![]() ![]() A composite transformation is available of WGS84 to GDA94 and GDA94 to GDA2020. The following diagram gives a representation of the relevant issue:Īt the time of writing in March 2021, there is no transformation between GDA2020 and WGS84 directly. Unfortunately, with the modernisation of the Australian datum to GDA2020 there is not a definitive relationship between GDA2020 and WGS84. Essentially, the problem presents itself as a misalignment of GDA94 or GDA2020 data when viewed in a WGS84 based projection. Although this has not presented significant issues to the end-user as data mapped in web maps typically had low accuracy – this is changing. That ‘Null’ parameter now creates a problem when projecting GDA2020 data to WGS84 Web Mercator as the differences between GDA94 datum (a static datum) and WGS84 (effectively, a dynamic datum) have increased over time. At this period both GDA94 and WGS84 were considered coincident as they were based on the same spheroid, with essentially the same origin. The Web Mercator Dilemmaĭuring the early 1990s, the governing bodies defining the transformation parameters between GDA94 and WGS84 set the transformation as ‘null’. Failing to address these challenges may result in data misalignment. ![]() The de facto WGS84 in web GIS brings some challenges not unique to Australia or ArcGIS. It is also important to understand GDA2020 and its relationship with web GIS. When publishing data to web GIS you need to consider what spatial reference will be used to store the data at a service level, how the publishing process may affect it, as well as what transformations (or lack of) will be applied when the layer is displayed in a web map with a datum. When you add layers to the Web Map Viewer, an on-the-fly reprojection is performed to align your data with the coordinate system of the basemap. Layers also have their own spatial reference and transformations defined at a service level. WGS 1984 Web Mercator (auxiliary sphere) is the spatial reference of basemap services in the default Esri basemap gallery, meaning this is the default web GIS coordinate system. ![]() In ArcGIS Online Map Viewer, all layers in a web map inherit the coordinate system of the basemap. ![]()
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