Plumb line

What this page shows

Geodetic latitude is the angle of the ellipsoid normal. Astronomical latitude is the angle of the gravity vector, which is the plumb line. Astronomical latitude and geodetic latitude differ by that deflection. The number here is only the difference of the two latitudes you enter. In the meridian, that difference is the north–south component of the deflection of the vertical.

A typical deflection is a few arcseconds, and it is larger near mountains. The page does not attach a measured deflection to a place. The 5″ preset is a round example of that size, not a survey.

Assumptions

• The angle is the astronomical latitude minus the geodetic latitude. A positive value means the astronomical latitude is the greater one.
• There is no geoid grid, no gravity model, and no terrain. Typing two latitudes does not look up a mountain.
• The east–west component is not on the page. Only the meridian difference is.
• The drawing exaggerates the angle. The cards are the difference, in degrees and in arcseconds.

Sources

• Heiskanen, W. A. and Moritz, H. (1967). Physical Geodesy. W. H. Freeman. The deflection of the vertical is the angle between the gravity vector and the ellipsoid normal. The meridian component is the difference of astronomical and geodetic latitude.
• The remark that the deflection is typically a few arcseconds, and larger near mountains, is that general size. No station value is used.