How to estimate distance by a telescope

[zli034]

Guys

I don't know if people could estimate distance to a target object by looking through a telescope. I believe telescopes for this kind use must be calibrated some how. When the time people sailed in the ocean and when they spotted another boat by the telescope, how did they tell the distance?

thanks

 

[zhermes]

Well, if you know how big a boat is, and you know how big it looks in a telescope... combining that with the magnification of the telescope tells you how far away it is.

Maybe I'm unclear on what you're asking.

 

[Studiot]

Many instruments, such as the telescopes in surveyors' theodolites possesses calibrated marks which give a distance factor of 100 when a staff or ruler is viewed. These are called stadia marks.

http://en.wikipedia.org/wiki/Stadiametric_rangefinding

Surveyors also use dedicated instruments which can provide both horizontal and vertical measurement by this means. these are special theodolites, called tacheometers.

http://en.wikipedia.org/wiki/Tacheometry

go well

 

[zli034]

The curve that shows target viewing size at a distance is what I really interested in. Anyone knows how it calculated.
Please see this pic from wikipedia
http://upload.wikimedia.org/wikipedia/commons/a/a9/Distance_meas.png
and this pic
http://upload.wikimedia.org/wikipedia/commons/d/da/M67_sight_full-stadia_picture.png

 

[turbo]

Some rifle scopes have calibrated reticles (typically with spacing conforming to the body size of a deer or similar) to help with aiming (elevation). Golfers have scopes calibrated to standard flag-pin heights so that golfers can estimate the distance to the hole and make their club selections. These are forbidden in pro golf so golfers have to rely on their caddies' notes and experience instead.