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 Geodesy

# Geodetic Misunderstanding

Dec 2005 | Comments Off on Geodetic Misunderstanding

1. Spheroid is considered equivalent to an ellipsoid.
2. The longitude at the North or South Pole is zero degree.
3. The Mean Sea Level (MSL), a time-variant “average”, is
considered as time-invariant and time-independent.
4. The measured “dn” along a spirit level loop sum up to zero.
5. Imagine a freely ?owing ocean or sea channel under
a continent and it represents the Geoid.
6. The geoidal undulation or height (N) is equivalent to orthometric height (H).
7. There are “yours” and “mine” geoids. In other words every country has its own geoid, creating a “political” boundary value problem for gravity.
8. When one has to re-observe at any station, it does not make any difference so long one is observing within “few” meters.
9. If one asks for and gets a height as “123.4 m”. It is OK to add zeros at the end, e.g., 123.4000, to make it more accurate.
10. A sea can be below the sea level.
11. The origin of WGS 84 is at the center of gravity of the Earth.
12. A map or chart can be plotted on any projection and a “different” grid can be super imposed. The extreme example here is the use of UTM grid over a stereographic projection in the polar region.
13. A map or chart can have TWO grids without TWO datums
or ellipsoids and be printed in the SAME color.
14. The standard deviation (or sigma) can be computed with “n = 1”.
15. The ellipsoid is used as zero reference for orthometric heights (H).
16. In a 70 GPS station network adjustment, 40+ or even more stations can be held “?xed”.
17. Two points on a “level” surface will have the SAME heights.
18. At any point on the Earth, the latitudinal arc depicts E-W direction.
19. Zero longitude still passes through Greenwich.
20. Everyone can teach geodesy.
Last but not the least:
If you have GPS, who needs a GEODESIST?
If anyone wants to know “why” about any misunderstanding, I will be very willing to provide the geodetic reason(s) or “why” it is still lingering.