Point Radius Method Uncertainty and Best Practices

Sources of uncertainty:  Coordinate Uncertainty  Map scale  The extent of the locality  GPS accuracy  Unknown datum  Imprecision in direction measurements  Imprecision in distance measurements (1km vs. 1.1 km) 20° 30 ’ N 112° 36 ’ W ScaleUncertainty (ft)Uncertainty (m) 1:1, ft1.0 m 1:2, ft2.0 m 1:4, ft 4.1 m 1:10, ft8.5 m 1:12, ft10.2 m 1:24, ft 12.2 m 1:25, ft12.8 m 1:63, ft32.2 m 1:100, ft50.9 m 1:250, ft127 m

Best Practices Resources MaNIS/HerpNET/ORNIS Georeferencing Guidelines Guide to Best Practices in Georeferencing training-manuals (

Sources of Uncertainty  The extent of the locality  GPS accuracy  Unknown datum  Imprecision in distance measurements  Imprecision in coordinate measurements  Map scale  Imprecision in direction measurements

Precision vs. Accuracy Disambiguated Precision is… …the level of detail contained in or described by the data. Example: 42, precise 42.1, more precise 42.01, even more precise Precision can help to minimize uncertainty.

Precision vs. Accuracy Disambiguated Accuracy is… …a measure of how close a given value is to the true value.* Example: Truth = = more precise, less accurate *We may never actually know the true value in georeferencing, but we do our best to reproduce the location of the true location.

Precision vs. Accuracy Disambiguated Final Answer? The sum of what we know… about what we don ’ t know… is the uncertainty.

Precision vs. Accuracy Disambiguated More Precise Less Precise More Accurate Less Accurate x

The Extent of the Locality

GPS Accuracy 5m 9m

Unknown Datum Error assuming NAD27, NAD83, or WGS84

Imprecision in Distance Measurements

Imprecision in Coordinate Measurements 20° 30’ N 112° 36’ W vs. 20° 30’ N 112° 30’ W Uncertainty based on coordinate precision using the WGS84 reference ellipsoid

Imprecision in Coordinate Measurements 52° 30’ N 112° 15’ W? Uncertainty based on coordinate precision using the WGS84 reference ellipsoid

Map Scale USGS Standard: "For maps on publication scales larger than 1:20,000, not more than 10 percent of the points tested shall be in error by more than 1/30 inch, measured on the publication scale; for maps on publication scales of 1:20,000 or smaller, 1/50 inch." USGS Map Accuracy 1/30th (.033) inch 1/50th (.02) inch

Imprecision of Direction Measurements Error: N > NE > ENE