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...@commonmap.info
<mailto:morb...@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 <acrosscanadatra...@gmail.com
<mailto:acrosscanadatra...@gmail.com>>
Cc: "osm-f...@googlegroups.com <mailto:osm-f...@googlegroups.com>"
<osm-f...@googlegroups.com <mailto:osm-f...@googlegroups.com>>,
frie...@commonmap.info <mailto:frie...@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speaking, out of date as soon as you measure it.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...@commonmap.info <mailto:morb...@commonmap.info>
http://commonmap.org/
Queensland Incorporated Association 37762
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