Thanks All!
E,
I'm going to hit some webinars by Dave Doyle this morning. Also some NOAA pages on Tidal Datums.
Dave
Dave, You posted lots of very good questions. If you (or anyone else lurking out there) would like to discuss these issues in greater detail call me at 301-704-9578 after you've had the opportunity to look at several of the related webinars posted on the NGS On Line Learning Resources page:
Introduction to Tidal and Geodetic Vertical Datums
Modernization of the National Spatial Reference System
Dave Doyle
Thanks, Dave! My cup runneth over!
I'm about a third of the way through this stack. Thanks for adding to my reading/watching list.
Dave
Is a Geoid just a Vertical Datum? Is there any way a Geoid can influence Horizontal Position?
Dave, I didn't see anyone directly address this; it might be in all those great links people have given you. The short answer is yes - scale factor. Your spherical coordinates (lat, long) do not change but ground coordinates do. Remember, SPCS even has to be adjusted for elevation. Think of slices of a pie, the angle doesn't change (lat, long) but the arc distance does as your radius changes. Where I worked at in Iraq, the geoid separation is small and the elevation is within 400 feet of "sea level" so we would often just work in ellipsoid. Up north around the mountains would have been a very different matter, as well in my home state of Colorado where elevation is such a big factor. So your question must be asked in each environement for a final answer, and consider how large the geoid separation is.
Your question has generated some great discussion, thanks for posting it and have fun learning.
Teresa
Live as if you were to die tomorrow, learn as if you were to live forever.
Thanks, Teresa! I love that name, "Geonerd."
I chip away at it. Everything I read gives me an "Aha!"
Dave
FWIW,
Interesting discussion. Like other posters, I find the questions not suited to short responses. I provide yet more links but also some definitions.
There have been a number of similar threads on this site. They can be reviewed using the search box. See for example: "Ellipsoid and Datum Equations" at https://surveyorconnect.com/index.php?mode=thread&id=236530 for more details about changes in reference ellipsoids and related issues.
Definitions (developed at the US NGS).
For the geoid see: http://www.ngs.noaa.gov/GEOID/geoid_def.html In addition to the definition, copied below, there is some history and references.
geoid: The equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level
Even though we adopt a definition, that does not mean we are perfect in the realization of that definition. For example, altimetry is often used to define "mean sea level" in the oceans, but altimetry is not global (missing the near polar regions). As such, the fit between "global" mean sea level and the geoid is not entirely confirmable. Also, there may be non-periodic changes in sea level (like a persistent rise in sea level, for example). If so, then "mean sea level" changes in time, and therefore the geoid should also change in time. These are just a few examples of the difficulty in defining "the geoid".
For more details see materials at:
For datum, geodetic datum, and geodetic coordinate systems see DATUM the NGS Geodetic Glossary at: http://www.ngs.noaa.gov/CORS-Proxy/Glossary/xml/
From this glossary I copy the following:
datum - (1) Any quantity or set of such quantities that may serve as a referent or basis for calculation of other quantities.
In particular, a geodetic datum, chart datum, or tidal datum. The plural form is datums.
(2) The singular of data, thus, a single, isolated piece of information.
datum, geodetic - (1) A set of constants specifying the coordinate system used for geodetic control, i.e., for calculating coordinates of points on the Earth.
At least eight constants are needed to form a complete datum: three to specify the location of the origin of the coordinate system, three to specify the orientation of the coordinate system, and two to specify the dimensions of the reference ellipsoid. Before geocentric geodetic datums became possible, it was customary to define a geodetic datum by five quantities: the latitude and longitude of an initial point, the azimuth of a line from this point, and the (two) parameters of a reference ellipsoid. In addition, specification of the components of the deflection of the vertical at the initial point, or the condition that the minor axis of the ellipsoid be parallel to the Earth's axis of rotation provided two more quantities. The datum was still not complete because the origin of the coordinate system remained free to shift in one dimension. This meaning does not conform to modern usage. The term datum alone is often used as a synonym for geodetic datum.
(2) The datum, as defined in (1), together with the coordinate system and the set of all points and lines whose coordinates, lengths, and directions have been determined by measurement or calculation.
coordinate system, geodetic - A coordinate system consisting of an ellipsoid, the equatorial plane of the ellipsoid, and a meridional plane through the polar axis. The coordinates of a point in this system are given by the perpendicular distance of the point from the ellipsoid, the angle between that perpendicular (the "normal") and the equatorial plane, and by the dihedral angle between the meridional plane and a plane perpendicular to the equatorial plane and containing the normal.
Another resource of possible interest is Dr. Peter Dana's site at: http://www.colorado.edu/geography/gcraft/notes/datum/datum.html
See also my introduction to geodesy course at: http://geodesyattamucc.pbworks.com/w/page/13931102/FrontPage This page was developed to support the course and includes lectures, labs and links. It was not intended to be self contained. Any reasonable requests for clarification/correction will be accommodated.
Cheers,
DMM
GeeOddMike,
Excellent, excellent, excellent! Thanks for the explanations and links. Coffee is on and I'm diving into the links.
The NGS Glossary link came up "Service Temporarily Unavailable." I'll try it later.
Dave
