6. The London Marathon Course this year had course changes from the East Parapet of Tower Bridge, through the Isle of Dogs and back to pick up the old course at the Traffic Light in Upper Thames Street, after Southwark bridge. Here is the Course Map which was prepared by Hugh Jones:

I rode the changed section in six rides. Some rides were in the direction of running, and some for traffic reasons where measured in reverse. At two points there were pedestrian railings which would be removed on race day, where I had to stop my measurement and resume on the other side. When this occurred I finished one ride with my front wheel touching the railings, and started the next ride with my back wheel touching the other side of the railings. I measured the length of my bike plus the thickness of the railings and found it to be 1.80m. Each the ride was less than 5 miles. My rides in the order I made them (not race order) gave the following counts at the exact reference points given:

95422 : Marsh Wall, Lamp Post M0142 outside Britannia Hotel
63770 : Front wheel touching peed. railing at the Highway

64668 : Back wheel touching ped. railing at the Highway
89664 : East parapet Tower Bridge

90300 : East Smithfield, west side of railing to crossing at Tower Bridge
06183 : Traffic Light in Upper Thames Street, after Southwark Bridge

22574 : East Smithfield, west side of railing to crossing at Tower Bridge
47831 : Direction sign S end Butcher Row

69196 : Marsh Wall, Lamp Post M0142 outside Britannia Hotel
89263 : Ride interrupted at white line to negotiate removal temporary road works obstruction ahead
90097 : Ride resumed at white line after removal of obstruction ahead
10596 : Front wheel touching ped. railings at West India Dock Road

11272 : Back wheel touching ped. railings at West India Dock Road
27754 : Direction sign S end Butcher Row

a) Using my calibration data given in Question 1.1 above, calculate the length of the new course of the London Marathon from the east parapet of Tower bridge to the traffic light in Upper Thames Street after Southwark Bridge.

b) Two IAAF measurers, and one US measurer measured with me. Our results were all within a span of 4.6m.
Calculate what this span is as a percentage of the distance we rode.