[Loadstone] Loadstone seizing up

monty at loadstone-gps.com monty at loadstone-gps.com
Mon Mar 19 10:33:53 GMT 2007


Hello Cearbhall,,

See below...

On Sat, 17 Mar 2007, Cearbhall O Meadhra wrote:

> Firstly, I think we can discard any thoughts of inaccurate data as we
> checked the co-ordinates in the points file with the co-ordinates displayed
> under an aerial picture of the location. They matched perfectly.

In this case accuracy directly relates to the scale of the aerial image. 
I'm not disputing what you say as you would know the source better than I, 
but it would be a factor!

> The co-ordinates are available to us in ten decimal places but we are 
> only taking seven because that seems to be the max. that Loadstone will 
> take.

Loadstone works with coordinates with up to 7 decimal places.  The  7th 
decimal place would yield a linear accuracy of less than 9 cm.  Since most 
consumer grade GPS receivers have accuracies of between 3M and 15M that 
means that anything after the 5th decimal point is normally of little use 
in terms of achieving better accuracy.

Since you say your coordinates are available to you with 10 decimal places 
I would have to assume that some significant rounding has happened 
somewhere since that would yield an accuracy of less than 1 millimetre.  I 
bet the millitary would like to get their hands on whatever receiver your 
map surveyor is using! *smile*

> I am interested in the drift idea. I have been complaining to Aedan and
> Ronan since we began this work that my front door swings up to 30m away and
> comes back again whilst I stand still. This is, I understand, more likely to
> happen when standing near a vertical wall such as the front of a house.
> However, it also happens when I stand on the Bray seafront where there are
> no buildings for 100m around!

You are experiencing a well-known effect of the SiRF III GPS chip set 
commonly known as drift!  Whether you're standing in the middle of a 
farmer's field or standing in a phonebooth  you will still get drift.

Remember, Loadstone is simply interpretting data from your GPS receiver, 
performing a generally accepted distance calculation (or several) with 
coordinates in a database  and sending that result to your phone's screen.

The output will only ever be as good as the information provided by the 
GPS receiver as input and the database it is using to compare to.

The accuracy value contained in the Loadstone database will only make a 
difference  when Loadstone is calculating for an approach alarm on a 
checkpoint, not when using the navigation keys Etc.

HTH

Best regards,
   Monty


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