point radius method
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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