My favourite way of chanelling out an area above an already dug underground is as follows (for a square 5x5 area... modify as necessary for bigger and rectangular areas).
1 Bare 2 Channel 3 Now do 4 Continue 5 Voila!
Ground a cross remainder to 5x5
of 3x3 square
square
....... ....... ....... ....... .......
....... ._..._. . ... . . ___ . . .
....... .._._.. .. _ .. ._ _. . .
....... ..._... .._ _.. ._ _. . .
....... .._._.. .. _ .. ._ _. . .
....... ._..._. . ... . . ___ . . .
....... ....... ....... ....... .......
Though probably the easiest way is to just do 'stripes':
1 Bare 2 Channel 3 Now the 4 Then the 5 Result!
Ground the middle channels next set
either
side
....... ....... ....... ....... .......
....... ....... ....... ._____. . .
....... ....... ._____. . . . .
....... ._____. . . . . . .
....... ....... ._____. . . . .
....... ....... ....... ._____. . .
....... ....... ....... ....... .......
You can micromanage the process, or leave it ages between reviewing it. You can also short-cut the process by setting to channel any tile that is not the final one supporting any other so-far unchannelled tile. Which means that in the second example you could set a block of 2x5 channels (because the tiles-to-be-dug in the other row of 1x5 are supported by the other side's not-yet-channelled tiles, IYSWIM), and in the first example you'll probably find they end up (with you designating more to be done, as and when it can be) progressively removing the left-hand triangle quicker than the right-hand one, but it's probably easier to see how that happens than for me to explain the process.
Even without a room below the surface to cause complications, I sometimes find it's handier to work in this way with channels at the top. You can see where there are trees (which would cause problems if trying to "ramp" from below) and you can't even designate a channel on a tree tile (so are also proof from the basic mistake of trying to undermine one).
Plus, if I'm digging a long ditch at the surface (or anywhere else things grow), I'll tend to make a 1xN (or 2xN) designation of channels, then go over the exact same area once or twice more with other designations, once (certainly) to assign Tree-felling duties to any trees that happen to be there, the second perhaps to designate Plant-gathering for any harvestable natural plants. These override the channelling designation (or, in the case of trees, just apply where the channel does not even get considered...), then your woodcutters and herbalists zoom in there and later on you can re-do the same rectangle over the same area for channelling operations to get your miners to pick out all the tiles that are still unchannelled but at least now do not have any significant plant or tree association. Again, it's probably easier to see it happen than for me to explain properly.