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

[Brent Fraser]

	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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