Pioneer venus radar mapper experiment.
Altimetry and radar scattering data for Venus, obtained from 10 of the first 13 orbits of the Pioneer Venus orbiter, have disclosed what appears to be a rift valley having vertical relief of up to 7 kilometers, as well as a neighboring, gently rolling plain. Planetary oblateness appears unlikely to exceed 1/2500 and may be substantially smaller.
Pettengill
G H
GH
Ford
P G
PG
Brown
W E
WE
Kaula
W M
WM
Keller
C H
CH
Masursky
H
H
McGill
G E
GE