georeferencing collections & determining uncertainty

Georeferencing CollectionsGeoreferencing Collections& &

Determining UncertaintyDetermining Uncertainty

Australian Biodiversity Information Services

Arthur D. Chapman

Chapman & Wieczorek (eds) (2006)http://www.gbif.org/orc/?doc_id=1288

• The document provides guidelines to World’s Best Practice for georeferencing, including guidance on– determining a georeference– determining the spatial uncertainty– recording the georeferences and uncertainties

• Several earlier projects provided a good starting point– Biogeomancer Classic – MANIS-HerpNet – MapSteDI – Diva-GIS – HISPID – CRIA– ERIN

Introduction

A Best Practices Document provides

• Speed and efficiency• Consistency• Training of new data entry operators• Improved data quality• Transparency and documentation• Scrutiny of management and external/internal auditors• Scrutiny of users of the information

Principles of Best Practice

• Accuracy – a measure of how well the data represent true values.– e.g. represented by an uncertainty radius (or polygon of uncertainty) in

georeferencing

• Effectiveness – the likelihood that a task achieves its desired objectives.– e.g. the percentage of records for which the latitude and longitude can be

accurately determined.

• Efficiency – the ratio of output to input. – e.g. the effort that is needed to produce an acceptable output. Also the

amount of input data the user has to obtain to produce an acceptable result (e.g. gazetteers, collectors itineraries, etc.)

• Reliability – refers to the consistency for which results are produced– e.g. refers to the repeatability for which a georeference can be produced

by the user for the same locality.

Principles of Best Practice

• Accessibilty – how accessible the results are to the users/ public– e.g. the ease with which users and other institutions can access the

georeference for a particular locality that has already been georeferenced.

• Transparency – an annunciation of the procedures for collection, analysis, reporting and update.

– e.g. refers to the transparency of methods used to georeference a particular locality.

• Timeliness – relates to the frequency of data collection, its reporting and updates.

– e.g. includes update frequency of gazetteers, new methodologies, and when records are georeferenced and made available to others.

• Relevance – the data collected should meet the needs of the user – i.e. should fulfill the principle of “fitness for use”.

– e.g. refers to the format of the output (does it include Datum etc.) and does it include good metadata on the above topics)

Collecting Data in the Field

Using a GPS– Accuracy variable

(seldom as good as ‘reported’ on the GPS Unit!)• DGPS• Wide Area Augmentation Systems (WAAS)• Local Area Augmentation Systems (LAAS)

– Document• Coordinates• Datum• Accuracy reported by GPS

Recording Datums– Different datums can mean a difference in location of

from a few cms to 3.552 km.

Recording Localities– The most specific localities are those described by

• a) a distance and heading along a path from a nearby and well-defined intersection or fixed persistent point, or

• b) two cardinal offset distances from a single persistent nearby feature of small extent.

Collecting Data in the Field

Recording Coordinates– Decimal degrees are preferred when reading

coordinates from a GPS• have higher precision and thus accuracy

– But note some recent GPS units record in Decimal seconds to three decimal points

• Reduces transcription error (most databases store data in DD)

Recording Extent– extent is a measure of the size of the area within

which collecting or observations occurred for a given locality – the distance from the point described by the locality and coordinates to the furthest point where collecting or observations occurred in that locality

Collecting Data in the Field

Documentation

Example:• Locality: 2 nm NNE of North Head Light House off

Sydney Heads• Lat/Long: −33.79916, 151.32054 • Datum: WGS84 • GPS Accuracy: 6 m • Extent: 50 m • Remarks: Garmin Etrex Summit GPS for

coordinates and accuracy

Database Fields

See: Geospatial Element Definitions v1.4(extension to Darwin Core)

• Decimal Latitude• Decimal Longitude• Geodetic Datum• Maximum Uncertainty Estimate• Maximum Uncertainty Unit• Verbatim Coordinates• Verbatim Coordinate System• Georeference Source (e.g. USGS Gosford Quad map 1:24000, 1973) • Verification Status (e.g.: "requires verification", "verified by collector")• Validation Status• Georeference Determined by• Georeference Determined date• Remarks

• [Spatial Fit]

Spatial Fit

From J. Wieczorek, in Chapman and Wieczorek (eds) (2006)

A measure of how well the geometric representation matches the original spatial representation.

For an area where the original spatial representation of a locality is the red polygon with area ‘A’. The spatial fit is:

(2*r22)/A

(Pi*r22)/A

1.0

0

(Pi*r12)/A

Geodetic Datums

Traditional Horizontal Datums

NAD 27(Clarke Ellipsoid )

ED 50(International Ellipsoid)

From US Navy (n.dat.)

Datum Shifts

Vertical Datums

High Tide

Low Tide

Mean Sea Level

Like horizontal measurements, elevation only has meaning when referenced to some start point.

MSL Elevation

Mean sea level is the most common vertical datum.Mean sea level is the most common vertical datum.From US Navy (n.dat.)

MaNiS Georeferencing Calculator

http://www.manisnet.org/gc.html

Locality TypesFeatures

• Named Place (Feature)• Near a Feature• Between Two Features• Street Address

Paths• Path• Between Two Paths

Offsets• Offset Distance• Offset at a Heading• Offset along a Path• Offset in Orthogonal Directions• Offset from two distinct Paths

Coordinates• Longitude and Latitude Coordinates• UTM Coordinates

Areas• Township, Range, Section• Map Sheet

Do not georeference• Captive or Cultivated• Dubious• Cannot be Located

Locality Types

1. Named Place» Example 1: “Bakersfield”» Example 2: “Point Lookout”» Example 3: “Isla Tiburon”» Example 4: “Junction of Dwight Avenue and Derby

Street ”

Georeference: Use centre of named placeExtent: Use the distance from the coordinates of the named place to the furthest point within the named place Uncertainty: Use the MaNIS Georeference Calculator

Example

Locality: “Bakersfield” Suppose the coordinates for Bakersfield came from the GNIS database (a gazetteer) and the distance from the center of Bakersfield to the furthest city limit is 3 km.

Coordinate System: degrees minutes seconds Latitude: 35º 22' 24" N Longitude: 119 º 1' 4" W Datum: not recorded; 79 m uncertainty Coordinate Precision: nearest second; 40 m uncertainty

Coordinate Source: gazetteer Extent of Named Place: 3 km Distance Units: km

From Georeferencing CalculatorDecimal Latitude: 35.37333 Decimal Longitude: -119.01778 Maximum Uncertainty Distance: 3.119 km

Locality Types

Between two Named Places» Example 1: "between Point Reyes and Inverness"

Georeference: Find the coordinates of the midpoint between the centres of the two named places (e)

Extent: Use the extent of A or B, whichever is greater, plus one-half the distance between the centres of A and B.

Uncertainty: Use the MaNIS Georeference Calculator Calculate the same as for ‘Named Place’.

BioGeomancer

• Introduction - Web Site

www.biogeomancer.org

BioGeomancer Workbench

• Need to register and Log in• Submit a project

– Use the “Help” to see how to go about submitting files

– (See next slide for formatting)– NB – files must be in UTF-8

• Results downloaded as XML file

BioGeomancer Workbench – Batch Georeferencing

Format for batch georeferencing

Similar to BioGeomancerCan use line/polygon features such as distance along a road/river etc.Many features only available (as yet) for USA and Spain.Desktop version, batch mode and collaborative georeferencing

GeoLocate

http://www.museum.tulane.edu/geolocate/

Acknowledgments:

Chapman & Wieczorek (eds) (2006)http://www.gbif.org/orc/?doc_id=1288