Hi Peter,
Drop the Cities layer into the Imagery Basemap layer again to display those warning icons. Then right-click on the Imagery Basemap layer in the Table of Contents and choose Analyze Basemap Layer from the context menu. A table opens at the bottom of ArcMap to show errrors, warnings, and messages (as in attached graphic).
The specific meaning of the warning is that the Cities layer is in a different coordinate system than the basemap. Coordinate system mismatches are an issue that surfaces several times in the book and isn't tackled head-on until Exercise 3b.
If you double-click directly on the code number for this error (10001) in the table, the ArcGIS help system will open a topic explaining the warning. The risk is that the basemap layer will take longer to draw because it is having to project the Cities layer on the fly.
This seems like an odd "error" to draw attention to (we project layers on the fly all the time!) until you know that the purpose of analyzing basemap layers is to publish map services with ArcGIS Server. In other words, this error isn't relevant to our local use of the map document. It's important only in the context of publishing a map service.
All this seemed like so much to explain at such an early stage of the book that we took the easy way out and just said "ignore the warning" in a caption. But it never pays to be sneaky, does it? 😛 It might have been better to skip the layer sandwich thing entirely, except that it's a fun and useful thing to do.
Does that clarify it? Let me know if you have follow-up questions.
Thanks,
Tim
