An odometer is a mechanical device that records revolutions. An odometer wheel has an odometer and a drive train that records the distance traveled, Figure 12.1.

The common odometer wheel counts the revolutions of a wheel and shows the distance traveled on a multiple dial readout, Figure 12.2. The dial can be designed to produce readings in whole feet, decimal feet, feet and inches and meters. The principle of an odometer wheel is circumference of a wheel times the number of

FIGURE 12.2. Odometer wheel reading of 1,850 ft.

revolutions equals distance, therefore any wheel can be used as long as the diameter is known and the number of revolutions are counted.

When used correctly, a wheel is more accurate than pacing. An error of 1 ft per 100 ft can be expected. To be accurate, the wheel must roll along the ground without slipping. Experience has shown that the accuracy of odometer wheels is also influenced by the surface and the topography. An odometer wheel designed to be used on hard surfaces will not be as accurate on grass or in tall weeds. In these situations, the accuracy is improved if the wheel is recalibrated for the surface on which it will be used. See the section 12.4 & 12.5 for an example of the calibration procedure.

When an odometer is used, the readout should be reset to zero before starting; then the distance traveled can be read directly. If another type of wheel is being used, the distance traveled is equal to pi (n) times the diameter multiplied by the number of revolutions:

where D = Measured distance; n = 3.14; D = Wheel diameter (ft); N = Number of wheel revolutions.

Problem: A bicycle wheel completed 155.2 revolutions as it was rolled along the boundary of a field. The wheel diameter is 2.16 ft. What is the length of the field

Solution:

D(ft) = n x D x N = 3.14 x 2.16 — x 155.2 rev rev

One advantage of odometer wheels over pacing is the increased accuracy, but they still must be rolled along the surface in a straight line to measure the distance.

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