page_title: Optical square
section_title: Optical square in surveying
hierarchical_section_title: Optical square / Optical square in surveying
caption_attribution_description: Foto eines Doppelpentaprismas wie es zum Abstecken rechter Winkel bei der Vermessung verwendet wird.
context_page_description: The optical square uses a pentaprism to reflect and refract a beam or sighting 90 degrees, it is used in pairs in surveying and in a singular block in metrology.
  In optical square
  A Horizon glass is placed at an angle of 120° with the horizon sight.
  The Index glass is placed at an angle of 105° with the Index sight.
  Angle between Index glass and Horizon glass is 45°.
context_section_description: In surveying it is used both as a hand held tool for sighting between two poles (often with a plumb bob hung from the handle) and also mounted on a Jacob's staff.
caption: Double pentaprism as used in surveying.
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