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How does the GPS correlator work?

Last Updated: Jun 02, 2017 10:03AM CDT

The GPS tab in ActiLife 6.3.0 and later gives the user the option to correlate their epoch level actigraphy data with time-stamped GPS data.

Supported GPS devices and input formats include:

  • Garmin (*.tcx)
  • Qstarz (*.csv)
  • iTrail (*.csv)
ActiLife's correlator can generate the following output types:
  • A CSV file having actigraphy data correlated with GPS data (includes GPS latitude, longitude, elevation, speed, and distance if available).
  • An Activity intensity map (*.kmz file compatible with Google Earth and Google Maps)

In addition, the source GPS data will be saved in the actigraphy data file (*.agd).  This eliminates the need to re-import the GPS data for future re-calculations. The correlation process does not affect the actigraphy data in any way.

Using the GPS Correlator

To begin, click the "Select Dataset" button and select an *.agd file that contains epoch-level actigraphy data from an ActiGraph device that was worn simultaneously with a GPS logging device.  Any epoch level greater than 1 second is allowed.  If this file contains GPS correlated data, the GPS correlator tool will give the option to delete or select another GPS dataset to replace the existing GPS data.

In the GPS Data section, select a device type and browse to the GPS file that contains the time-stamped GPS data.  ActiLife will prompt with an alert if this data does not align with actigraphy data in the *.agd file selected above.

In the results section, select the desired output(s).  "Create a CSV correlation output" will create a CSV file that contains both the epoch level actigraphy data and GPS data in separate columns, correlated by timestamps.  The "Create an intensity map (KMZ) output" option will create a KMZ file which contains correlated GPS data points and cut point intensity levels (see image below).  Users can open the KMZ file in various programs including Google Maps.  Select the desired cut point set (defaults in ActiLife - read more here) or create a new custom set by clicking the "edit" link to the right of the cut point selection combo box.  Checking the box "Open with Google Earth" will automatically load the intensity map in Google Earth after the correlation is complete (Google Earth must be installed on the same computer as ActiLife).  

 

gps.jpg

Time Correlation

Often, the epoch size of a selected *.agd accelerometry file will not align with the timestamp from a GPS monitor.  To accommodate this anomaly, the GPS correlator will align the GPS points and *.agd epochs by aligning each epoch with the next closest GPS point where the correlated GPS point is before the next epoch.  The table below illustrates.

ActiGraph Epoch Timestamp Example GPS Timestamp
10:03:00 10:03:20
10:04:00 10:04:30
10:05:00 10:05:42
10:06:00 None (no 10:06:xx data)
10:07:00 10:07:49
10:08:00 10:08:01
10:09:00 10:09:07
10:10:00 10:10:37
10:11:00 10:11:40

 

A word about outputs

The GPS correlator produces potentially three (3) outputs:

  1. The dataset (*.agd file) will be modified to include ALL of the GPS data.  For example, if GPS data and actigraphy are at different resolutions (e.g., 10s epochs and 1s GPS data), the correlator tool will insert ALL of the GPS data into the *.agd file.  The original actigraphy data will not be affected.  ActiLife will properly handle the file regardless of the presence of the GPS data.  The GPS data can be deleted from the *.agd file at any time.
  2. If selected, a CSV output will be generated.  Epoch-level actigraphy data always takes priority during the correlation process.  If present, excess data from the higher resolution GPS file is discarded.  For example, a 10s epoch actigraphy file correlated with a 1s epoch GPS file will result in a 10s epoch CSV file.  The correlator tool will use the closest matching GPS timestamp to correlate with each 10s epoch actigraphy count.  In addtion, GPS data points that are not within the timespan of the first and last activity data point are not output.
  3. If selected, a KMZ output file will also be generated.  Similar to the generation of a CSV file, the KMZ output will be driven by the epoch-level actigraphy resolution.  A 10s epoch actigraphy file correlated with a 1s epoch GPS file will result in a 10s resolution KMZ file; excess GPS data will be discarded in this file.

If you need any further assistance, please visit our support portal at https://help.theactigraph.com/home. From there, you can view many helpful FAQ's and Troubleshooting Articles; log in to check any existing Support Requests; or create a new Support Ticket if you are unable to find a solution. You can also email us at support@theactigraph.com or give us a call at (877) 497-6996, Monday through Friday, 8 am to 5 pm CST.

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