[Talk-ca] Canadian data - GeoGratis (and an accuracy rant)

[Michel Gilbert]

2008/12/10 Matt Wilkie <matt.wilkie at gov.yk.ca>

> Hi Brent, thanks for bringing up the oft-overlooked and even more often
> misunderstood confusion of accuracy over precision.
>
>  > Most (some? all?) of the CanVec data originally came from the
>  > 1:50,000 NTS topographic maps .
>

Precision and accuracy is a subject that comes back all the time in
Geomatics. Thanks to
Wikipedia<http://en.wikipedia.org/wiki/Accuracy_and_precision>to help
me explain why the CanVec, Geobase, NTDB documents are rigth when
they describe the metadata. Precision as defined by Brent is realted to the
computer world not "scientific" world. Precision is about repetition
measures not the number of decimals.

Michel


>
> For clarity: which had their coordinates rounded off to the nearest
> metre (in UTM coordinate space), as per the NTDB specification.
>
>
> matt wilkie
> --------------------------------------------
> Geographic Information,
> Information Management and Technology,
> Yukon Department of Environment
> 10 Burns Road * Whitehorse, Yukon * Y1A 4Y9
> 867-667-8133 Tel * 867-393-7003 Fax
> http://environmentyukon.gov.yk.ca/geomatics/
> --------------------------------------------
>
>
> Brent Fraser wrote:
> > I'm sure they mean accuracy instead of precision.  Precision is just
> > the number of digits stored/displayed, whereas accuracy is how well
> > the data reflects reality.  Just because you chose to display
> > coordinates to the nanometer doesn't mean they are that accurate.
> > Not that I want to confuse the issue, but it can be important.
> >
> > Most (some? all?) of the CanVec data originally came from the
> > 1:50,000 NTS topographic maps .  Within the past few decades some
> > have been updated from medium-resolution satellite imagery, and some
> > have been updated with data from the various Provincial 1:20,000
> > mapping initiatives.  At any rate, the "Quantitative Horizontal
> > Accuracy Value" is given in the metadata for each NTS sheet, with a
> > number 30 meters being common.
> >
> > To stir the pot even more, the Manitoba government
> > (https://mli2.gov.mb.ca//) has it's 1:20k topographic maps available
> > for free (and the license looks libre too).  Their metadata gives
> > accuracy values (Horizontal_Positional_Accuracy) of 1.25 meters, 2.5
> > meters, etc.  Wow!  I expected 20 meters at best.
> >
> > And don't get me started on the accuracy of hand-held
> > navigation-grade GPS receivers...
> >
> > Best Regards, Brent Fraser
> >
> >
> >
> >
> >
> > Richard Weait wrote:
> >> Hi folks,
> >>
> >>> From the good folks at Natural Resources Canada (GeoBase).
> >>
> >> "Yes you can use the data found on GeoGratis site .  The licences
> >> are identical.  The only differences  are the copyrights, one is
> >> GeoBase, the other one is NRCan (GeoGratis).
> >>
> >> Please note: The data found on GeoGratis could have different
> >> planimetric precision and could not fit exactly with the precise
> >> GeoBASe data or OSM data.  Please refer to the metada info of the
> >> files you will be using."
> >>
> >> I'm sure that we are all excited about the additional data.  Please
> >> note the guidance regarding precision.
> >>
> >> Best regards, Richard
> >>
> >>
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> >
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