Course Measurement Methods
The Detweiller Park cross-country course was measured using a number of different methods. An explanation of each follows.
Steel Tape. The course is measured by pulling a steel measuring tape around the course. The tape is pulled tight over its entire length on straight sections of the course and laid on the ground along the appropriate radius around curves. A pin or flag is inserted into the ground at the end of each tape segment to mark the end of the tape and provide a starting point for the next segment. In the case of the Detweiller Park course measurement, a 50-meter tape was used, and the flags were numbered to allow the measurers to keep track of the number of tape pulls. Measurement using a steel tape is accurate but can be time consuming. One of the advantages of this method on an uneven surface like a cross-country course is that it tends to smooth out small surface bumps while accurately conforming to more significant terrain features that are of more consequence to the participants in the event. Surface conditions do not affect the accuracy of the measurement since the method does not depend on any kind of contact or friction with the ground.
Calibrated Bicycle. The front wheel of a bicycle is fitted with a counter that counts fractions of a revolution of the front wheel. The bicycle is first ridden at least four times over a straight course of known distance, called a calibration course, and the results of the calibration are used to calculate a constant, usually expressed in counts on the counter per unit of measure. The course is measured and the bicycle is calibrated again. The course distance is calculated based on the number of counts recorded on the course and the two sets of calibration rides.
The Detweiller Park course was measured using a bicycle calibrated over the first 300 meters of the race course as measured by the steel taping crew. The calibrated bicycle method is used to measure almost all road racing courses in the United States and has been adopted my many other countries around the world. It is not quite as accurate as measuring with a steel tape, but is generally faster and easier. The only two methods recognized by USATF for measuring a course with anything other than a single straight line segment are the calibrated bicycle and steel tape.
Measuring Wheel. A wheel that is usually mounted to a handle, and turns a counter that reads out in units of measure. The wheel is pulled or pushed around a course. Most high school cross-country courses are measured this way. While reasonably accurate on hard paved surfaces, a number of issues affect the accuracy of these devices on grass. They can be more accurate if calibrated, but most users simply accept the length shown on the counter.
GPS: The course was measured with a Garmin 305 GPS mounted to a bicycle. Although more accurate GPS units are available, consumer grade GPS is generally accurate within 5 to 10 meters. GPS units calculate distance by connecting readings taken at regular intervals around the course. USATF's experience with handheld GPS units is that they generally produce measurements between one and two percent long. This is most likely because GPS units connect readings taken along the route. Due to the limits of accuracy inherent to the device, some of these points may be on one side of the measured line, and some may be on the other.
The result is a zigzag line, which will naturally be longer than the straight line the device actually followed. Units mounted on a bicycle are generally more accurate than those carried by hand, possibly because they move faster, thus smoothing out the curves more gracefully. Bicycle mounted GPS units also may get better satellite reception because the satellite signals are not blocked by the runner carrying them. It should also be noted that consumer GPS models generally only read to the hundredth of a kilometer, or 10 meters. If greater accuracy is desired, they must be augmented with another measuring device capable of more accurate readings.
Comparing the methods. Of the methods used, there is little doubt that the steel tape is the most accurate method. The calibrated bicycle is accurate but there can be issues with a bicycle calibrated on a hard surface such as asphalt or concrete that is used to measure a course on a softer surface such as grass. The calibrated bicycle should be more accurate than a measuring wheel because it is recalibrated no less often than for each day's measurements and that calibration reflects the weight and riding style of the measurer. It will approach the accuracy of a steel tape if the calibration and measurement are both performed over a smooth, hard consistent surface. Measuring wheels also appear to lose accuracy when used over grass courses, especially if the surface is wet or uneven. The accuracy of GPS is unknown; test results have varied. A GPS is probably accurate enough to measure training runs, but not accurate enough to measure race courses. If a coach or race director is interested in an accurate measurement of his or her cross-country course, the safest way would be to measure it with a steel measuring tape.