Tag Archives: Coordinate Systems

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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Understanding Spatial References in Australia

GIS data must accurately represent real-world locations to be useful. To do this, you need a standard framework for defining location – a coordinate system. All spatial data utilises a coordinate system to position itself relative to your map and other data. However, coordinate systems are frameworks. They are models, spheroids, imperfect approximations of the shape of the Earth with all its bumps and curves as it changes with time. When working with spatial data you need to consider how a GIS positions this data against reality using coordinate systems.

By better understanding coordinate systems, you will be able to manage your data in a way that increases the accuracy of your maps and the work that you do with them. This blog is the first in a series taking a closer look at using Australian spatial references in ArcGIS. We will start by looking at geographic coordinate systems, projections, and transformations.

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The Pleasures and Sorrows of Coordinate Systems

Ever tried to add points from an Excel spreadsheet or GPS log, and had them not appear where they were supposed to? Chances are you’ve got a case of muddled coordinate systems.

When you view x,y data in ArcGIS (using the “Display XY data…” dialog or the “create feature layer” tool), you must tell ArcGIS what coordinate system the x,y coordinates are in. However, in its quest to make your life easier, it often auto-selects a coordinate system for you – which, despite the best intentions, can cause headaches down the track. Read on for the cause, and more importantly, the solution… Continue reading