We sometimes get inquiries about determining the actual angle of inclination that our devices measure. Some researchers may need more information than just the category of inclination (i.e. sitting, standing, lying, or off). While this information is not available in ActiLife itself, you can determine this from the raw data by using some trigonometric calculations as explained below. ActiLife does not provide this information as ActiLife currently only analyzes Epoch Data.
The inclination angle(s) can be determined from the raw data. Both pitch and roll may be calculated but at this time, there is not enough data to determine yaw/heading.
Please refer to the following paper from Analog Devices that explains inclination calculations using 1-, 2- or 3-axis accelerometer data:
Note that the Analog Devices paper assumes that measured acceleration is only due to gravity. So, any accelerations due to the subject must be filtered out. This can be done using a digital low-pass filter since gravity remains constant while subject accelerations change rapidly.
Tuning of a low-pass filter for gravity is dependent upon your desired results. A high-order filter with a low cutoff frequency will yield very smooth results, but will respond slowly to changes in inclination. The opposite is true as the frequency is increased. So, you have to weigh reducing noise against responsiveness. You will need to construct your own low-pass filter.
You can also refer to the Inclination whitepaper we've posted here.
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 firstname.lastname@example.org or give us a call at (877) 497-6996, Monday through Friday, 8 am to 5 pm CST.