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

[Brent Fraser]

Michel,

  Great Wikipedia link!  I've found that most people get a glazed look in
their eyes if I say precision is about repeatability, so I usually just
stick with the number-of-digits explanation, or use the dartboard/target
 example if they show any interest at all.

  Coming up with a way to state the accuracy so most people will
understand can be a challange.  One way is to say "within 20 meters at 2
sigma", another (less rigorous) way is "suitable for mapping at
1:20,000". And finally, the rather ambiguous "+/- 20 meters".  And while
these are similar, they are not really the same.  But on the other hand,
they're close enough for non-rigorous discussions...

  But anyway, I wanted to get people thinking about mixing 1:50k CanVec
data (+/- 50m) with GPS-gathered data (+/- 5m).  Is tagging with the
"source" of the data enough?

Thanks!
Brent Fraser

> 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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