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Georeferencing Workshop

Georeferencing Workshop. Rebecca J. Rowe University of Chicago Committee on Evolutionary Biology & Division of Mammals The Field Museum. Outline. What is georeferencing? Introduce the point-radius methodology Introduce georeferencing tools Work through some examples .

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Georeferencing Workshop

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  1. Georeferencing Workshop Rebecca J. Rowe University of Chicago Committee on Evolutionary Biology & Division of Mammals The Field Museum

  2. Outline • What is georeferencing? • Introduce the point-radius methodology • Introduce georeferencing tools • Work through some examples

  3. Georeferencing is the expression of a spatial description in coordinates within a frame of reference

  4. Coordinates alone are not enough • Latitude and longitude (in a consistent format) • Uncertainty (maximum error distance) • Frame of Reference (i.e. datum)

  5. Retrospective Georeferencing From textual descriptors: • determine coordinates • assign spatial uncertainty From collector determined coordinates: • convert coordinates (if necessary) • assign spatial uncertainty

  6. Textual Descriptors • Bakersfield • Near Bakersfield • 10 mi E (by air) Bakersfield • 5 mi from Bakersfield • 2 mi E and 1.5 mi N of Bakersfield • 13 mi E (by road) Bakersfield • 10.2 mi E of Bakersfield

  7. Coordinate Systems • decimal degrees 35.3733 , -119.0178 • degrees minutes seconds 35° 22 23.88 N, 119 1 4.08 W • degrees, decimal minutes 35 ° 22.398 N, 119 1.068 W • UTM Zone 11S 316695E 3916111N

  8. The Ultimate Goal • Spatially explicit link between the identity of a voucher, and the collection event • Increases data for biological distribution & diversity studies • Enables computer based geographic analyses

  9. The Ultimate Goal • Spatially explicit link between the identity of a voucher, and the collection event • Increases body of data for biological distribution and diversity studies • Enables computer-based geographic analyses

  10. The Ultimate Goal Spatial Uncertainty • The geographic area encompassing all possible locations for the point

  11. Georeferencing, the how to

  12. Choosing the right georeferencing method is a compromise between precision, accuracy, and speed

  13. The Point-Radius Method • Uses one set of coordinates, the datum, and a bounding radius to describe a locality • Can incorporate both manual and automated georeferencing methods • Is a reproducible process

  14. Davis, Yolo County, California

  15. Davis, Yolo County, California

  16. Accuracy & Precision Accurate but not precise: • Illinois, Chicago Precise but not accurate: • Kelley Canyon, Z11 625200E, 4368390N • Correct UTM zone is 12

  17. Accuracy & Precision The georeferencing process aims to increase both the accuracy of the locality descriptor and the locality precision

  18. Spatial Uncertaintya.k.a maximum error distance • Captures the original inaccuracy and/or imprecision documented by the collector • Additional error introduced during data entry or the georeferencing process

  19. What contributes to uncertainty? • Locality extent • GPS accuracy • Unknown datum • Imprecision in distance • Imprecision in direction • Imprecision in coordinates • Map scale • Combinations of the above

  20. Out of context, you can’t assume what the collector meant!

  21. Locality Extent • Named places are not single points • Use the geographic center of a place & measure the greatest extent for the uncertainty • In general, current extents will be greater than historical ones, yielding conservative measures

  22. Davis, Yolo County, California

  23. GPS Accuracy • Deliberate signal scrambling before May 2000, artificial inaccuracies of ~ 100 m • Accuracy recorded on the GPS unit can be misleading • Best to assume 30m accuracy for retrospective GPS coordinates after May 2000

  24. Unknown Datum • Geodetic datums define the size and shape of the earth and the origin and orientation of the coordinate systems used to map the earth • Without a datum, coordinates are like measures without units.

  25. Datums Frames of Reference: • Australian Geodetic 1984 • Japanese Geodetic Datum 2000 • North American Datum (NAD) 1927 • North American Datum (NAD) 1983 • South American 1969 • World Geodetic System (WGS) 1984

  26. Datums, how do they vary?

  27. Why is the datum important?

  28. Why is the datum important? Extent of Offset

  29. Distance • 10.5 mi N of Bakersfield - the fraction is ½, uncertainty = 0.5 mi • 10.6 mi N of Bakersfield - fraction is 6/10, uncertainty = 0.1 mi • 10.75 mi N of Bakersfield - fraction is ¾, uncertainty = 0.25 mi In general, use 0.5 times powers of ten

  30. Direction • 10 mi N of Bakersfield - uncertainty 45 degrees in either direction • 10 mi NE of Bakersfield - uncertainty is 22.5 degrees, ENE - NNE • 10 mi N and 5 mi E of Bakersfield - can ignore directional imprecision

  31. Coordinate Precision • Too few digits = unnecessary uncertainties • 1° does not equal the same distance everywhere on the surface of the earth

  32. Map Scale • Each map has an inherent level of accuracy USGS Map Accuracy (1/30 to 1/50 inch) • For non-USGS use NOAA standard of 1 mm

  33. Georeferencing Calculator http://elib.cs.berkeley.edu/manis/gc.html Example: 1.3 km N, 0.6 km E Confluence Brumley & Pack Creeks, La Sal Mtns, Utah.

  34. Sources of Coordinates • Maps • Gazetteers • Localities already recorded with coordinates http://www.indexmundi.com/zp/ http://geonames.usgs.gov/

  35. Example N side Clear Creek Campground, Raft River Mtns, Utah T13N R13W Sec 10 Options: • TRS Converter & pre-established uncertainty http://www.esg.montana.edu/gl/trs-data.html • Use a map and measure section geocenter • Georeference to the campground Demonstrate TOPO program

  36. Elevation • Collector recorded elevation values are not incorporated in the georeferencing process • There is often no record of source & precision • Can be helpful guide in identifying errors

  37. Before you start, plan • Georeferencing is time-consuming • Georeferencing errors will occur • Certain descriptors won’t be georeferenced • A common framework for data collection is essential

  38. Increasing Efficiency What not to do: • Do not georeference specimen by specimen! • Do not treat the collections individually!

  39. Increasing Efficiency What to do: • Compile unique localities across all collections • Divide that list geographically and redistribute • Use your expertise • Assign georeferencing tasks by small spatial units (e.g. counties or provinces) • Within those units sort localities by place name • Coordinate the purchasing of georeferencing aids

  40. Example

  41. MaNIS Framework

  42. Error detection Map the coordinates (ArcView): • Do the coordinates have the correct + or – sign? • Is the decimal place shifted over (e.g. -111.90000 vs. -1119.0000) • Does the latitude = longitude or vice versa? • Do they fall within the correct geographical unit?

  43. Error Checking Egypt

  44. Error Checking Egypt http://www.cia.gov/cia/publications/factbook/

  45. Extra slides if TOPO fails

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