Changing the field route

By sunrise I had numbered the sample bags to match the order of the soil pits and copied every grid reference into a waterproof notebook. Halfway across the moor, a flooded channel cut off the next site. I did not try to preserve the route merely because it was written down: I circled the accessible pits, reversed the sequence and recalculated how long the return walk would take. Preparation, I realised, was not loyalty to a plan; it was what made changing the plan possible.

From the passage, select one word or phrase that shows the writer's careful preparation and one word or phrase that shows the writer's reasoned response to the unexpected obstacle.