Dodgey distance sensors + Dodgey encoders = bad bad map

The distance sensors are even worse than we already discovered. I finally got the encoders to work correctly such that the robot sits almost in the middle of the square at every move, and have been frustrated trying to find out why its mapping still has occasional bugs. Then I discovered that the distance sensors aren't very reliable, infact, it is a coupling of both the wheel encoders and the distance sensors together that causes the problem. The robot doesn't stop close enough to the centre to 'always' get an accurate reading, and the distance sensors fluctuate too greatly. So i'm exploring slightly dif
Solutions:
One method one of the other contestants had been doing was to walk diagonally, which I had thought was rather funny looking until now, but finally understand why. They walk straight into the wall until it bumps so that it knows its there, and it guesses the structure of the path. This means that they know where every 2nd wall on each side is, but they don't know the gaps for sure.


So now i'm trying to get a vision based-wall sensor to work instead. I'm splitting the bottom half of the image into 4 sections and counting the number of dark pixels (non-grey, high black count). For example the following picture shows a wall 2 squares ahead and a wall 1 square ahead