Question

Answer

Web Mercator, also known as Spherical Mercator, is the coordinate system used by web mapping tools such as Google Earth or Virtual Earth.

The FME name for Web Mercator is SPHERICAL_MERCATOR, EPSG: 3857, EPSG: 3785 or EPSG: 900913 (Google spelled with numbers!).

When writing to KML, you should always use LL84, which is FME's name for LL-WGS84. Apparently Spherical or Web Mercator is only used internally within Google Earth, and also with other tiling systems such as Virtual Earth. However, all data output to KML, including raster tiles, should be reprojected to LL84. This will happen automatically unless the source coordinate system is unknown.

Note that EPSG 900913 is a projected system with ground units in meters, not degrees. So perhaps one use for this system would be if you had to do some intermediate processing that required a projection in ground units of meters for data that comes from several different projected sources. For example, you could reproject multiple UTMs to 900913, do whatever processing that was required in the projected system, such as clipping with other projected data or buffering in meters, then reproject to LL84 before writing to KML (e.g. UTM to 900913 to LL84.). This would eliminate any problems that you might otherwise experience on the UTM zone boundaries.

However, if your results are acceptable using LL84 alone, and all you are doing is writing to KML, you probably don't need to worry about 900913 / Web Mercator.