[Loadstone] Question about Accuracy

Rob Melchers rob at loadstone-gps.com
Tue Feb 27 23:59:35 GMT 2007

Here's a table that can be used while reading key 2 - accuracy in Loadstone 
(from a car driver's point of view):

1: Ideal
This is the highest possible confidence level to be used for applications 
demanding the highest possible precision at all times.
2-3: Excellent
At this confidence level, positional measurements are considered accurate 
enough to meet all but the most sensitive applications.
4-6: Good
Represents a level that marks the minimum appropriate for making business 
decisions. Positional measurements could be used to make reliable in-route 
navigation suggestions to the user.
7-8: Moderate
Positional measurements could be used for calculations, but the fix quality 
could still be improved. A more open view of the sky is recommended.
9-20: Fair
Represents a low confidence level. Positional measurements should be 
discarded or used only to indicate a very rough estimate of the current 
21-50: Poor
At this level, measurements are inaccurate by as much as half a football 
field and should be discarded.

For pedestrian use 4 is as worse as you want it to get. More on similar 
issues, as well as this table, can be found at:

At 2/27/2007, you wrote:
>On Tue, Feb 27, 2007 at 01:32:34PM -0800, Tom Lange wrote:
> > Keypad 2 shows 2.6.
> > Often I've seen numbers  as low as 1.0, 1.1, 1.2, and so on. What do those
> > numbers represent?  Are these numbers on a scale of 1 to 10, for example,
> > where 10 represents the highest possible accuracy?
>What Loadstone calls accuracy is actually called HDOP or
>horizontal dilusion of precision.  Put simply it's an error
>factor which is based on the accuracy of the GPS device.
>In other words, the hdop can vary for the same accuracy
>given two gps devices with different unenhanced precisions.
>A rough idea of accuracy can be had by multiplying the hdop
>by the accuracy of the device.  For example, the sirf3
>units have a stated positional accuracy of 10 metres so an
>hdop of 2.6 means you should be within 26 metres of your
>target.  If you are getting waas corrections, the hdop can
>go below 1 to let's say 0.8 giving an accuracy of 8 metres
>on a 10 metre precision unit.  hdop can go as high as it
>likes but I wouldn't trust anything above 5 for pedestrian
>use on a Sirf3.
>Loadstone mailing list
>Loadstone at loadstone-gps.com
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