[OSGeo-Discuss] A technical geodesic note about CommonMap - national vs international datums

[Brent Fraser]

Sam,

   I suppose a good way to minimize datum-related systematic errors in 
geographic data would be to:
     - specify your "storage" datum (e.g. WGS84(G1150) ) 
(http://www.geod.nrcan.gc.ca/faq_e.php#23)
     - give approved methods of transforming data from various datums to 
your storage datum.

On the other hand,  the magnitude of non-systematic/random positioning 
errors in the incoming data is about 3 to 5 meters (at best) for 
navigation grade GPS receivers and 5 to 50 meters for NAD83 CANVEC 
Canadian topo data, so the NAD83/WGS84 1.5m difference would not have 
much of an impact.

Looking at the OpenStreetMap FAQ, their attitude is "if you don't like 
the accuracy, fix it", which is pragmatic.  Personally I'd like some 
kind of accuracy estimate on each geographic feature, but requiring that 
would likely bring the crowdsourcing to screeching halt.

You might get some more opinions on the Proj4 mailing list...

Best Regards,
Brent Fraser


On 3/22/2011 12:03 AM, Sam Vekemans wrote:
> Hi, OSGeo-discussion list,
>
> I just wanted to forward this message from Brendan,  hopefully someone 
> who knows enough about projections can help with this, so to decide on 
> which coordinates system to use, and how to handle coordinates shifting.
>
> Thanks,
> Sam
>
> ---------- Forwarded message ----------
> From: *Brendan Morley* <morb_au at commonmap.info 
> <mailto:morb_au at commonmap.info>>
> Date: Mon, Mar 21, 2011 at 5:05 AM
> Subject: [OSM Fork] A technical geodesic note about CommonMap - 
> national vs international datums
> To: Sam Vekemans <acrosscanadatrails at gmail.com 
> <mailto:acrosscanadatrails at gmail.com>>
> Cc: "osm-fork at googlegroups.com <mailto:osm-fork at googlegroups.com>" 
> <osm-fork at googlegroups.com <mailto:osm-fork at googlegroups.com>>, 
> friends at commonmap.info <mailto:friends at commonmap.info>
>
>
> Sam etc.,
>
> I've been thinking a bit further about which datum to use for 
> coordinates in CommonMap.  As you know it can mean 1-2 metres 
> difference between your current national datum ( NAD83(CSRS) ) and the 
> GPS datum ( WGS84(G1150) ~ ITRF2000 ).
>
> While WGS84 seems attractive because of direct compatibility with the 
> GPS system, it's actually seems to be quite horrid from a geodesist's 
> point of view.
>
> This is because NAD83(CSRS) coordinates are basically tied to the 
> North America continental plate, therefore are expected to converge to 
> finer coordinates over time, but hopefully never drifting.
>
> However, WGS84 coordinates are basically tied to the average of all 
> continental plates.  Therefore a WGS84 coordinate is, strictly 
> speaking, out of date as soon as you measure it.
>
> This doesn't work so well in a geo database where, once you capture a 
> coordinate, it's expected to be pretty much stable.  Otherwise, how 
> often do you run through the database to update the coordinates?
>
> So, what I'm now suggesting is to run the imports in the current 
> national datum, e.g. NAD83(CSRS), and make a note of the import datum 
> as part of the changeset.  The results should agree to 2 metres today, 
> but will get worse over time.
>
> Then, we set up an additional function in the rails code to transform 
> the national datum to the international WGS84 datum, "just in time". 
>  In other words, predict what the WGS84 coordinates should be at the 
> time you call the CommonMap API.  Currently this would be the Helmert 
> transformation from NAD83(CSRS) to WGS84(G1150) ~ ITRF2000, then the 
> continental drift factors from (I think) ITRF2000's epoch 1997.0 to 
> today.  I think we would ignore tidal factors, they are not really of 
> any cadastral significance, and in any case I don't think PROJ4 is set 
> up to go that far.
>
> Manually entered coordinates would still assume the epoch of the time 
> they were entered, and if they were entered in the Canadian area, they 
> would be transformed back into the NAD83(CSRS) datum for stable 
> storage of coordinates.
>
> To help classify which coordinates belong on which continental plates 
> (and therefore calculate drift back to the NAD83(CSRS), GDA94 datums 
> etc) we can take advantage of previous work, of which this seems to be 
> the most comprehensive to date:
>
> http://peterbird.name/publications/2003_PB2002/2001GC000252.pdf
>
>
> Thanks,
> Brendan
>
> -- 
> Brendan Morley
> President, CommonMap Inc.
> morb_au at commonmap.info <mailto:morb_au at commonmap.info>
> http://commonmap.org/
> Queensland Incorporated Association 37762
> Also find us on Facebook, Twitter and LinkedIn
> --
>
>
>
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