Coincidence rangefinder (Wiki)
A single point on the target is brought into alignment to get the range, so the point of the triangle is at the target. This has the advantage that you know need to know
any dimensions of the target, but the disadvantage that the instrument gets very large if you need to range up to1,000 yards or longer.
The range finder works by having a known separation of the two sight lines in the instrument. So, the separation between the sight line needs to increase as the range increases.
For example a camera focusing rangefinder may have an inch or so separation to range to maybe 30 feet, but to range to 100 yards (300 feet) requires about 6 feet separation.
The Edscorp Field Range Finder has a separation of a couple of inches and can range to about 100 feet.
1168650 Binocular telescope, Frederick L G Kollmorgen, Keuffel and Esser Co, 1916-01-18, 359/417; 359/726 -
Also see these related web pages:
Panoramic Telescope M12 patents
Image property range finders are used in modern digital cameras. One way these work is to look at the image contrast as the lens is set a various focal lengths. The highest contrast image is deemed to be the best focused. The position of the lens is calibrated in distance to the subject. There are a number of other methods employed in digital cameras that analyze the pixels and the focus motor changes the lens focus. When they are done the best focus has (most of the time) been achieved and the camera and the EXIF data (Wiki) in the photograph data file reports the distance.
A single point in the instrument forms a triangle where the
other two points are at the target. The disadvantage is
that you need to know some dimension of the target, such as
the height of the top of a ships. main mast above the water
line, the height of a man's head, the height or length of a
vehicle, etc. The advantage is that the instrument can
be small like a rifle telescopic sight (Wiki: angular mil,
Dot sight), surveyor's telescope or the ship's
The surveyor's method of stadia
depends to tick marks on the telescope reticule that include
an angle of 10 milli radians (Wiki)
so that if there's a separation on the rod of 1 foot (between
the top and bottom stadia marks) then the rod is 100 feet
|Fig 2 Adjustment Instructions
||Fig 3 Adjustment Step 2 Peep hole and
Index Glass (3)
(telescope has been removed)
Fig 4 Horizon Glass (4) adjustments step 2 and step 3 & step 4.
Fig 5 Index Glass Adjustment step 3.
|Stadimeter Mk III
||Stadimeter Mk V (photo from Wiki)
Sextant type (see patents below)
U.S. Navy-Bu. Ships
Mark III-Mod.[ ]-1942
Rotate knob until object is spanned
between line on glass and movable sight.
Read Range opposite known height.
(If length instead of height is known,
use instrument on its side).
Rotating knob rotates cam that raises post sight.
Glass with line is fixed. At top there's an adjustment screw to set zero.
Near top you can see line on glass just above dot of light from
pinhole. Near bottom you can see post sight.
Measurement Devices Ltd may have been purchased by Renishaw,
including their Boretrak
system. The handheld laser rangefinder was part of
the Boretrak system.
|Fig 1 Has optional Magnetic compass and
||Fig 2 uses 2 each AA batteries
||Fig 3 Photo did not pick up red dot seen in
The following description from US patent 8471906
Miniature celestial direction detection system (See Stellar
A retroreflector (Wiki) is a device that returns incident light back to the source. A simple one can be made by taping three flat mirrors at right angles to each other making a corner reflector. Another type is a half silvered sphere, this is the cause of "cat's eye" (Wiki) or "red eye" (Wiki) or Raised Pavement Markers (Wiki).
The range of surveying total stations is limited when the target is non cooperative, like a tree or building, but when a retroreflector is used the range is much greater because of the efficiency of light return. The below reflector is about 60 mm (2.36") in diameter and is what's called a "Zero Offset" design. That means the range you measure is the same as if the target was on the point shown in Fig 2 and Fig 3. Other designs have some offset distance that needs to be backed out of the measured range.
The retroreflector prisms are specified by how close they come to sending the light back to it's source.
Here are some Hollow Retroreflectors - with no chromatic aberrations - between 1 and 20 arc seconds of angle.
Army TrainingU.S. Army Simulation Training and Instrument Command Retroreflector, Small
Approximately 1" active diameter
posted here: June 2017
Fig 1 U.S. Army Simulation Training and Instrument Command Retroreflector, Small
DVC No. 17-172/1
Contr No. N 61339-92-C-0114
Fig 3 Saab Training Systems
Ser: 000000 123
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