Determination The deflections reflect the undulation of the geoid and gravity anomalies, for they depend on the gravity field and its inhomogeneities. VDs are used in astro-geodetic levelling, a geoid determination technique. As a vertical deflection describes the difference between the geoidal and ellipsoidal normals , it represents the horizontal  gradient of the  undulations of the geoid (i.e., the separation between geoid and reference ellipsoid) . Given a starting value for the geoid undulation at one point, determining geoid undulations for an area becomes a matter for simple integration .
Determination The deflections reflect the undulation of the geoid and gravity anomalies, for they depend on the gravity field and its inhomogeneities. VDs are used in astrogeodetic levelling: as a VD describes the difference between the geoidal and ellipsoidal normal direction , it represents the horizontal spatial gradient of the geoid undulations of the geoid (i.e., the separation between geoid and reference ellipsoid) .
R
astro-geodetic levelling, a geoid determination technique. As a vertical deflection
astrogeodetic levelling: as a VD
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normals
normal direction
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spatial
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geoid
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. Given a starting value for the geoid undulation at one point, determining geoid undulations for an area becomes a matter for simple integration