Excerpt from the document, 'Darwin Core Key Spatial
Concepts'
Written by Simon Bennett, Atlas of Living Australia, 29
November 2011.
The full document, includes definitions and examples for each of
the terms: http://tinyurl.com/6t6zyaa
Darwin Core Key Spatial Concepts
The following discussion provides a perspective on Darwin Core
(TWDG 2009) based occurrences records as they relate to the
fitness-for-use of occurrence data for spatial analyses such as
species distribution mapping and spatial modelling.
Fitness-for-use often refers to a scale of spatial data quality
that changes with varying data accuracy, precision and intended use
(GBIF 2010). For some uses, such as continental scale distribution
mapping relatively imprecisely located spatial can be used. While
if the intended use is for fine-scale habitat mapping, then only
reliable, precisely located spatial occurrences should be used.
The Location
In order to assess the fitness-for-use of a species occurrence
record it is essential to know the:
(i) geographic coordinates;
(ii) precision at which coordinates where collected;
(iii) extent of the location where the occurrences is reported
from; and
(iv) geodetic datum (spatial reference system) under which the
geographic coordinates where collected.
(v) Uncertainties associated with determining the coordinates
Terms (ii), (iii), (iv) and (v) are added together to produce an
overall measure of the uncertainty associated with a coordinate
pair. See the following diagram as an example.
Spatial Concepts text in Google Maps above
The location is a 2 ha survey plot. The coordinates are at
the centre of the location. The geocode uncertainty is the error in
recording the coordinates: typically 5m with a GPS. The location
extent is the radius of a circle encompassing the actual location.
If the datum is not recorded then there may be an additional 200m+
datum shift error. Coordinate precision relates to the number of
decimal places the coordinates have (7 places or 0.0000001 with
Google Maps).
The overall coordinateUncertaintyInMeters is geocoding
uncertainty + location extent + potential datum shift (if datum
unknown) + coordinate precision (<0 in this case) = 105m or 305m
if datum is unknown. Note: Darwin Core does not contain terms for
location extent or geocode uncertainty.
Data generalisation
Sometimes coordinates are generalised to protect sensitive
species or to protect intellectual property. Common methods are to
(i) round coordinates to fewer decimal places, (ii) grid the data
or (iii) randomly move the location. If generalised then the method
of generalization must be recorded in DwC:dataGeneralizations.
Where spatial generalisation does occur a new overall
uncertainty should created by adding the uncertainty introduced by
the generalisation process to the original
coordinateUncertaintyInMeters. The original
coordinateUncertaintyInMeters should be preserved as a 'verbatim'
field.
Explicit verification of location
Where an occurrence record is on the edge of its range, or is a
range extension, then it is useful to know whether the location
coordinates have been verified
(DwC:georeferenceVerificationStatus).
Determining Overall Spatial Uncertainty
The overall measure of spatial uncertainty is
DwC:coordinateUncertaintyInMeters: 'The horizontal distance (in
meters) from the given decimalLatitude and decimalLongitude
describing the smallest circle containing the whole of the
Location'. Typically it is additive of distances/errors and
associated spatial uncertainty: typically the extent of the
location and any uncertainty related to the datum and coordinate
precision. John Wieczorek (in lit.) notes that
DwC:coordinateUncertaintyInMeters is exactly equivalent to Maximum
Error Distance used in his Georeferencing Calculator.
A knowledge of the coordinates, their precision and geodetic
datum, and the extent of location is essential to determine the
overall spatial uncertainty of a location. Wieczorek (2007)
demonstrates the calculation of the potential error in these terms
to provide an overall measure of spatial uncertainty, the Maximum
Uncertainty Distance = DwC:coordinateUncertaintyInMeters. The
methods used in the Georeferencing Calculator are outlined in the
MaNIS/HerpNet/ORNIS Georeferencing Guidelines, available at http://manisnet.org/GeorefGuide.html
The Georeferencing Calculator (http://manisnet.org/gc.html) is an interactive
tool that determines the Maximum Uncertainty Distance (=
DwC:coordinateUncertaintyInMeters) based on coordinate precision
(DwC:coordinatePrecision in decimal degrees), the extent of the
locality (locationExtent), and knowledge of the geodetic datum
(DwC:geodeticDatum) and other uncertainties. Note that terms
'Coordinate Precision' and 'Coordinate Uncertainty' are sometimes
confused and have been used interchangeably. So take care when
using these terms.
Determining fitness-for-use for spatial analysis
GBIF (2010) notes that fitness-for-use refers to a scale of data
quality that changes with the varying data accuracy, precision and
intended use. For some applications, data quality can be relatively
low and still be fit for use. In the context of geospatial data, we
can split fitness-for-use into two broad categories: (i) Are the
geospatial data correct? (ii) Are the geospatial data usable at the
geographic scale of the question? For example, coarse scale
geospatial data may only be usable for continental or global
analyses but certainly not for local analyses. They also note that
the minimum information needed to assess spatial fitness for use is
a latitude-longitude-geographic uncertainty triplet.
Other important factors to consider in assessing overall
fitness-for-use:
- Taxon name
- Institution and occurrence record identifier
- Date
- Elevation and Depth
- Is the record a natural occurrence?
Future Developments
The focus of further work will be development of additional
methods for validation of occurrence data, and development of
explicit metrics and summaries to enable a user to make informed
choices on spatial, taxonomic and temporal fitness-for-use.
Robyn Lawrence