On Wed, 30 May 2012 16:20:28 -0700, Darryl Ramm wrote:
On Wednesday, May 30, 2012 3:52:41 PM UTC-7, Martin Gregorie wrote:
[snip]
On a slightly different topic: GPS altitude. I've always known that all
GPS altitudes are relative to the WG-84 geoid but have never known how
precisely that corresponds sea level, so I finally did some research
and it turns out that its within +/- 1 metre of AMSL.
Actually I'm not sure where you get +/- 1m difference between the WSG-84
geoid and AMSL. It's potentially larger than that (but still that does
not mean GPS altitude is inherently not usable). e.g. see this article
http://www.esri.com/news/arcuser/0703/geoid1of3.html
He "The new World Geodetic System was called WGS 84. It is currently
the reference system being used by the Global Positioning System. It is
geocentric and globally consistent within ±1 m. Current geodetic
realizations of the geocentric reference system family International
Terrestrial Reference System (ITRS) maintained by the IERS are
geocentric, and internally consistent, at the few-cm level, while still
being metre-level consistent with WGS 84." - from:
https://en.wikipedia.org/wiki/World_Geodetic_System
yes, I know its only Wikipedia, but IME it seems to be generally OK for
technical details, but its backed up by this source:
http://kartoweb.itc.nl/geometrics/Re...faces/body.htm
IOW the 'best fit' geometric elipsoid, which can deviate from AMSL by up
to 105m is adjusted for gravimetric factors (the reference data set seems
to be ITRF96) and the result is the WGS84 geoid. According to the second
reference: "The World Geodetic System of 1984 (WGS84) datum has been
refined on several occasions and is now aligned with the ITRF to within a
few centimetres worldwide." which appears to be referring to the
deviation from AMSL though this is nowhere stated explicitly but appears
to be the meaning in the given context.
--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |