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I use HTML5's geolocation API and the position object has a "accuracy" property which is a number that may vary depending on the accuracy of the positioning.
So far so good. But the value is an unspecified unit.. sometimes it's 60, or 1250 or even 18 000.
I've read the W3C Geolocation API Specification[1], they talk about this property but never mention how it should be interpreted.
The only place I've found a possible answer is in the Department of Defence World Geodic System[2] technical report.. They mention accuracy in cm units with a 1σ (standard deviation).
But that doesn't tell me what's the accuracy unit that the HTML5 geolocation API returns me.. I can only suppose it's in cm..
[1] http://dev.w3.org/geo/api/spec-source.html
[2] http://earth-info.nga.mil/GandG/publications/tr8350.2/wgs84fin.pdf
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Depending on the availability of GPS on the device and the quality of the mobile/WIFI signals, HTML5 geolocation can be very accurate i.e. to a street level. Hence it can be used to pin point a device's location making it a very useful technology for websites or apps that require the exact user's location to work.
Geolocation sources Mobile devices tend to use triangulation techniques such as GPS (accurate to 10m and only works outside), WiFi and GSM / CDMA cell IDs (accurate to 1000m).
The HTML Geolocation API is used to get the geographical position of a user. Since this can compromise privacy, the position is not available unless the user approves it. Note: Geolocation is most accurate for devices with GPS, like smartphones.
IP-based geolocation services provide 55 percent to 80 percent accuracy for a user's region or state. And they provide 50 percent to 75 percent accuracy for a user's city. In practice, the actual accuracy may vary from provider to provider and depending on the location of the device.
From the documentation:
The accuracy attribute denotes the accuracy level of the latitude and longitude coordinates. It is specified in meters and must be supported by all implementations. The value of the accuracy attribute must be a non-negative real number.
So its measurement is meters.
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