Georeferences
Definition: http://rs.tdwg.org/dwc/terms/index.htm#coordinatePrecision
Translations: http://terms.tdwg.org/wiki/dwc:coordinatePrecision
Related issues: https://github.com/tdwg/dwc-qa/issues/23
Related webinar: Where am I, exactly? Darwin Core Georeferencing Terms
Further reading: There is a nice treatment of accuracy and precision, including references to GPS capabilities, at http://gis.stackexchange.com/questions/8650/measuring-accuracy-of-latitude-and-longitude.
Question: Does anyone record or serve coordinatePrecision? If so, can you explain how researchers might find this information useful?
Answer: The term coordinatePrecision is populated fairly often, especially in contexts that use grid cells based on geographic coordinates (e.g., 1 degree grid cell). In this context, in the absence of coordinateUncertaintyInMeters or footprintWKT, a point-radius or bounding box could be determined from the decimalLatitude, decimalLongitude and coordinatePrecision. The term coordinatePrecision could be used in records that have coordinateUncertaintyInMeters as well to highlight that the coordinates, as given, are of a particular precision that cannot be captured by the coordinates themselves (e.g., the coordinates 34.5, -117.25 could refer to a location with indeterminate very high precision, or roughly, to a location with a precision of a 15 minutes). If the coordinateUncertaintyInMeters is given and calculated faithfully, the coordinatePrecision adds an indicator of how much of the uncertainty is due to imprecision.
Researchers would find coordinatePrecision particularly useful to understand how specific the coordinates are in the absence of a point-radius or other geometry.
Question: What are acceptable values for coordinatePrecision?
Answer: Following is a table showing the English language equivalent and the coordinatePrecision value. The coordinatePrecision value of any missing precisions should be possible to interpolate.
| coordinate precision | coordinatePrecision |
|---|---|
| nearest degree | 1 |
| nearest half degree | 0.5 |
| nearest quarter of a degree | 0.25 |
| nearest tenth of a degree | 0.1 |
| nearest hundredth of a degree | 0.01 |
| nearest thousandth of a degree | 0.001 |
| degree to four decimal places | 0.0001 |
| *degree to five decimal places | 0.00001 |
| nearest 10 minutes | 0.1666667 |
| nearest minute | 0.0166667 |
| nearest tenth of a minute | 0.0016667 |
| nearest hundredth of a minute | 0.0001667 |
| *nearest thousandth of a minute | 0.0000167 |
| nearest 10 seconds | 0.0027778 |
| nearest second | 0.0002778 |
| nearest tenth of a second | 0.0000278 |
| *nearest hundredth of a second | 0.0000028 |
* these three levels of precision are commonly found on commercial GPS devices in the three modes: decimal degrees, degrees decimal minutes, and degrees minutes seconds.
Question: Does coordinatePrecision represent the precision of the physical instrument (e.g. GPS) used to "read" the decimalLatitude and decimalLongitude? If yes, does it represent the manufacturer declared value or the value read on the unit when the event occurred?
Answer: Strictly speaking, no. The coordinatePrecision is a numerical value to represent the precision of the coordinates provided in decimalLatitude and decimalLongitude, with values such as those given in the table above. It may or may not have anything to do with a GPS, and almost certainly has nothing to do with the accuracy of the GPS, even if a GPS was used to determine the coordinates. Though poor coordinate precision can affect accuracy, GPS accuracy is a distinct measure. Both accuracy and precision contribute to coordinateUncertaintyInMeters. The distinction between these concepts, and how they contribute to uncertainty is taught in georeferencing workshops and is not trivial. These concepts are covered in the Georeferencing Quick Reference Guide, The point-radius method for georeferencing locality descriptions and
calculating associated uncertainty, and the Guide to Best Practices for Georeferencing. The Georeferencing Calculator can be used to explore the effects of precision and accuracy on 'coordinateUncertaintyInMeters`.
Manufacturer's tend not to declare an accuracy for a GPS unit, because accuracy is more dependent on environment and satellite configuration at the place and time of the Event than anything inherent in the device. Some GPS units do declare "accuracy" in their interfaces, but this is a theoretical accuracy based on current conditions, not a measured accuracy.
Question: Can coordinatePrecision be calculated retrospectively?
Answer: Strictly speaking, no. However, one can infer the minimum coordinatePrecision of a coordinate pair, or the coordinatePrecision for the coordinates in a data set, if they are consistent. For example, if the decimalLatitude and decimalLongitude values in an entire data set are given to two decimal places and all of the decimal parts are multiples of a quarter degree (.00, .25, .50, .75), one could infer that the coordinatePrecision for each record was to the nearest quarter of a degree, or 0.25. Note that this inference could be much weaker for a single coordinate pair.
Definition: http://rs.tdwg.org/dwc/terms/index.htm#footprintWKT
Translations: https://terms.tdwg.org/wiki/dwc:footprintWKT
Related issues: https://github.com/tdwg/dwc-qa/issues/112
Further reading: "Guide to Best Practices for Georeferencing", Chapman and Wieczorek, eds. 2006.
Well-known Text: Wikipedia
Well-known Text: OGC/ISO specification
OBIS Geocoding tool: With option to encode in WKT Tutorial video
Question: The ALA has a potential data provider who has a high quality weed survey and treatment dataset that contains fairly precise polygon data and we would like to include it. Does Darwin Core allow for polygon-based occurrence data currently?
Answer: The term footprintWKT is meant specifically to hold geographic shape information that is more complex than the simple point-radius representation of a coordinate pair (decimalLatitude, decimalLongitude, geodeticDatum) with coordinateUncertaintyInMeters. The footprintWKT field can be used to capture explicit corners of a bounding box, for example, or complex polygons with many vertices. The recommended format for the geometric information is Well-known Text (WKT), which is a joint international standard (ISO 19162) of the Open Spatial Consortium (OGC) and the International Standards Organization (ISO).
A path with three points, starting at longitude 30, latitude 10 and ending at longitude 40, latitude 20 would be expressed as multiple line segments with a LINESTRING as follows:
LINESTRING (30 10, 35 15, 40 20)
A one-degree bounding box with opposite corners at (longitude=10, latitude=20) and (longitude=11, latitude=21) would be a polygon expressed as follows:
POLYGON ((10 20, 11 20, 11 21, 10 21, 10 20))
POLYGON ((-98.368768 40.067848, -98.368982 40.066765, -98.37057 40.067339, -98.371901 40.067996, -98.373295 40.068128, -98.374519 40.067701, -98.37527 40.067799, -98.376128 40.068341, -98.37645 40.067832, -98.377651 40.067389, -98.378059 40.067947, -98.379561 40.067733, -98.3816 40.067126, -98.382651 40.066272, -98.383574 40.067356, -98.382115 40.068128, -98.37969 40.068981, -98.377201 40.069195, -98.375463 40.068949, -98.374948 40.068604, -98.374733 40.068456, -98.372909 40.068916, -98.371343 40.068752, -98.370098 40.068242, -98.368768 40.067848))
The Darwin Core term footprintSRS is meant to capture the spatial reference information associated with the footprintWKT, and without it, the footprintWKT is ambiguous.
The Ocean Biogeographic Information System (OBIS) uses footprintWKT for polygons (e.g., habitats) and line strings (e.g., trawls). OBIS has a geocoding tool (http://iobis.github.io/map/), which has an option to generate WKT. See short video tutorial at https://www.youtube.com/watch?time_continue=84&v=XB4rUYhb_ok.
The OBIS recommendation is to provide the midpoint and radius in addition to footprintWKT for polygons and line strings. For recommendations on what to do when the midpoint is outside the polygon, see https://blog.mapbox.com/a-new-algorithm-for-finding-a-visual-center-of-a-polygon-7c77e6492fbc. Another option is to show these polygons as points on a map (see https://i.stack.imgur.com/cTLuj.jpg).
Definition: http://rs.tdwg.org/dwc/terms/index.htm#geodeticDatum
Translations: http://terms.tdwg.org/wiki/dwc:geodeticDatum
Related Issues: https://github.com/tdwg/dwc-qa/issues/59
Question: How do I find / select the correct EPSG code for my data when the EPSG Regsitry website provides a large list.
Answer: (in development, see Related Issues ticket)
Definition: http://rs.tdwg.org/dwc/terms/index.htm#georeferenceVerificationStatus
Translations: http://terms.tdwg.org/wiki/dwc:georeferenceVerificationStatus
Related issues: https://github.com/tdwg/dwc-qa/issues/32
Lookup table: For a list of distinct values of georeferenceVerificationStatus and recommended standard values for them, see https://github.com/tucotuco/DwCVocabs/blob/master/kurator/georeferenceverificationstatus.txt
Question: Do you use the georeferenceVerificationStatus field? Do you know if or how researchers have used this field?
Answer: The intention of the georeferenceVerificationStatus field is to avoid repeating work unnecessarily. Darwin Core documentation of georeferenceVerificationStatus recommends exactly three possible non-null values for this field ('verified by collector', 'verified by curator', and 'requires verification'). The utility of this field is best realized in the source data to distinguish between georeferences that are as good as they could ever be ('verified by collector'), from those that have had lesser degrees of validation. To be 'verified by collector' means that the collector has seen the occurrence mapped and verified that no more specific georeference can be made. In the absence of the collector's verification, 'verified by curator' indicates that all available resources (which ought to be captured in georeferenceSources) have been use to make the georeference as specific as it can be. Both of these verification statuses indicate to a researcher (and indeed to the data custodian) that there is little that can be done to improve on the georeference, thus avoiding unnecessary work in those cases where questions about the veracity of the georeference arise. Any other state of a record should have a georeferenceVerificationStatus of 'requires verification'.
In all cases, the field georeferencedBy should indicate the person responsible for asserting the veracity of the georeference as given. The georeferenceVerificationStatus field is in common use in collections that value georeferences and the effort expended to determine them, because georeferencing well is not trivial.
Definition: http://rs.tdwg.org/dwc/terms/index.htm#pointRadiusSpatialFit
Translations: http://terms.tdwg.org/wiki/dwc:pointRadiusSpatialFit
Related issues: https://github.com/tdwg/dwc-qa/issues/31
Further explanation: Guide to Best Practices for Georeferencing Chapter "Georeferencing Legacy Data" Section 6, "Determining Spatial Fit".
Question: Does anyone record or serve pointRadiusSpatialFit? If so, can you explain how researchers might find this information useful?
Answer: This field is rarely used. It is meant to give a measure of how well the point-radius matches the actual geometry of the locality. A researcher could use it as a filter to distinguish between localities that are well-represented by the point-radius (pointRadiusSpatialFit very close to 1.0) from those that are not (pointRadiusSpatialFit = 0, null, or much greater than 1). Unless the pointRadiusSpatialFit is 1, 0, or null, or the original geometry is a bounding box, the pointRadiusSpatialFit can be onerous to determine.