[time-nuts] DGPS at home

Chris Albertson albertson.chris at gmail.com
Sat Nov 26 22:32:05 EST 2011

On Sat, Nov 26, 2011 at 2:35 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> Pure brute force would compute the center of mass and then scan all the data
> points computing the distance...  That's an N-squared process which might
> take too long with a large clump of data.  For offline research like this, it
> might be OK.

For off line work your "shrink the bounding box (or bounding ellipse)"
method would work fine.   I think even with a few million points it is
nearly trivial on a modern PC.

But you can do something like this continuously in real time and keep
the processing time constant.
Keep a running mean of each point (actually two means as you need one
for Y and one for X direction.  three for Z if you include altitude.
Along with the running means keep a running "sigma" (To do this you
need keep the count "N", and the sum and the sum of the squares.)  Now
as you get each point test if it falls outside a three sigma limit.
Discard it if it does.  You can play with the size and shape of the
bounding box.   If the robot moves then it can update the running
means by dead reckoning and errors in the dead reckoned estimate will
get corrected eventually by GPS

GPS is never going to be exact.  Or I should say you don't know the
exact lat. long. for every place you want to go.  So to find something
like a bear bottle in your refrigerator you need vision

Chris Albertson
Redondo Beach, California

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