GGIR (version 2.3-0)

g.calibrate: function to estimate calibration error and make recommendation for addressing it

Description

Function starts by identifying ten second windows of non-movement. Next, the average acceleration per axis per window is used to estimate calibration error (offset and scaling) per axis. The function provides recommended correction factors to address the calibration error and a summary of the callibration procedure.

Usage

g.calibrate(datafile, spherecrit = 0.3,
minloadcrit = 72, 
printsummary = TRUE,chunksize=c(),windowsizes=c(5,900,3600),
selectdaysfile=c(),
dayborder=0, desiredtz = "", ...)

Arguments

datafile

name of accelerometer file

spherecrit

the minimum required acceleration value (in g) on both sides of 0 g for each axis. Used to judge whether the sphere is sufficiently populated

minloadcrit

the minimum number of hours the code needs to read for the autocalibration procedure to be effective (only sensitive to multitudes of 12 hrs, other values will be ceiled). After loading these hours only extra data is loaded if calibration error has not been reduced to under 0.01 g.

printsummary

if TRUE will print a summary when done

chunksize

number between 0.2 and 1 to specificy the size of chunks to be loaded as a fraction of a 12 hour period, e.g. 0.5 equals 6 hour chunks. The default is 1 (12 hrs). For machines with less than 4Gb of RAM memory a value below 1 is recommended.

windowsizes

see g.getmeta

selectdaysfile

see g.part1

dayborder

see g.part1

desiredtz

see g.getmeta

...

Please ignore. Only used by the code internally when called from within g.part1 with selectdaysfile specific.

Value

scale

scaling correction values, e.g. c(1,1,1)

offset

offset correction values, e.g. c(0,0,0)

tempoffset

correction values related to temperature, e.g. c(0,0,0)

cal.error.start

absolute difference between Euclidean norm during all non-movement windows and 1 g before autocalibration

cal.error.end

absolute difference between Euclidean norm during all non-movement windows and 1 g after autocalibration

spheredata

average, standard deviation, Euclidean norm and temperature (if available) for all ten second non-movement windows as used for the autocalibration procedure

npoints

number of 10 second no-movement windows used to populate the sphere

nhoursused

number of hours of measurement data scanned to find the ten second time windows with no movement

meantempcal

mean temperature corresponding to the data as used for autocalibration. Only applies to data collected with GENEActiv monitor.

References

• van Hees VT, Fang Z, Langford J, Assah F, Mohammad A, da Silva IC, Trenell MI, White T, Wareham NJ, Brage S. Auto-calibration of accelerometer data for free-living physical activity assessment using local gravity and temperature: an evaluation on four continents. J Appl Physiol (1985). 2014 Aug 7

Examples

# NOT RUN {
datafile = "C:/myfolder/testfile.bin"

#Apply autocalibration:
C = g.calibrate(datafile)
print(C$scale)
print(C$offset)
# }