Talk:Geodesy

From Wikipedia, the free encyclopedia

	This article is supported by the Geography WikiProject, a collaborative effort to improve Wikipedia's coverage on Geography and related subjects on Wikipedia. Please participate by editing the article Geography, or visit the project page for more details on the projects.
B	This article has been rated as B-Class on the assessment scale.

I am not sure that the Earth's magnetic field is the subject of geodesy. Don't think so.

No, it isn't. Geodesists make just use of it, but it belongs to Geophysics or Potential theory. --Geof 05:27, 27 September 2006 (UTC)

I created 'reference ellipsoid'. Some of the stuff in this page should IMO go there.

1 Traditional Surveys vs Space Geodesy
2 Merge, Organize?
3 Link to a link collection?
4 Vertical datum transformations
5 Vandalism

[edit] Traditional Surveys vs Space Geodesy

I did not write the (unsigned) item above.

It is my impression that many datums are still the result of surveys of limited geographic regions (with classical instruments like the theodolite and plumb bob) and that they often do not patch together very well. While there is the attempt to reference them to an ellipsoid, the use of the plumb bob or equivalent references them to the geoid, which then must be further referenced to a chosen ellipsoid. I suggest that the "datums" article be filled out some more with examples - perhaps with newer ones determined by spacecraft - and that it be OWN3D! separate. There are already so many ellipsoid models (WGS84, Fischer, Mercury, Everest, .....) and so many datums that one might do best to add some table of the most used of these in each article; putting them together could result in a hodgepodge. Pdn 23:40, 19 July 2005 (UTC)

This sentance doesn't make sense. I don't know anything about geodesy, or i'd fix it myself. "Geodesy is primarily concerned with positioning and the gravity field and geometrical aspects of their temporal variations, although it can also include the study of the Earth's magnetic field." 169.237.97.2 00:19, 18 February 2006 (UTC)

@ Pdn: The different reference of terrestrial and space based Geodesy is a problem and a chance, too. Terrestric height MUST refer to geoid, otherwise "water will flow" between points of the same height. But satellite measurements are mainly geometric and not influenced by the gravity field like the level of a theodolite. Therefore the plumb line deflections do not affect SLR-Laser or GPS measurements => the results are more accurate and can be treated on a global basis. This gives the chance to reduce all local data to a global system (if required) up to some mm..cm accuracy.
As for the different ellipsoids: some 200 exist for all countries, but in german Wiki I wrote a table of the most important de:Referenzellipsoids in 2004. You may transfer it to reference ellipsoid. If you are interested in numerical examples, it's difficulte on a general basis. On the border of 2 countries, the national geodetic systems may differ by 100 or 500 m (caused by different fundamental points and their vertical deflections). But by empirical transformation formulas we can solve the "problem" up to ±1-2 cm. --Geof 05:27, 27 September 2006 (UTC)

[edit] Merge, Organize?

I am thinking about adding some info that seems to be missing from WP:

geodetic coordinates, geocentric coordiantes, ECEF coordinates etc.
geodetic height, geoid height, etc
datum conversion, 3 parameter, 7 parameter, Molodensky etc.
lists of reference ellipsoids and datums

and add some SVG diagrams to help explain everything. It looks like Datum has already been merged into Geodetic system and may be merged into this article. Geodetic vs. geocentric is covered in Lattitude but could use some pics. Any thoughts on how organize this? Rebuild the Datum article? Add to this article? Cover everything in Datum conversion? EricR 18:58, 21 March 2006 (UTC)

Diagrams are welcome, but Merging is not a good idea. The geodetic SYSTEM theory (which may be expanded, and also supplemented by international frames like ETRF/ITRS/ITRF 99-05, VLBI, plate tectonics etc.) doesn't fit to a general GEODESY article - and would make it bulky, too. If you understand german, look there (in Germany & Austria we have very active Wiki-Geodesists and the Category tree is increasing in quality). But if you really like to merge the articles, please give me a note on my german Discussion page. --Geof 05:27, 27 September 2006 (UTC)

[edit] Link to a link collection?

The newly added [1] link should be deleted, or replaced with the most useful links found on that page. I suggest deleting it, just take a look on that crap of a page. Just wanted to ask your opinion first... --V. Szabolcs 12:14, 29 September 2006 (UTC)

Spam removed. Vsmith 23:29, 29 September 2006 (UTC)

[edit] Vertical datum transformations

In the Geodetic Data section, on the subject of datum transformations, it says: "In the case of vertical datums, this consists of simply adding a constant shift to all height values." But that cannot be true, except locally. In Scandinavia, for example, the land is still rising after the last ice age, but not uniformly. Parts of north Sweden are rising by 9 millimeters per year, while south Sweden is almost still. Therefore, when shifting height values from a vertical datum of the year 1900 into a datum of the year 2000, there should be a one-meter change in north Sweden, but much less in south Sweden. --Mikael R 15:19, 20 February 2007 (UTC)