[sac-forum] Re: GPS Reading
- From: stanlep@xxxxxxxxxxxxx
- To: sac-forum@xxxxxxxxxxxxx
- Date: Wed, 27 Oct 2010 21:59:43 -0700 (MST)
Rick,
I used 9 meter as an example, although that is about the best I get in the
geographic area I am in. I have an older ETREC which cost me about $125 at
the time. I do not remember when I bought it but it must be at least 7 or
more years ago. I bought it at the Popular Surplus that was between Union
Hills and Bell Road on 27th ave (whatever the street is that turns north
by Hooters on Bell). They are no longer in business and if you are
familiar with that location you might remember when it was closed.
So, my ETREC is an old unit and maybe that is why I am not getting much
better accuracy. However, that is not much of a problem if one can
increase the sample size and then take an average. However, for this to
work, the data (readings) have to be unbiased (or very-very low bias). It
would serve no purpose to average 30 readings if each one is systemically
off from the true position by 20 feet (for example) in the same direction.
From your response I could infer that each GPS reading is unbiased, but
since you did not specifically say this I am not sure.
Stan
> Stan,
>
> GPS derives position by timing signals from the satellites. Each satellite
> essentially draw a sphere based on the distance calculated. With three
> satellites, there will be on point of common intersection, add a fourth
> and
> a 3d Solution (Altitude) can be derived.
>
> You say you're getting 9 meter accuracy? Most consumer grade GPS units
> will
> easily give an EPE of less than 10 feet. My Etrex Vista routinely gives 8
> feet, My old Magellan unit also give at the worst 15 feet. Essentially
> this
> means the position returned is within an 8 foot radius of where you are
> standing. Unless you can get your hands on a survey grade GPS unit you're
> not gonna get much better than that.
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