Calculating an estimate of wind resistance experienced by a cyclist

1. A method comprising:

collecting a plurality of time-stamped measurements of position, speed, heading, and altitude of a wearer engaged in an activity;

splitting the plurality of time-stamped measurements of position, speed, and heading into a plurality of straight-line segments representing segments of travel of the wearer during which the wearer maintained a straight-line path of travel;

for each of the plurality of straight-line segments, determining a separate one of a plurality of time-stamped grade estimates based on one or more of the plurality of time-stamped position, speed, heading, and altitude measurements;

for each of the plurality of straight-line segments, determining a separate one of a plurality of time-stamped total power outputs of the wearer engaged in the activity;

determining a wind speed;

determining a wind direction; and

estimating one or more of power output due to wind resistance, relative air velocity, coefficient of rolling resistance, coefficient of drag, and exposed surface area based on at least one of the plurality of time-stamped total power outputs, the plurality of time-stamped measurements, the determined wind speed, and the determined wind direction.

2. The method of claim 1 , further comprising detecting, by a wearable device, that the activity the wearer is engaged in is a known activity.

3. The method of claim 1 , wherein the activity is riding a bicycle.

4. The method of claim 1 , wherein the time-stamped measurements are collected at irregular intervals.

5. The method of claim 1 , wherein the measurements collected at a particular one of the plurality of time-stamped measurements comprise a subset of the position, speed, heading, altitude, and total power output of the wearer engaged in the activity.

6. The method of claim 1 , wherein determining a plurality of time-stamped total power outputs comprises: measuring a plurality of time-stamped heart rates of a wearer of the device; and estimating a plurality of time-stamped total power output of the wearer of the wearable device based on the heart rate of the wearer.

7. The method of claim 6 , further comprising:

increasing an interval between measurements of time-stamped heart rates after estimating one or more of relative air velocity, coefficient of rolling resistance, coefficient of drag, and exposed surface area; and

estimating total power output based on the plurality of time-stamped measurements and the estimated one or more of relative air velocity, coefficient of rolling resistance, coefficient of drag, and exposed surface area.

8. The method of claim 1 , further comprising:

estimating a direction of gravity relative to a wearable device based on motion information from the wearable device;

estimating a hand position of the wearer of the wearable device based on the estimated direction of gravity; and

changing the coefficient of drag based on the estimated hand position of the wearer.

9. The method of claim 1 , wherein determining a wind direction comprises retrieving a wind direction from a remote server.

10. The method of claim 1 , wherein determining a wind direction comprises estimating a direction of the wind based on the plurality of time-stamped total power outputs and the plurality of time-stamped measurements of heading.

11. The method of claim 1 , wherein the estimated coefficient of rolling resistance is changed based on a determination of surface roughness made by analyzing a plurality of accelerometer measurements.

12. A wearable device comprising:

a positional sensor for obtaining position, speed, heading, and altitude information of a wearer of the wearable device;

a power output sensor for obtaining data correlated to the total power output of the wearer of the wearable device; and

a processor communicatively coupled to the positional sensor and the power sensor, wherein the processor is configured to:

receive a plurality of time-stamped measurements of position, speed, heading, and altitude information from the positional sensor;

receive power output data from the power output sensor;

split the position, speed, and heading information into a plurality of straight-line segments representing segments of travel of the wearer during which the wearer maintained a straight-line path of travel;

for each of the plurality of straight-line segments, determine a separate one of a plurality of time-stamped grade estimates based on one or more of the plurality of time-stamped measurements of position, speed, heading, and altitude;

for each of the plurality of straight-line segments, determine a separate one of a plurality of time-stamped power output measurements from the power output data;

determine a wind speed;

determine a wind direction; and

estimate one or more of power output due to wind resistance, relative air velocity, coefficient of rolling resistance, coefficient of drag, and exposed surface area based on at least one of the plurality of time-stamped total power outputs, the plurality of time-stamped measurements, the determined wind speed, and the determined wind direction.

13. The wearable device of claim 12 , wherein the processor is further configured to determine that the wearer is riding a bicycle.

14. The wearable device of claim 12 , wherein the time-stamped measurements are received at regular intervals.

15. The wearable device of claim 12 , wherein the power output sensor is a heart rate sensor.

16. The wearable device of claim 12 , wherein the processor is further configured to:

estimate a direction of gravity relative to the wearable device based on motion information from a motion sensor communicatively coupled to the processor;

estimate a hand position of the wearer of the wearable device based on the estimated direction of gravity; and

refine the estimate of the coefficient of drag based on the estimated hand position of the wearer.

17. The wearable device of claim 12 , further comprising a network interface communicatively coupled to the processor, wherein determining a wind speed and determining a wind direction comprise retrieving a wind direction and a wind speed from a remote server via the network interface.

18. The wearable device of claim 12 , wherein determining a wind direction comprises estimating a direction of the wind based on the plurality of time-stamped total power outputs and the plurality of time-stamped measurements of heading.

19. The wearable device of claim 18 , further comprising one or more accelerometers communicatively coupled to the processor, wherein the processor is further configured to:

receive a plurality of acceleration measurements from the one or more accelerometers;

derive a plurality of high-frequency acceleration measurements from the plurality of acceleration measurements; and

change the estimated coefficient of rolling resistance based on the derived plurality of high-frequency acceleration measurements.

20. The wearable device of claim 12 , wherein the positional sensor is a companion device with a GPS receiver.