NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Latitude by Talcott-Horrebow Method
From: Peter Monta
Date: 2018 Nov 5, 16:01 -0800
From: Peter Monta
Date: 2018 Nov 5, 16:01 -0800
> Which implies that measuring the latitude and claiming significance to 0.1 arcseconds is an empty claim
Depends on the definition of latitude. The ITRF notion of latitude is self-consistent at the level of a few millimeters. As you point out, earth orientation has its uncertainties, but strictly speaking, terrestrial (geodetic) latitude need not involve earth orientation at all. Just assume perpetually cloudy skies; now we need a notion of latitude without reference to celestial bodies. Geodetic latitude is a function of monuments anchored to bedrock (nowadays, the monuments are GNSS and VLBI and SLR antennas). Now, bedrock is not perfect either, but plate motion has been modelled for a long time, and even the episodic earthquakes are accounted for now in the newest ITRF (ITRF14). So unless you're really close to a large earthquake in both space and time, you're good to a few millimeters (assuming excellent monumentation and months to years of averaging time). If you're unlucky in the earthquake department, your coordinates degrade to decimeters or worse until the motion has a good retrospective model.
But with the Wobble, you're talking about astronomic latitude. Astronomic quantities are problematic. Earth orientation is a part of the problem, but if your measurement involves gravity, there are also seasonal and secular variations in the Laplace correction from e.g. the water table. Astronomic measurements are great for geodesy (and astrogeodetic enthusiasts), but not so good for reference systems anymore.
Cheers,
Peter
Depends on the definition of latitude. The ITRF notion of latitude is self-consistent at the level of a few millimeters. As you point out, earth orientation has its uncertainties, but strictly speaking, terrestrial (geodetic) latitude need not involve earth orientation at all. Just assume perpetually cloudy skies; now we need a notion of latitude without reference to celestial bodies. Geodetic latitude is a function of monuments anchored to bedrock (nowadays, the monuments are GNSS and VLBI and SLR antennas). Now, bedrock is not perfect either, but plate motion has been modelled for a long time, and even the episodic earthquakes are accounted for now in the newest ITRF (ITRF14). So unless you're really close to a large earthquake in both space and time, you're good to a few millimeters (assuming excellent monumentation and months to years of averaging time). If you're unlucky in the earthquake department, your coordinates degrade to decimeters or worse until the motion has a good retrospective model.
But with the Wobble, you're talking about astronomic latitude. Astronomic quantities are problematic. Earth orientation is a part of the problem, but if your measurement involves gravity, there are also seasonal and secular variations in the Laplace correction from e.g. the water table. Astronomic measurements are great for geodesy (and astrogeodetic enthusiasts), but not so good for reference systems anymore.
Cheers,
Peter
