A numerical description of a place that can be mapped. What is a georeference?
A numerical description of a place that can be mapped. What is a georeference? In other words…
IDSpeciesLocality 1Lynx rufusDawson Rd. N Whitehorse 2Pudu pudacerca de Valdivia 3Canis lupus20 mi NW Duluth 9Ursus arctosBear Flat, Haines Junction 4Felis concolorPichi Trafúl 5Lama alpacanear Cuzco 6Panthera leoSan Diego Zoo 7Sorex lyelliLyell Canyon, Yosemite 8Orcinus orca1 mi W San Juan Island What we have: Localities we can read
Darwin Core Location Terms –higherGeography –waterbody, island, islandGroup –continent, country, countryCode, stateProvince, county, municipality –locality –minimumElevationInMeters, maximumElevationInMeters, minimumDepthInMeters, maximumDepthInMeters
What is a georeference? A numerical description of a place that can be mapped.
Davis, Yolo County, California point method Coordinates: 38.5463 -121.7425 Horizontal Geodetic Datum: NAD27
Data Quality data have the potential to be used in ways unforeseen when collected. the value of the data is directly related to the fitness for a variety of uses. as data become more accessible many more uses become apparent. – Chapman 2005 the MaNIS/HerpNET/ORNIS guidelines follow best practices (Chapman and Wieczorek 2006) to enhance data quality and value
What is an acceptable georeference? A numerical description of a place that can be mapped and that describes the spatial extent of a locality and its associated uncertainties.
Davis, Yolo County, California bounding-box method Coordinates: 38.5486 -121.7542 38.545 -121.7394 Horizontal Geodetic Datum: NAD27
Davis, Yolo County, California point-radius method Coordinates: 38.5468 -121.7469 Horizontal Geodetic Datum: NAD27 Maximum Uncertainty: 8325 m
What is an ideal georeference? A numerical description of a place that can be mapped and that describes the spatial extent of a locality and its associated uncertainties as well as possible.
pointeasy to produce no data quality bounding-boxsimple spatial queries difficult quality assessment point-radiuseasy quality assessment difficult spatial queries shapeaccurate representation complex, uniform Method Comparison probabilityaccurate representation complex, non-uniform
MaNIS/HerpNET/ORNIS (MHO) Guidelines http://manisnet.org/GeorefGuide.html http://manisnet.org/GeorefGuide.html uses point-radius representation of georeferences circle encompasses all sources of uncertainty about the location methodology formalizes assumptions, algorithms, and documentation standards that promote reproducible results methods are universally applicable
Geodetic Datum: defines the position of the origin, scale, shape, and orientation of a 3- dimensional model of the earth. Example: WGS84. Coordinate System: defines the units of measure of position with respect to the datum. Example: latitude, longitude in degrees, minutes, seconds Geographical Concepts:
Map Projections: mathematical transformations of the 3-D model of the surface of the earth onto a 2- D map. there are many (e.g., conical, cylindrical, azimuthal) - they all suffer from distortions (area, shape, distance, or direction), but some preserve areas or distances. When measuring distances on paper maps, use an equal distance projection, if available, otherwise understand the implications.
Named place: a place of reference in a locality description. Example: Davis in 5 mi N of Davis Areal extent: the geographic area covered by a named place (feature). Example: the area inside the boundaries of a town. Linear extent: the distance from the geographic center to the furthest point of the areal extent of a named place. Georeferencing Concepts
Offset: the distance from a named place. Example: 5 mi in 5 mi NE of Beatty. Heading: the direction from a named place. Example: NE in 5 mi NE of Beatty. Georeferencing Concepts
coordinateUncertaintyInMeters: The horizontal distance (in meters) from the given decimalLatitude and decimalLongitude describing the smallest circle containing the whole of the Location. Leave the value empty if the uncertainty is unknown, cannot be estimated, or is not applicable (because there are no coordinates). Zero is not a valid value for this term. (from Darwin Core) Maximum Error Distance: same as coordinateUncertaintyInMeters, except the units are the same is in the locality description, not necessarily meters. Georeferencing Concepts
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. 1.1 km) 20° 30 N 112° 36 W ScaleUncertainty (ft)Uncertainty (m) 1:1,2003.3 ft1.0 m 1:2,4006.7 ft2.0 m 1:4,80013.3 ft 4.1 m 1:10,00027.8 ft8.5 m 1:12,00033.3 ft10.2 m 1:24,00040.0 ft 12.2 m 1:25,00041.8 ft12.8 m 1:63,360106 ft32.2 m 1:100,000167 ft50.9 m 1:250,000417 ft127 m
Georeferencing Calculator Example Locality: 0.5 km N of Little mermaid, Copenhagen, DK Start with original coordinates for the mermaid: 55° 41' 34.18" N 12° 35' 56.73" E Then use the Georeferencing Error Calculator to determine the final coordinates AND the uncertainty.
Georeferencing Error Calculator: 0.5 km N of Little mermaid, Copenhagen, DK 55° 41' 34.18" N 12° 35' 56.73" E
Capture georeferences in database or spreadsheet (we will use an Excel template for examples) Georeferencing Templates