[Tilecache] tiles for no-data areas

[Richard Greenwood]

I am not sure of the correct term for my question (which may be why my
searches haven't been successful) so my apologies if this has been
discussed previously.

I have 8 layers of historic aerial photography from different years
for a region. Unfortunately not all of the years cover all of the
region. So a user might zoom into their area of interest viewing the
1945 imagery, then select the 1967 imagery which does not cover the
area they are looking at, and see nothing but white. I'd like to
notify the user that 1967 imagery is not available for the area that
they are zoomed into. Something like Google Maps' notice that imagery
is not available at the scale you are viewing.

One idea I have is to create a vector layer for the no-data areas of
each layer/year and label it appropriately. If I tilecache the layer I
would eventually end up with many tiles that said the same thing, e.g.
"1967 imagery is not available in this area". Which seems rather a
waste. I could not run the vector layer thru tilecache and avoid that
problem.

I wondering if there is a more elegant solution to my problem.

Thanks for reading.

Rich

-- 
Richard Greenwood
richard.greenwood at gmail.com
www.greenwoodmap.com