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Hewlett-Packard (USA)


HP 3820A

Serial Number



Length 239 mm, Width 162 mm, Height 298 mm


HP 3820A Electronic Tacheometer


This Hewlett-Packard HP 3820A Electronic Tacheometer is a combination of a digital theodolite for angle measurements, an EDM instrument for distance measurements and a microcomputer for basic survey calculations such as determining height differences and calculating horizontal distances from observed slope distances and zenith angles. The microcomputer also has the responsibility of monitoring the measuring systems and preparing the measured values for display. The measured distances can be displayed in either metres or feet.

The optical system serves as transmission and receiving optic, as well as the aiming telescope. The plate and index levels usually found on theodolites, are replaced by a dual-axis level sensor. This dual-axis level sensor automatically checks for the dislevelment of the vertical axis and applies the necessary corrections. The plumbline is given by a mercury horizon, dampened with silicone oil. 

The instrument uses a digital phase measuring method with an infra-red 830 nm Ga-Al-As-Diode (Gallium-Aluminium-Arsenide-Diode) as its radiation source. The modulation fine measurement frequency is 15 MHz and the power requirement for the distance measurement is 15 Watts. The maximum range of distance measurement for the instrument is approximately 1 km using a single prism and 5 km using 6 prisms. The standard deviation of a distance measurement is (5mm+0.000001D), where D represents the distance in km.

The magnification of the telescope is 30x, the diameter of the objective lens of the telescope is 66 mm, and the shortest pointing (minimum focusing) distance is approximately 5 m. The instrument uses the method of electronic 'decoding' for the circle reading, and provides a horizontal and vertical accuracy of 2" and 4" respectively. The measured angles can be displayed in either degrees or grads.

History & comments

This is one of the first instruments with a 'dual-axis' level sensor. Instrument was purchased by the School of Surveying, UNSW, in the late 1970's.


In a hard plastic case



  • Upright telescope image.
  • The weight of the instrument is 11 kg.
  • Catalogued by T. Ko
  • Updated by F. Pall


Manufactured in 1974 (approx). Catalogued in 2000.

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