NOTE: we do not encourage measuring pixels off our Web Mercator imagery. Instead, we recommend you try the Great Circle distance or the Haversine formula for a ready-to-use formula that you can simply plug into your app to calculate the desired distance.
PhotoMaps™ in the MapBrowser are shown using the same spherical Pseudo-Mercator projection as in Google Maps, Bing Maps, OpenStreetMap, MapQuest, and Yahoo Maps.
The codes EPSG:3857, EPSG:3785, and 'EPSG':900913 all represent this projection. The projection assumes that the Earth can be represented as a rectangle and that the length of the equator is the same as the length of, say, the Arctic Circle. This is the reason Greenland looks as large as Africa in these online maps.
There are many benefits in using this projection for online maps. However, because a rectangle does not represent the Earth accurately, one must not take measurements directly off this 3857 projection (don't worry, measurements taken using the MapBrowser measurement tool are accurate).
Our georeferenced Save Image function in Classic MapBrowser saves images using this 3857 projection so that what you save appears exactly as it did in Classic MapBrowser. If you import your image into a GIS program to take measurements, you need to ensure the software imports the image as 3857, but then reprojects the image into a more appropriate projection for the specific region you are working with. GDA94 projections are popular for Australia.
Note that you select your projection when exporting georeferenced content from New MapBrowser.
Appropriate instructions for importing images into compatible software packages can be found elsewhere on this site or in the packages' own documentation.
For a projection comparison, differences can be seen in the following images:
Australia in EPSG:3857
Australia in EPSG:4283 (a GDA94 Projection)