Link to ICSM video : Metadata : What is it, and why is it so important
Datum Related Metadata Requirements
Spatial location data is incomplete without knowledge of the datum or reference frame upon which it based as coordinates without datum are at best ambiguous and at worst meaningless.
The multi-reference frame era, catering for high accuracy real-time positioning and datasets, results in other metadata routinely being required to appropropriately indicate usage limitations of spatial data.
Put simply, the aim is for all to "know your data, know your datum"
Land Tasmania requires the following minmum metadata to be included with every spatial dataset:
- Datum / Reference Frame e.g. GDA94, GDA2020, ATRF2014, AGD66, AHD83, King Island Local Datum, Flinders Island Local Datum, WGS84 Ensemble, etc
- Projection e.g MGA Zone 54. MGA Zone 55, Lambert Conformal Conic, Mercator, Web Mercator, etc
- Epoch (where appropriate) e.g. 2021.25, 2017.50, etc
- Transformation applied (where appropriate) e.g. GDA94<>GDA2020 Conformal and distortion NTv2 grid, AGD66<>GDA94 NTv2 grid method, AUSgeoid09, etc
The accompanying metadata should also include a statement regarding the accuracy (estimated or known) of the data.
Examples of Metadata Statements
The ICSM Metadata Working Group has published a comprehensive document entitled "Preparing Metadata for the AGRS" to outline ICSM's recommended metadata for Australian Geospatial Reference System elements in accordance with the international standard for metadata (ISO 19115-1). In particular, it provides guidance on how to treat "epoch" of measurment for spatial datasets on the ATRF2014 in advance of the pending upgrade of SO 19115-1 to specifically cater for the epoch of measurement and time-dependant reference frames.
The following examples of portions of a metadata statement of GDA2020 data are adapted from that document and utilise the concepts of GDA2020 Compatible and GDA94 Compliant outlined in the document. They are provided to indicate how information in the metadata adds context for subsequent users of the data.
Source Data / Method : GNSS capture, localised to survey control mark on static datum
Projection: MGA Zone 55
This dataset is GDA2020 compliant.
The dataset was collected using GNSS positioning technique xxx, by user(s) xxx, localised to SPMxxxx
GDA94 MGA Zone 55 Exxxxxx.xxx Nxxxxxxx.xxx AHD83 xxx.xxx
Captured on yyyy-mm-dd
The technique employed to capture the spatial data component provided feature positions with reference to GDA94 (horizontal) and AHD83 (vertical, via application of the AUSGeoid09 model (required for use with GDA94) within the equipment).
The horizontal (and vertical) accuracy of the data upon collection is estimated [or known] to be xxx (and xxx).
The horizontal component of this dataset was subsequently transformed to GDA2020 via the 'Conformal Plus Distortion' NTv2 grid method with a nominal (additional) transformation accuracy / uncertainty of 0.05m.
Source Data / Method: Image, originally GDA94, low accuracy
Projection: MGA Zone 55
This dataset is GDA2020 compatible.
This dataset was collected from features evident in an existing georeferenced Image using technique xxx, by user(s) xxx.]
The source image was obtained from / via xxx.
The source image was captured yyyy-mm-dd [or appropriate temporal range if mosaic].
The source image and therefore the captured features are georeferenced as GDA94 (horizontal only).
The horizontal accuracy of the geo-referencing and/or data collection method for this example are low (5 metre). The resulting dataset has been nominated as a low-accuracy GDA2020 dataset. As this data has not directly measured in GDA2020, nor transformed to GDA2020 the resulting data is 'GDA2020 Compatible', not Compliant.