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POWhat does horizontalAccuracy exactly mean?
 

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  1. POWhat does horizontalAccuracy exactly mean?
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    <p>I am working on an iOS application using location services. Having a background in experimental physics, I am wondering what exactly <code>horizontalAccuracy</code> in a <code>location</code> found in <code>locationManager:didUpdateToLocation:fromLocation:</code> stands for. The documentation is a bit sparse...</p> <p>I assume that the accuracy gives a confidence interval based on a gaussian (or poisson?) distribution. Thus, with a certain probability, the actual position is within a circle with a radius of <code>horizontalAccuracy</code>, but could as well be outside that area. The question is then: how big is that probability? If <code>horizontalAccuracy</code> corresponds to 1&sigma;, I'd have a probability of 68% to be within that circle with <code>horizontalAccuracy</code>, but looking the other way around, in nearly one third of the cases, the actual position will be outside that area. Thus, in certain cases, I'd rather use 2&sigma; (<code>2*horizontalAccuracy</code>) or even 3&sigma; (<code>3*horizontalAccuracy</code>) to calculate with.</p> <p>To put it short: is there any indication somewhere, which confidence interval <code>horizontalAccuracy</code> has?</p> <p>Comment to all who respond "Apple says it is within": Well - the measurement can not be exact. It must have a certain level of uncertainty. If you repeat the measurement very often, you will get a distribution of results - probably a gaussian distribution. This gaussian has a certain width, which corresponds to the level of uncertainty of the measurements. Measuring the position more often will reduce the uncertainty and thus increase accuracy, but never will give you a distinct interval where the actual position is guaranteed to be in. You will only get a probability. But if the accuracy is 3sigma, we have 99,7% - which is close to certain. To put it short - I doubt the documentation from Apple.</p>
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      1. POWhat does horizontalAccuracy exactly mean?
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    1. COwoooo man we all are programmers here this stuff is above my head.I just know that it is the approximate radius that user can be within this circle. That's it.
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      1. POWhat does horizontalAccuracy exactly mean?
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    2. CORe your edit - are you talking about the accuracy of the location, or the accuracy of the accuracy? It's not clear what practical problem you are trying to sove here.
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    3. COWell - http://en.wikipedia.org/wiki/68-95-99.7_rule has a good explanation. If the accuracy has a confidence level of 3sigma, in my calculation, I can be pretty sure that the actual position is indeed in that circle. If it was only 1sigma, in 1 of 3 cases, the position is actual outside the circle. I then would have to calculate with 3* horizontalAccuracy (corresponding to 3sigma), which makes quite a difference in some cases.
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