Calibrating a pressure sensor

1. A method comprising:

determining a first plurality of calibration values for a first plurality of visits to a first revisit zone to which a mobile device repeatedly returns;

determining a first relative calibration adjustment value based on the first plurality of calibration values;

determining an adjusted absolute calibration value based on i) an absolute calibration value used to calibrate pressure measurements made by a pressure sensor of the mobile device, and ii) the first relative calibration adjustment value; and

calibrating future pressure measurements made by the pressure sensor of the mobile device using the adjusted absolute calibration value.

2. The method of claim 1 , further comprising:

determining the first relative calibration adjustment value based on a difference between a first calibration value and a second calibration value of the first plurality of calibration values.

3. The method of claim 1 , further comprising:

determining the adjusted absolute calibration value based on a sum of the absolute calibration value and the first relative calibration adjustment value.

4. The method of claim 1 , wherein:

the first revisit zone is a 2D horizontal location.

5. The method of claim 1 , wherein:

the first revisit zone is determined based on a battery of the mobile device being charged.

6. The method of claim 1 , wherein:

the first revisit zone is determined by using a global navigation satellite system.

7. The method of claim 1 , wherein:

the first revisit zone is determined based on a connection with a WiFi access point.

8. The method of claim 1 , further comprising:

determining a second plurality of calibration values for a second plurality of visits to a second revisit zone to which the mobile device repeatedly returns;

determining a second relative calibration adjustment value based on the second plurality of calibration values;

determining an overall relative calibration adjustment value based on the first relative calibration adjustment value and the second relative calibration adjustment value; and

determining the adjusted absolute calibration value based on the absolute calibration value and the overall relative calibration adjustment value.

9. The method of claim 8 , further comprising:

determining the second relative calibration adjustment value based on a difference between a first calibration value and a second calibration value of the second plurality of calibration values.

10. The method of claim 8 , further comprising:

determining the adjusted absolute calibration value based on a sum of the absolute calibration value and the overall relative calibration adjustment value.

11. The method of claim 8 , further comprising:

determining that the second plurality of visits to the second revisit zone do not overlap in time with the first plurality of visits to the first revisit zone; and

determining the overall relative calibration adjustment value based on a sum of the first relative calibration adjustment value and the second relative calibration adjustment value.

12. The method of claim 8 , further comprising:

determining that the second plurality of visits to the second revisit zone overlap in time with the first plurality of visits to the first revisit zone; and

determining the overall relative calibration adjustment value based on the first relative calibration adjustment value adjusted by a first weight value and the second relative calibration adjustment value adjusted by a second weight value.

13. The method of claim 1 , further comprising:

determining an absolute calibration confidence for the absolute calibration value;

determining an aged absolute calibration confidence based on an age of the absolute calibration value;

determining a first plurality of calibration confidences for the first plurality of calibration values;

determining a relative calibration confidence based on the first plurality of calibration confidences; and

determining an adjusted absolute calibration confidence for the adjusted absolute calibration value based on the aged absolute calibration confidence and the relative calibration confidence.

14. The method of claim 13 , further comprising:

determining the relative calibration confidence based on the first plurality of calibration confidences adjusted by a plurality of weight terms.

15. The method of claim 13 , further comprising:

determining a relative calibration time period being between a first visit and a second visit of the first plurality of visits; and

determining the aged absolute calibration confidence by propagating the absolute calibration value during an unknown calibration time period being outside of the relative calibration time period.

16. The method of claim 1 , further comprising:

determining a first plurality of data fixes collected during a first visit of the first plurality of visits;

identifying first anomalous data fixes in the first plurality of data fixes;

forming a first subset of the first plurality of data fixes by deleting the first anomalous data fixes from the first plurality of data fixes; and

determining a first calibration value of the first plurality of calibration values for the first visit of the first plurality of visits based on the first subset of the first plurality of data fixes.

17. The method of claim 16 , further comprising:

determining a second plurality of data fixes collected during a second visit of the first plurality of visits;

identifying second anomalous data fixes in the second plurality of data fixes;

forming a second subset of the second plurality of data fixes by deleting the second anomalous data fixes from the second plurality of data fixes; and

determining a second calibration value of the first plurality of calibration values for the second visit of the first plurality of visits based on the second subset of the second plurality of data fixes.

18. The method of claim 16 , further comprising:

determining a number of clusters of the first plurality of data fixes; and

identifying the first anomalous data fixes for the clusters of the first plurality of data fixes.

19. The method of claim 18 , further comprising:

when a total number of the first anomalous data fixes is greater than a maximum threshold value, incrementing the number of clusters and repeating the identifying of the first anomalous data fixes.