3/8. Harry Beck 's London Underground Map. The tube guide revolutionised the face of underground mapping and travel for ever. Having realised two years before that circuit diagrams were a perfect model for a new map of the network, electrical engineer Harry Beck created the blueprint that's wholly relevant today. Why did he exclude everything at street level? What dictated his choice of colour for each line? Minding the gap, Nicholas Crane checks out the facts. Director/Series producer Michael Waterhouse (AD)