Accuracy Assessments (and their attribute accuracy counterpart) were provided in response to this request. A producer identifies the test used by name and the value obtained from the test. The derivation of these values should be described in the accuracy reports.

What does SDTS say about positional accuracy?

Positional Accuracy:

The quality report portion on positional accuracy shall include the degree of compliance to the spatial registration standard (see section 4.1.3.5 [of SDTS]). Quality of control surveys shall be reported by using the procedures established in the geodetic standard. If a separate control survey has been used, it shall be described in the standard form, even if results fall below the recognized classification thresholds.

Descriptions of positional accuracy shall consider the quality of the final product after all transformations. The information on transformations forms a part of the lineage portion of the quality report.

The report of any test of positional accuracy shall include the date of the test. Variations in positional accuracy shall be reported either as additional attributes of each spatial object or through a quality overlay (reliability diagram).

Measures of positional accuracy may be obtained by one of the following optional methods.

Deductive Estimate

Any deductive statement based on knowledge of errors in each production step shall include reference to complete calibration tests and shall also describe assumptions concerning error propagation. Results from deductive estimates shall be distinguished from results of other tests.

Internal Evidence

Federal Geodetic Control Committee procedures will be used for tests based on repeated measurement and redundancy such as closure of traverse or residuals from an adjustment.

Comparison to Source

When using graphic inspection of results ("check plots"), the geometric tolerances applied shall be reported and the method of registration shall also be described. Use of check plots shall be included in the lineage portion.

Independent Source of Higher Accuracy

The preferred test for positional accuracy is a comparison to an independent source of higher accuracy. The test shall be conducted using the rules prescribed in the "ASPRS Accuracy Standards for Large Scale Maps" (see 1.3.3 [of SDTS]). When the dates of testing and source material differ, the report shall describe the procedures used to ensure that the results relate to positional error and not to temporal effects. The numerical results in ground units, as well as the number and location of the test points, shall be reported. A statement of compliance to a particular threshold is not adequate in itself. This test may only be applicable to well-defined points.

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