How to measure the gradient/height differen of a long slope?

[Frimley111R]

We are putting in a long zip wire system for a customer and need to calculate the gradient of the slope. What I think we need is some type of GPS device that allows us to stand at to the top of the filed and the bottom of the field and show us the height difference, we can do the rest. So, where do I get one (will a smatphone do it?)