[Talk-ca] NAD83-SCRS vs WGS84 Reference systems

[Brendan Morley]

Daniel,

I was musing on a similar topic just recently, which I've cc'd to:

http://www.commonmap.org/blog/commonmaps-common-datum/2011-03-27

I assume you mean NAD83(CSRS) coordinates, by the way?

My understanding is the de facto datum of OSM is WGS84/ITRF with epoch 
varying with the age of each entered coordinate, and planimetric 
accuracy at best 10 metres (at 10m, WGS84=ITRF).

Given NAD83 and WGS84 are also within the 10m error, you could just 
argue that you can just upload the coordinates to OSM as is.  Leave a 
note in the changeset (maybe per boundary way?) noting where the 
rigorous values can transformation methods can be found.

However if you want to adjust the coordinates, you could try the 
NAD83->ITRF (WGS84) 7(+3 dynamic) parameter similarity (Helmert) 
conversion for areas without a better option.

This paper is the best I've found on the matter; see Section 4 of:
http://www.geod.nrcan.gc.ca/publications/papers/pdf/nad83csrs.pdf

Remember that the coordinates start drifting with 2-3cm/annum velocity 
as soon as you enter then into OSM!  Do you want to be this rigorous?


Brendan

On 27/03/2011 5:55 AM, Daniel Begin wrote:
> Hi all,
>
> I need someone to confirm the following about reference system...
>
> Context: Paul and I are uploading US-Canada boundary monuments/turning
> points to get a stable and verifiable information. The data is available
> from their web site and I got the confirmation that the data can be used
> without any restriction.  The data can be found here...
> http://www.internationalboundarycommission.org/products.html
>
> and it is available for NAD27 and NAD83-SCRS reference system.
>
> Context: For what I understand, The difference between NAD83-SCRS and WGS84
> is 0-2 meters.  To get a rigorous transformation from NAD83-SCRS to WGS84 we
> need to use shift grids describing the shift between NAD83 and NAD83-SCRS.
> These grids should be available through provincial agencies but I have been
> told that not all provinces have them available.
>
> Question1: Do I understand it properly?
>
> Question2: Considering that provided coordinates value/reference numbers can
> be read directly from their web site, it make sense for me to use NAD83-SCRS
> directly even if there is a 0-2 meter offset.  Does it make sense for
> everyone?
>
> Cheers,
>
> Daniel
>
>
>
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