I’m going to assume an awareness of what coordinate systems and projections are but this article covers the basics for anyone who is already confused. When I started in GIS there wasn’t much cause to know about projections, beyond knowing that UK data was generally stored in a coordinate system called British National Grid or BNG. At the time I did some work integrating GPS devices directly with ArcMap and had to learn some of the mysteries of geodesy. I felt better after a little lie down. ArcGIS Online is a global platform and in order to provide basemaps that are useful to a global community of users the default projection has to work globally. ArcGIS Online uses the WGS 1984 Web Mercator (Auxiliary Sphere) coordinate system as the default Projected Coordinate System (PCS). This is a Mercator based map projection combined with the WGS84 Geographic Coordinate System (GCS). All projections involve an element of compromise and those based on the Mercator projection exaggerate the size and shape of areas as you approach the extreme Northern and Southern latitudes of the earth, this article describes the effect. For many use cases in the UK this distortion isn’t a problem. Some GIS data used within the UK is in a WGS84 based coordinate system but much of it is stored in British National Grid (also known as Great Britain National Grid or BNG). The British National Grid PCS is based on a different geographic coordinate system, usually GCS_OSGB_1936 (there are some other variants). Data stored in one PCS can be re-projected into another. As with other parts of the ArcGIS platform, ArcGIS Online will re-project layers in different projections on the fly to the PCS of the basemap. Simples? Yes, but there are things to consider depending on your work flows. If you will be storing and using all your data within ArcGIS Online then it makes sense to use Web Mercator across the board. Any services created from a CSV or a Shapefile* or via Esri Maps for Office will be in Web Mercator. For those of you that have both ArcGIS for Desktop (ArcMap) and ArcGIS Online it may be appropriate to mix and match things a little. If you already have an established body of GIS data in BNG then you are likely to want to continue to use that as your main coordinate system. Also, if you need a high degree of accuracy (sub-metre) then BNG is already a better choice. If you want to create a BNG based service in ArcGIS Online you will need to publish them from ArcGIS for Desktop (read more here). If you are creating datasets that will be stored and consumed primarily in ArcGIS Online then it may be better to set these up as Web Mercator services. If the data is originating from a BNG source then you should do the re-projection to Web Mercator in ArcGIS for Desktop, where you can select an appropriate Transformation, before publishing to ArcGIS Online. Remember, if you mix coordinate systems in a webmap, web application or in Desktop then the system will need to re-project on the fly. This puts a load on the system and can make the draw times slower, depending on the data volumes. If you can work with all your data in one projection it will result in a slicker overall experience when zooming and panning on the map. If your business data is predominantly in BNG then you will benefit from using a BNG basemap within ArcGIS Online. You can either subscribe to the Esri UK datahub services or create your own tiled service by building and publishing it from ArcGIS for Desktop. The considerations here are very similar to the previous section but it is likely that you already have a larger investment, in terms of existing data, in a particular coordinate system. You may well also be using a mixture of in-house services, from ArcGIS for Server, and ArcGIS Online services. The issues with on the fly re-projection would also apply here. Hopefully this article has given you an understanding of where there are decisions to be made in relation to the coordinate system you are using and what the right options might be. If you’d like any further advice on this then please do get in touch with us at Esri UK. If you create your data by geocoding some address data then the coordinate system of the points will be determined by the locator you use. The World Geocoding Service uses WGS84 (ID = 4326) whereas the Esri UK Datahub locators use BNG (ID = 27700). *There is a known issue with the accuracy of the re-projection of BNG data from a Shapefile which results in the data in the ArcGIS Online service being offset by up to 100 metres. Source.