Is it impossible?!
Online maps are visual, there is no getting round this basic premise. The use of a summary attribute for a map element in a page is the only thing I can think of that would be of any use. This works in the same was as an alt attribute for an image – screen readers will read this text out to users who have problems visual impairments.
As the majority of online mapping servers generate images as output there could probably be further development in customising the ALT attributes of these images to provide more detailed information of the current map state, so rather than alt=”A Map”, alt=”Map showing towns and major roads.” This alt text could be derived from visible layers, and the centre coordinates of the current map location.
I tried running one of my map URLs on the online validator at http://webxact.watchfire.com/ which tests “single pages of web content for quality, accessibility, and privacy issues.” It timed out…
http://wave.webaim.org/wave/Output.jsp performs similar checks..what can best be described as a mess was returned along with lots of “page not found errors.”
Fortunately clients do not seem to demand that accessibility standards are met for mapping pages, but with increasing legislation in many countries for websites to be usable by people with disabilites it is definitely an area that needs further work.