Varied interval between measurements

Issue Name: NOAA-CrowdMag_12_2014 - Varied time interval between measurements
Device Make: iPhone
Device Model: 6
Device Operating System Version: 8.1
Browser: ? not applicable
Carrier (or WiFi): Verizon/wifi
Severity: hi
Description:
When I review my exported data, the intervals between measurements vary. How often are measurements collected, and is there a user-based trigger for data collection (such as movement of the phone)? See sample data below:
12/23/2014 17:39
12/23/2014 17:49
12/23/2014 17:59
12/23/2014 18:13
12/23/2014 18:23
12/23/2014 18:32
12/23/2014 18:53
12/23/2014 19:13
12/23/2014 19:21
12/23/2014 19:31
12/23/2014 19:39
12/23/2014 19:51
12/23/2014 20:02
12/23/2014 20:10
12/23/2014 20:21
12/23/2014 20:32
12/23/2014 20:56
12/23/2014 21:07
12/23/2014 21:17
12/23/2014 21:26
12/23/2014 21:37
12/23/2014 21:49
12/23/2014 21:59
12/23/2014 22:10
12/23/2014 22:21
12/23/2014 22:31
12/23/2014 22:39