Shaded relief missing at larger scales in USGS TNM Topo maps outside US

[emxsys]

The USGS TNM Topo maps have shade relief coverage for the world, but when zooming in, the shaded relief disappears from maps outside of the US and its properties. This is not a bug in World Wind, but rather an issue with the USGS Topo Base Map service endpoint from The National Map. The National Map should be made aware of this issue.

Compare the screen shots below made at slightly different zoom levels to see the issue: