By Kenneth Field, Esri Research Cartographer
The default projection for all basemaps in ArcGIS Online is Web Mercator. In a previous blog entry, Bern Szukalski explained using custom basemaps as an alternative to Web Mercator and the ArcGIS Resource Center provides information on how to build online basemaps. Mercator projections are very useful for certain mapping tasks but not so good for others because they dramatically exaggerate northern and southern latitudes. This exaggeration deforms the shape and size of areas. Whilst Web Mercator has become a widely adopted web mapping standard (because of the tiling system being well suited to this spatial reference), there are situations where you may want a different map projection. In thematic mapping, for instance, you might want to preserve areas or shapes or even use a more pleasing view of the world than Web Mercator provides at small scales. This blog entry describes how you can switch out the default basemap for your own, simplified thematic basemap.
When you open the ArcGIS.com map viewer the default terrain with labels basemap appears in Web Mercator (figure 1).
Figure 1. Default terrain with labels basemap in ArcGIS.com map viewer
There are a couple of different ways to switch out the basemap. One option is to click Add and then select Add layer from Web. If you know the URL of the service that you want to use (perhaps you have hosted an alternative basemap on a Server) then enter it into the dialog box and check Use as Basemap (figure 2).
Figure 2. Add layer from Web
Alternatively, you can search for services in ArcGIS Online by selecting the Search for Layers option (figure 3) and then entering your search term. Once you have located the service you are interested in, click on its name and then select Use as Basemap (figure 4).
Figure 3. Search for Layers
Figure 4. Using an ArcGIS Online service as a basemap
The default Web Mercator basemap will be replaced by your new basemap. Once the new basemap is added to the map viewer, any new layers you add are re-projected on-the-fly to the new basemap.
For thematic mapping, the Esri Light Gray Canvas map provides a neutral background for operational overlay information. In this example, we built a generalized world map in a similar style in ArcGIS for Desktop but projected it using the Robinson projection. This compromise projection is neither equal area nor conformal but gives a good overall view of the whole globe. We then published a cached map service at a range of small scales suitable for global and continental scale web maps and used it to build a web map version of the famous English Coal Exports in 1894 map by Charles Joseph Minard (figure 5).
Figure 5. English Coal Exports in 1894 presented using a Robinson projection in ArcGIS.com map viewer