Hi, all.

I need to run an overflow pipe from an airing cupboard in the centre of a
house to outside.

I've got a hatch in the airing cupboard floor, and if I stick my head into
the hatch, I can see the external wall 12 feet away along the void space.
( parallel with the rafters, fortunately. ) I don't have a 12 foot long
drill bit, so...

I'd like to drill into this space from the outside, and shove the pipe from
the outside all the way in to my hatch.

The trick is knowing the right spot to drill, from the outside!

What I'd really like is a 'super laser' I could shine from the hatch all the
way through external wall, which would shine through to the outside, showing
me where to drill!

The only way I can think to do it is a series of intricate X-Y measurements,
from the hatch all the way to a window frame as a reference between inside
and out, and then measure X-Y from the window frame on the outside.
However, I'm not confident that the cumulative error would be good enough,
and that I would not end up drilling into another room! The X error is not
too important, but I only have a few inches of tollerance in the Y axis
before I end up above the floor or below the ceiling!

I could of course lift the carpet in the room with the outside wall, and
make an other hatch next to the wall and drill from the inside out, but I
was hoping for a clever way to avoid that...

--
Ron