[Top] [Prev] [Next] [Bottom]

3.2 Positional Accuracy

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

Descriptions of positional accuracy must 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 must include the date of the test. Variations in positional accuracy must 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.

3.2.1 Deductive Estimate.

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

3.2.2 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.

3.2.3 Comparison to Source.

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

3.2.4 Independent Source of Higher Accuracy.

The preferred test for positional accuracy is a comparison to an independent source of higher accuracy. The test must be conducted using the rules prescribed in the "ASPRS Accuracy Standards for Large Scale Maps" (see 1.3.3). When the dates of testing and source material differ, the report must 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, must 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.