This article was originally published on 19 Oct 2020 by The Data WA Team
Stage 3 of the Landgate GDA2020 implementation project has commenced. This project will run over the 2020/21 financial year and focus on systems delivering data to our customers. Redevelopment of the Shared Location Information Platform (SLIP) data downloads solution will deliver GDA2020 compliant data while continuing legacy support for GDA94.
SLIP is now prepared to received data in GDA2020 from data custodians. We have also recently implemented updates to our data upload tool (DUT) to capture extra metadata when data is uploaded.
This additional information has also been added to the SLIP Data Update API, including the datum data was upload in, before any transformations are applied to store and publish data to web services. This API is also used by Data WA to update the ‘Data last updated’ and ‘Geospatial source projection’ fields for SLIP datasets.
For more information on GDA2020, technical implementation, and Landgate’s GDA2020 project please see the GDA2020 page on the Landgate website.
WGS84 and SLIP Web Services
SLIP web services will continue, for the foreseeable future, to provide data in WGS84@1994.0 (EPSG:3857) (the nominal equivalent of GDA94), unless otherwise specified, to align with existing GDA94 data.
GDA2020 data supplied to SLIP will be transformed to WGS84 for storage using the 2D conformal + distortion NTv2 grid file [EPSG: 8447] by default. Data will be stored as WGS84 and GDA2020 data will be transformed to WGS84 via GDA94 to avoid data misalignment.
WGS84 is a dynamic datum, GDA94 and GDA2020 are static datums. GDA94 and GDA2020 data, when displayed together using the de facto standard WGS84 Web Mercator (WGS84 WM) projection can be misaligned by 1.8 metres. This affects all web-mapping software and has been recognised as an international problem. Until web-mapping standards (EPSG registry and ISO standards) are updated it is necessary, in the short term, to choose an appropriate static datum and/or epoch for web-mapping, to avoid metre-level misalignments.
For advice on web mapping and the use of WGS84 WM please see the ICSM website and GMIWG Advisory on WGS84 and Web Mapping.