Location, Part 2: What’s Accuracy?
Posted by Jeremy Bante
This is Part 2 in a multi-part series about using the location functions in FileMaker Go.
Where Am I?
Where’s My Stuff?
In my last post, I described the basics of using the Location and LocationValues functions in FileMaker Go. Now I want to focus on something that’s been bugging me for a while about these functions: accuracy.
GPS is pretty good at pinpointing your location, but there are several likely sources of error to account for, so we can’t realistically expect it to be exactly right — thus the accuracy component of a location. Accuracy is accepted by both the Location and the LocationValues functions as a parameter, and accuracy is returned by both functions as a result. But what exactly does FileMaker mean by “accuracy”?
There are a few different measures of GPS accuracy in common use, including CEP, R95, and 2DRMS. CEP and R95 accuracies specify that your true position is within the given radius of the returned coordinates a certain percentage of the time, 50% and 95%, respectively. 2DRMS is twice the expected root mean squared error of the returned coordinates from your true position. None of FileMaker’s documentation describes what measure of accuracy the location functions are using. Neither does Apple’s documentation of the CLLocation class.
In the absence of documentation, I set out to figure it out for myself. I built an accuracy testing file to repeatedly get location samples from the same spot to see what kind of variability we get for different accuracy values. I asked other folks to help gather more data in more locations.
I don’t have a whole lot to go on yet, but here’s what I’ve figured out so far:
FileMaker is not reporting CEP, R95, 2DRMS, or anything else consistent. For some reported accuracies, FileMaker appears to be reporting the 90% confidence radius (true position is within that radius 90% of the time); for others, it looks like the 75% confidence radius. For some positions, the reported accuracy is orders of magnitude bigger than the actual variation in the coordinate positions.