What you dont know can hurt you:

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What you dont know can hurt you uncertainties
in georeferencing
John Wieczorek Museum of Vertebrate
Zoology University of California, Berkeley
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What we haveLocalities we can read
ID
Species
Locality
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Lynx rufus
Dawson Rd. N Whitehorse
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Pudu puda
cerca de Valdivia
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Canis lupus
20 mi NW Duluth
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Felis concolor
Pichi Trafúl
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Lama alpaca
near Cuzco
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Panthera leo
San Diego Zoo
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Sorex lyelli
Lyell Canyon, Yosemite
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Orcinus orca
1 mi W San Juan Island
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Ursus arctos
Bear Flat, Haines Junction
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What we wantLocalities we can map
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What is a georeference?
A numerical description of a place that can be
mapped.
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Davis, Yolo County, California
Coordinates 38.5463 -121.7425
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Davis, Yolo County, California
Coordinates 38.5463 -121.7425
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Davis, Yolo County, California
Coordinates 38.5463 -121.7425 Horizontal
Geodetic Datum NAD27
point method
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What is an acceptable georeference?
A numerical description of a place that can be
mappedand that describes the spatial extent of a
locality and its associated uncertainties.
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Davis, Yolo County, California
Coordinates 38.5486 -121.7542 38.5450
-121.7394 Horizontal Geodetic Datum NAD27
bounding-box method
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Davis, Yolo County, California
Coordinates 38.5468 -121.7469 Horizontal
Geodetic Datum NAD27 Maximum Uncertainty 8325 m
point-radius method
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Point-radius Method
MaNIS/HerpNet/ORNIS Georeferencing
Guidelines now Global Biodiversity Information
Facility Best Practices for Georeferencing
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Sources of uncertainty

• Map scale
• The extent of the locality
• GPS accuracy (assume 30 m since Selective
  Availability ended 2 May 2000, previously gt100 m
  error)
• Unknown datum
• Imprecision in distance measurements (e.g.,1km
  instead of 1.1 km)
• Imprecision in direction measurements

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Georeferencing Calculator

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What is an ideal georeference?
A numerical description of a place that can be
mappedand that describes the spatial extent of a
locality and its associated uncertaintiesas
well as possible.
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Davis, Yolo County, California
shape method
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20 mi E Hayfork, California
probability method
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Method Comparison
point
easy to produce no data quality
bounding-box
simple spatial queries difficult quality
assessment
point-radius
easy quality assessment difficult spatial queries
shape
accurate representation complex, uniform
probability
accurate representation complex, non-uniform
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(a)
(b)
(d)
(c)
Rowe, 2005. Elevational gradient analysis
ofhistorical museum specimens a cautionary tale
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Rowe, 2005. Elevational gradient analysis
ofhistorical museum specimens a cautionary tale
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Weight 13
Weight 13 stone
Standardized Weight 81.65 kg
with uncertainty 81.65 3.17 kg