Coordinates of tiles

Jun 26, 2012 at 4:46 AM

I've gathered some tiles from Bing aerial in the southern hemisphere and I'm trying to understand the coordinate system in the tiles. Is this documented somewhere?
An example UTM (zone 18 south) coordinate pair is 572263E and 8442924N however when I look at the content of a JGW file for a tile (zoom 17) it reads as follows

This translates into a tile located at, as you might expect -8289595.7E by -1568487.8N which is meaningless.

When I tried again in unprojected WGS84 decimal degrees the problem persists.
However I moved the ArcBruTile directory onto a different machine it seems to work so it makes me think there's a coordinate system I'm not aware of that is guiding the placement of the tiles.  I see this Microsoft Bing Maps Tile System page but that doesn't seem to explain what I'm seeing.