What is the purpose of the "Overview Map" block?

The purpose is to show overview maps in the location of that block. See e.g. http://nightly.communitymapbuilder.org/mapbuilder/demo/mapViewer/index.html. In Drupal an overview map would fit nicely in a block which you put on a page.

Problem with the mapbuilder library is that you get javascript errors if an overview object in a config file cannot find the corresponding div element. Thus a way must be found to have an overview block that only shows when the config file specifies it, and vice versa.
At present, this flexibility is not yet implemented and generated configs have overview maps disabled because javascript errors would occur if people don't have the overview map enabled.

It is planned to have overview maps display only when it is enabled in a <map> tag, and when the user has access rights for it.

If the overview block is enabled, it will be shown for nodes that refer to it in the map tag, e.g.:

<map config="path/to/config" blocks="overview" />

Hi, openwereld, that's neat functionality:

I notice "blocks" is plural. Does that mean I can get the same map to appear in more than one block? (If I should check the docs, just say so ;-)

The plural 'blocks' is used because it can contain something like 'blocks="overview,legend,featurelist,scalebar"'. Blocks can be enabled for an overview map, a legend, a scalebar, or a featurelist. This functionality is not yet documented as far as I recall.

Interesting. As soon as I'm not drowning I'll look at it and let you know. Thanks.