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

[Sam Vekemans]

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>
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>
Cc: "osm-fork at googlegroups.com" <osm-fork at googlegroups.com>,
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  Therefore a WGS84 coordinate is, strictly speaking, out
of date as soon as you measure it.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
http://commonmap.org/
Queensland Incorporated Association 37762
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