Tag Archives: mercator

GDA2020, ArcGIS Online and the Web Mercator Dilemma

In ArcGIS Online Map Viewer, all layers in a web map inherit the coordinate system of the basemap. 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. Layers also have their own spatial reference and transformations defined at a service level. 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.

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.

It is also important to understand GDA2020 and its relationship with web GIS. The de facto WGS84 in web GIS brings some challenges not unique to Australia or ArcGIS. Failing to address these challenges may result in data misalignment.

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