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

[Matt Wilkie]

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 .

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
>> 
>> 
>> _______________________________________________ Talk-ca mailing
>> list Talk-ca at openstreetmap.org 
>> http://lists.openstreetmap.org/listinfo/talk-ca
>> 
> 
> _______________________________________________ Talk-ca mailing list 
> Talk-ca at openstreetmap.org 
> http://lists.openstreetmap.org/listinfo/talk-ca
>