[ros-users] SLAM_Gmapping Occupancy cell

Hello all,

I am working on slam_gmapping tutorial "How to build a Map using logged
data".
I have a case like,
A Robot while building its own map initially; if it comes across the
location where there are downstairs and so it will not find any boundary
line to draw. In such case what would be the cell occupancy?
Will it be treated as "-1 completely unknown"?
If so then, according to 'LRF readings method' it should be completely
'undecided area' and has to be treated always as an obstacle in future map
updates.
Also, while initial mapping, do we require encoder reading (==distance
travelled) to match with the laser input?

Looking forward for your help.

-Prasad Dixit