Point Radius Method Uncertainty and Best Practices Best

Point Radius Method Uncertainty and Best Practices

Best Practices Resources Ma. NIS/Herp. NET/ORNIS Georeferencing Guidelines http: //manisnet. org/Georef. Guide. html Guide to Best Practices in Georeferencing http: //www. gbif. org/participation/training/resources/gbiftraining-manuals (http: //www. gbif. org/orc/? doc_id=1288 h)

Sources of uncertainty: 20° 30’ N 112° 36’ W }Coordinate Uncertainty }Map scale }The extent of the locality }GPS accuracy }Unknown datum }Imprecision in direction measurements }Imprecision in distance measurements (1 km vs. 1. 1 km) Scale Uncertainty (ft) Uncertainty (m) 1: 1, 200 3. 3 ft 1. 0 m 1: 2, 400 6. 7 ft 2. 0 m 1: 4, 800 13. 3 ft 4. 1 m 1: 10, 000 27. 8 ft 8. 5 m 1: 12, 000 33. 3 ft 10. 2 m 1: 24, 000 40. 0 ft 12. 2 m 1: 25, 000 41. 8 ft 12. 8 m 1: 63, 360 106 ft 32. 2 m 1: 100, 000 167 ft 50. 9 m 1: 250, 000 417 ft 127 m

Determining uncertainties • • Precision vs. accuracy Extent of locality GPS accuracy Unknown datum Imprecision in distance measurements Imprecision in coordinate measurements Map scale Imprecision of 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 Less Precise Precision can help to minimize uncertainty. More Precise

Precision vs. Accuracy Disambiguated Accuracy is… …a measure of how close a given value is to the true value. * Example: Truth = 42 41. 999 = more precise, less accurate Less Accurate x More 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.

The Extent of the Locality

The Extent of the Locality over Time Yellow polygon from GEOLocate

GPS Accuracy 5 m 9 m

Unknown Datum Error assuming NAD 27 vs. NAD 83 or WGS 84

Imprecision in Distance Measurements

Imprecision in Coordinate Measurements Uncertainty based on coordinate precision using the WGS 84 reference ellipsoid 20° 30’ N 112° 36’ W vs. 20° 30’ N 112° 30’ W

Map Scale 1/30 th (. 033) inch 1/50 th (. 02) inch USGS Map Accuracy 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. "

Imprecision of Direction Measurements Error: N > NE > ENE