F9 Minimap Legend?

[tlhIngan]

What's the F9 minimap showing me while underground?
What do all the colours mean? There's a big area of orange right next to my shaft, I can't seem to reach it. There's also a couple of large blue areas, and "penciled in" grey ones. This occurs about 100 nodes under sea level.

[firefox]

the normal mode mini-map shows your surroundings from a top-down view.
because the "camera" is far above your actual height, it does not work underground :P
what you see is located far above you.

by pressing F9 again, you can cycle through the modes and scales.
when you go underground, it is recommended to use the radar mode.
it shows all nodes above and below you: solid is void (black), air is transparent green.
this let's you see the caves around you.
obviously it shows nothing if you are on the surface ;)

[tlhIngan]

the normal mode mini-map shows your surroundings from a top-down view.
because the "camera" is far above your actual height, it does not work underground :P
what you see is located far above you.

I did some flying around, and I am recognising some of the features the minimap is showing me. The camera is about 120 nodes above my present location, and it is showing me stuff that is in the walls. So far I've confirmed black is obsidian bright orange is probably lava and pale orange seems to be torches. I assume the other blips are minerals. Being 120ish nodes above my position, it's not terribly useful, but I'll go scavenging for a few of them minerals it's showing, to confirm.

[sorcerykid]

The top-down view is only approximately 120m overhead. I suspect that it resides along mapblock boundaries, so can only shift in 16m intervals along the y-axis.

[orwell]

Right, as you say, the map is generated by determining the topmost node in every x,z column that is known to the client. While being above ground, this determines the correct surface node, however, when being underground, the topmost known mapblock is somewhere below the surface, and thus the surface of this underground mapblock is shown. (showing such things as lava)