A Crossrail environmental specialist will organise a detailed survey to look at the trees along the new route. They will find out how many trees there are, what sort they are, their size and their age. They are then plotted on a map of the area.

The trees are then categorised into four groups:

• high (those trees most desired to be kept)

• moderate (trees desired to be kept)

• low (trees which could be kept)

• fell (trees for removal)

This information enables Crossrail to make important decisions about which trees must be most protected and those that can be felled (cut down) if necessary during the railway construction.