Point Radius Method

Uncertainty and Best Practices

Best Practices Resources

MaNIS/HerpNET/ORNIS Georeferencing Guidelines

http://manisnet.org/GeorefGuide.html

Guide to Best Practices in Georeferencing

http://www.gbif.org/participation/training/resources/gbif-training-manuals

(http://www.gbif.org/orc/?doc_id=1288h)

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. 1.1km)

20° 30’ N 112° 36’ WScale 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. AccuracyDisambiguated

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.

More Precise

Less Precise

Precision vs. AccuracyDisambiguated

Accuracy is…

…a measure of how close a given value is to the true value.*

*We may never actually know the true value in georeferencing, but we do our best to reproduce the location of the true location.

Example: Truth = 42

41.999 = more precise, less accurate

More Accurate

Less Accurate

x

Precision vs. AccuracyDisambiguated

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

5m

9m

Unknown Datum

Error assuming NAD27 vs. 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

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