DataClassesCoordinateQuality - AtlasOfLivingAustralia/ala-datamob Wiki

Introduction

2013/14 - i rescued this from the old sites portal, which is behind a closed door - maybe one day i'll revise it to read a little easier

Attributes of a quality coordinate location

Quality coordinate information has these attributes; without all of them, certainty is lost and the quality of the position is reduced:

  1. latitude or eastings
  2. longitude or northings
  3. datum(s) and method(s) of projection, if applicable
  4. time of recording
  5. method used to determine location
  6. indication of conversions performed, if applicable
  7. precision
  8. accuracy (uncertainty) judgement of measurement

In the case of coordinate locations, accuracy (uncertainty) should not be inferred from precision.

Occurrence data attribute – precision of coordinate locations

Coordinate data stored in the Atlas has often passed through a conversion process en route to the database. The conversion process to decimal degrees may well result in 5 or more digits after the decimal point, representing ~1 metre or less at the equator.

This implied precision is not warranted if we look at some common methods of determining location. When using GPS an error of +/- 10m must be allowed for, or, a location may have been recorded as a 6-figure grid reference; assuming skilled map-reading, this puts the location within a 100m x 100m square. These are the true numerical precision (or resolution) of the measurements, and should be kept in mind.

Occurrence data attribute – accuracy (uncertainty, error) of coordinate locations

Accuracy (uncertainty, error) of the measurements represents a judgement of the method used to obtain location information, and would generally be recorded by the observer as a distance. This accuracy judgement could also be made after the fact if the method of determining location is recorded, for e.g. the use of a GPS, the make & model, and the datum.

Accuracy would include any errors inherent in the method used to determine location; it is also affected by any conversions or manipulation of the data that has occurred downstream; any such adjustments to the accuracy should be recorded.

Here are metadata on two locations recorded using different methods (absent are the actual locations themselves). Note that both require knowledge of the method used to obtain the location:

GPS used for location Precision 10m Accuracy is 20m (+/- 10m) on each axis Notes: Garmin used on WGS84 datum Taken dd/mm/yyyy
Map read for 6-figure grid reference Precision 100m Accuracy is 300m (+/- 150m) or 1/10th of a grid either side Notes: map GDA94 – 8628-4 Binalong Taken dd/mm/yyyy

Further reading

Chapman, A. D. 2005. Principles of Data Quality, version 1.0; pp 3–5 http://www2.gbif.org/DataQuality.pdf

Bennett, S. 2011. Darwin Core Key Spatial Concepts https://docs.google.com/file/d/0B7rqu1P0r1N0MmU0ZjE2YzUtNTYwZS00ZTI4LTllMmUtNzFmMTYwZjdmYjc4/edit?pli=1&hl=en_US

For a primer on coordinate locations, see: ALAhandlingspatiallocationdata.pdf

One discussion around how to handle uncertainty when it isn't provided as metadata, see: https://github.com/AtlasOfLivingAustralia/ala-dataquality/wiki/UNCERTAINTY_NOT_SPECIFIED

TODO: link to A. Tindall's doco on georeferencing...