GPS receivers can give different distance results. Why?

A LESSON FROM TWO GPS RECEIVERS.

INTRODUCTION

During the Adelaide Canoe Club Coffin Bay Kayaking Expedition 13, 14, 15 and 16^th April 2008, discussion arose because the two GPS receivers, used to log daily progress, gave different distance results.  On one day one receiver showed a 26.8km paddle while the other receiver showed a 29.4km paddle.  It was discovered later that one instrument logged about 1km/hour while stationary, and that this instrument had been stationary and switched on for approximately 30 minutes during the day in question.  This would reduce the 29.4km to about 28.9km.

Comparing the GPS receivers directly

During another kayaking expedition 30 April- 1 May 2008 on the Coorong the same two GPS receivers were put together on the same kayak.  They both measured the same distance. 

 Conclusions

• The difference in the distance measured by the two GPS receivers is real.
• The difference was caused by one kayak traveling further than the other.
• There are two known reasons for the difference in distance traveled.
• The kayak traveling the least distance used a deck compass to adjust its heading during the passage.  It was therefore more sensitive to course alterations.
• The kayak traveling the longest distance was poorer at tracking and therefore wandered off course more often.

The general message is that the type of boat you paddle and attention to the detail of navigating can reduce the distance you paddle even when paddling within the same group.  An extra 2 km added onto a 27km paddle is worth avoiding!

Rob Tanner