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What is a depression rangefinder, who used it, and where?

Posted: Sat Jan 25, 2014 12:26 am
by steffen19k
I found this thing on one of my ebay walkthroughs, which I do when I'm bored and I'm looking for something to give me something to chew on. This one just took me aback by a mile or so. Any information that anyone would be willing to scare up on it would be greatly appreciated, so I can understand it better.

I've included the link below so that you can see it for yourself, if you're not sure what I'm talking about.

http://www.ebay.com/itm/Depression-Rang ... 3cdb53c92a

Re: What is a depression rangefinder, who used it, and where

Posted: Sun Jan 26, 2014 5:26 am
by Rick Rather
It was a device used by coastal artillery as a back-up method of range finding. The primary [pre-radar] method used "base end stations" to get a cross-fix on the target. Here is a training film showing how that worked. The BES method relied on at least two stations having visual on the target AND communications with the Plotting Room.

If, for whatever reason this was not the case, the individual stations and the battery's own observation post usually had a depression rangefinder. Since the elevation of the installed (and leveled) rangefinder could be precisely surveyed, then measuring the angle in degrees from horizontal to the waterline of the target could provide a trigonometric range to the target.

For example: Nagato & Mutsu are bombarding San Francisco. Fort Barry, on the Marin headlands has an observation post at the top of the cliffs, 70 meters above the waves. The depression rangefinder measures the angle from the horizon to Mutsu's waterline as 0.3 degrees. 70m / tan0.3 = ~13,369 meters. This was in-range for Battery Wallace's 12" barbetted guns, but just outside the reach of Battery Mendell's 12" disappearing guns and Battery Alexander's 12" mortars.

Note that, to get an accurate solution, the depression rangefinder had to see the target's waterline, which meant that it was effectively useless at night or in bad weather, or if the target was hull-down or too far away to get a decent angle measurement.

Hope this helps.

Re: What is a depression rangefinder, who used it, and where

Posted: Mon Jan 27, 2014 11:45 am
by steffen19k
Actually, Mr. Rather, that is everything I was looking for to explain it.

Thank you very much.