[Talk-ca] GPS inaccuracy

Richard Weait richard at weait.com
Mon Nov 19 16:35:18 GMT 2012


On Sun, Nov 18, 2012 at 10:43 AM, Tom Taylor <tom.taylor.stds at gmail.com> wrote:
> I had a frustrating experience last night, sorting out an area in my
> neighbourhood. I laid down a GPS track totalling some 4 km. The whole track
> was about 10 meters to the west of the Bing and Canvec data already in
> place. North-south accuracy varied, but wasn't anywhere near so bad.
>
> It doesn't seem worth uploading my track, though maybe I should investigate
> some more.

"Chin up, Buttercup!"  :-)

Don't be frustrated by that result.  EVERY source we use for mapping
will "lie" to us in one way or another.  Our GPSes will vary due to
atmospheric conditions, local interference or reflections, satellite
positions (yes, really), placement of the GPS device in your car or
backpack, etc.

Aerial imagery can be offset, badly referenced, out of date, distorted, etc.

External digital sources can be offset, badly referenced, out of date,
simplified counter to our requirements, just dead wrong, etc.

Our own survey observations can suffer from typo / spell-o errors,
poor handwriting (my fav. :- ) ), mapper distraction / confusion, etc.
 Our own survey photos can suffer from limited resolution,
under-/over-exposure, insufficient contrast, badly-timed truck
blocking the address, etc.

But.  Even with all of those errors we can consider all of our
available sources and improve the map.  Learning the limitations and
error modes of your GPS will only help your mapping over time.  Grab
another similar trace on another day and compare them.  It's fun.

Happy mapping,
Richard



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