Editing Vertical deflection (section)

{{Short description|Measure of the downward gravitational force's shift due to nearby mass}}
{{distinguish|Cathode-ray tube#Deflection}}
[[Image:GRAVIMETRIC DATUM ORIENTATION.SVG|thumb|upright=1.75|Earth's ellipsoid, geoid, and two types of vertical deflection.
{{legend-line|solid cyan|Ellipsoid of [[Geodetic astronomy|astro-geodetically]] oriented datum}}
{{legend-line|solid lime|[[Geoid]]}}
{{legend-line|solid red|[[Gravimetry|Gravimetrically]]-oriented ellipsoid}}
]]

The '''vertical deflection''' ('''VD''') or '''deflection of the vertical''' ('''DoV'''), also known as '''deflection of the plumb line''' and '''astro-geodetic deflection''', is a measure of how far the [[gravity direction]] at a given point of interest is rotated by local mass anomalies such as nearby mountains. They are widely used in [[geodesy]], for [[surveying]] networks and for [[geophysics|geophysical]] purposes.

The vertical deflection are the angular components between the true [[zenith]]–[[nadir]] [[curve]] ([[plumb line]]) [[tangent line]] and the [[normal vector]] to the surface of the [[reference ellipsoid]] (chosen to approximate the Earth's [[Sea level|sea-level]] surface). VDs are caused by [[mountain]]s and by underground [[geology|geological]] irregularities. Typically angle values amount to less than 10 [[arc-second]]s in flat areas or up to 1 [[arc-minute]] in mountainous [[terrain]].<ref name="t119">{{cite web | title=DEFLEC18 | website=National Geodetic Survey | date=2019-02-26 | url=https://geodesy.noaa.gov/GEOID/DEFLEC18/ | access-date=2025-03-23}}</ref>