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Position accuracy
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yeezhou


Newbie

Join Date: 2016-04-22
Posts: 5
Posted: 2016-12-04 02:32:25 AM

I have a questions about the position accuracy from a recent experience. After reading a few related posts, I have some general understanding about the accuracy but still would appreciate some further clarification.


Recently while attending a trade show in Orlando in a large indoor exhibition hall (Orange County Convention Center, the new/east building) I noticed that while I believe I was still on the exhibition floor, for a few times, my positions traveled to some outside locations about 1.6 km east to the show floor, at an address of Vista Cay Condo Rental. The concerned data points showed an accuracy of 165 meters, while most of the time, I realized the location accuracy were about 65 meters or less which showed I was still on the show floor. At that time, I was not using any wifi but my cell data was on. This happened on November 15th - 17th.


I read the Followmee forum and understand and one of the discussion is about the 68% confidence, as following "We define accuracy as the radius of 68% confidence. In other words, if you draw a circle centered at this location''''s latitude and longitude, and with a radius equal to the accuracy, then there is a 68% probability that the true location is inside the circle. In statistical terms, it is assumed that location errors are random with a normal distribution, so the 68% confidence circle represents one standard deviation. Note that in practice, location errors do not always follow such a simple distribution."


My question is, is such location error (physically 1.6 km away but showing accuracy of 165 meters) expected? If it is in the 32% probability and out of the 165 radius, would it be possible to be about 1.6 km away? Does this relate to being indoor? The roof? The cell towers? Or the wifi? Or 165 meters is simply indicates something very inaccurate? And my observation of my location back in my office recently, the 165 meters accuracy shows up and proved that the 68% chance within circle of radius 165 meters is mostly correct. However, what about in Orlando? If indoor, I assume GPS signal was not available so the positioning must be calculated by cell towers; is my understanding correct?


I’m using an iPhone 5s (iOS 10.1.1)

Many thanks.
Administrator


Best of the Best

Join Date: 2010-01-27
Posts: 1429
Posted: 2016-12-04 09:09:59 PM

Thank you for the detailed explanation. GPS does not work well in-door. Not enough satellites are visible to the GPS receiver and there are more interference inside a building. When in-door, GPS receiver tends to receive false signal and calculate a false coordinates. Because of this, the accuracy number could be a false number and not reliable.

yeezhou


Newbie

Join Date: 2016-04-22
Posts: 5
Posted: 2016-12-08 04:06:16 PM

Thanks for your explanation.

On the other hand, I also subscribed for 1 year history(assuming followMee can get me history going back one year from the date I subscribed, i.e. from Nov. 25, 2015-Nov. 25, 2016). It has been more than two weeks but I can only see history back to about 2.5 weeks ago. Is my understanding of the "want longer history" correct? If so, how long does it take to have the past year history shown on the Map along with the associated logs?

Thanks.
mteicher


Forum Superstar

Join Date: 2014-04-09
Posts: 506
Posted: 2016-12-08 07:26:55 PM

I believe history is going forward from when you subscribed to the added history retention and not in arrears.
Administrator


Best of the Best

Join Date: 2010-01-27
Posts: 1429
Posted: 2016-12-08 09:45:33 PM

mteicher is correct. The extended history starts from the date of your subscription and going forward.

mattd08060


Newbie

Join Date: 2017-03-26
Posts: 3
Posted: 2017-03-28 09:10:21 AM

On a related issue... I think?
Is it safe to assume that when cell phone is indoors and stationary, that the location might change slightly if signal switches between GPS and WIFI? And also, does this reflect in the accuracy?

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