An apparatus for observing a feature using dual charged particle beams(1018, 1043), comprising: a focused ion beam column configured to generate, focus, and direct a focused ion beam; an electron beam column configured to generate, focus, and direct an electron beam; one or more processors; and a computer-readable storage medium coupled to at least one of the one or more processors, the computer-readable storage medium comprising first executable instructions and second executable instructions, wherein the first executable instructions, when executed, cause the one or more processors to direct the focused ion beam to mill a trench in a surface of a substrate, the trench exposing a wall having an area around a feature to be observed, and wherein the second executable instructions, when executed, cause the one or more processors to: direct the electron beam to capture a first electron beam image of the wall while the electron beam column is maintained at a first angle of incidence relative to a longitudinal axis of the electron beam column; change the angle of incidence between the longitudinal axis and the wall from the first angle of incidence to a second angle of incidence; direct the electron beam to capture a second electron beam image of the wall while the electron beam column is maintained at the second angle of incidence; and approximate a topography of the wall based on differences between the first electron beam image and the second electron beam image. a focused ion beam column configured to generate, focus, and direct a focused ion beam; an electron beam column configured to generate, focus, and direct an electron beam; one or more processors; and a computer-readable storage medium coupled to at least one of the one or more processors, the computer-readable storage medium comprising first executable instructions and second executable instructions, wherein the first executable instructions, when executed, cause the one or more processors to direct the focused ion beam to mill a trench in a surface of a substrate, the trench exposing a wall having an area around a feature to be observed, and wherein the second executable instructions, when executed, cause the one or more processors to: direct the electron beam to capture a first electron beam image of the wall while the electron beam column is maintained at a first angle of incidence relative to a longitudinal axis of the electron beam column; change the angle of incidence between the longitudinal axis and the wall from the first angle of incidence to a second angle of incidence; direct the electron beam to capture a second electron beam image of the wall while the electron beam column is maintained at the second angle of incidence; and approximate a topography of the wall based on differences between the first electron beam image and the second electron beam image. direct the electron beam to capture a first electron beam image of the wall while the electron beam column is maintained at a first angle of incidence relative to a longitudinal axis of the electron beam column; change the angle of incidence between the longitudinal axis and the wall from the first angle of incidence to a second angle of incidence; direct the electron beam to capture a second electron beam image of the wall while the electron beam column is maintained at the second angle of incidence; and approximate a topography of the wall based on differences between the first electron beam image and the second electron beam image.