Position and work load in the field. Human measurements and estimations from GPS-data

Basic GPS-technique has been used for estimation of position, transportation speed and gradient. Energy metabolic rate has been calculated from the masses of the individual and of carried burden, speed, and terrain. The metabolic rate of one subject was also measured directly using a portable equipment. Comparison between measured and calculated metabolic rate was performed for various terrain; a path with moderate gradients, a flat running track, a hilly walk and motor road with a long slope. Generally the position error of the GPS was with its specification, i.e. ±15 meters. The terrain affects the accuracy of the GPS obtained position. The accuracy changes with time also. Yet, sufficient accuracy in position can be reached even in dense woods and close to rocks and even among high buildings. The lack of accuracy is more serious to the estimation of speed as a sudden error in position may give an impossible power demand. The gradient of the terrain was measured using the barometer of the GPS-unit. After averaging, its resolution, 1 meter, allowed rather accurate gradient estimations. Measured and calculated metabolic rates coincided fairly well. However, during shorter periods deviations occurred, with the magnitude depending on the dynamics of speed and gradient. Average metabolic rate, for each activity and terrain, differed not more than 3% between the techniques. The method of using a GPS-unit to estimate the physical load of an individual seems to be useful. Adding a heart rate and/or core temperature meter the physical load could probably be separated also giving information on the thermal load.