As mentioned in an earlier post on The Pocket Transit , I began looking for ways to estimate distance for the purpose of calculating the height of objects, like trees, buildings, cliffs, etc. Recall that if I could estimate the distance to the object and then determine the angle to the top I could use the tangent or sine, depending on whether I’m estimating the adjacent or hypotenuse sides, to calculate the height.
Estimate Distance using your Thumb – One method I found involved using your thumb. Apparently, your arm is approximately 10 times longer than the distance between your eyes. So, by looking at an object with your arm extended and closing one eye, and then closing it and opening the other, then noting how far your thumb moved, you can estimate the distance. Old Farmer’s Almanac has a description of how this works. If you search for estimating distance using your thumb you’ll find all sorts of sites which say pretty much the same thing. Who knew?
Laser Rangefinder – I could also buy a laser rangefinder, which are compact and easy to use. I did a quick search and found one could be purchased for as little as $130 and good to ranges out to 600 yards. You can, of course, spend more. A lot more.
But . . . I wanted something more basic than a laser rangefinder, but a little better than guestimating with my thumb, and a little less bulky than a stadiameter (which really works better if you know the distance or the size of the thing you’re measuring, like a ship or a person). Enter the coincidence rangefinder.
Coincidence rangefinder – Using mirrors and prisms the coincidence rangefinder presents the viewer with two images of the object. One of the mirrors is adjusted until the two images are coincident, at which point, the user reads distance. Plus, it requires no batteries.
I searched around for such a device and discovered that a company called Ranging, manufactured a product line of range finders back in the 1970’s. The one which looked most useful was the Rangematic 1000. I picked one up on ebay, made in 1978, for $20 and then a second one for $10 to use for parts and experimenting. The owner’s manual shows a price tag of $60 for one of these when they were new, way back in 1978.
Operation – The way it works is that you look through the 6x magnification viewfinder (which is detachable), noting the two images. One is blueish and the other yellow. It helps a lot if you focus the eye-piece. The camera didn’t focus very well when looking through the viewfinder, in the photo below.
Assuming the unit is calibrated, you adjust the large dial until the two images are coincident and then read the distance. I calibrated mine by a known distance of 50 yards according to the instruction manual. I also adjusted the unit to make the two images side-by-side as one should not be higher or lower than the other.
Disassembly – Both of the Rangematic 1000’s I acquired had lenses and mirrors which were very foggy on the inside making viewing difficult. I wasn’t sure how the two halves of the unit came apart, but this was the main reason for buying the busted-up junky one. First, a note: If you just plan to clean the lenses and mirrors, don’t bother taking out the two screws on the back of the unit in the middle. All they do is hold the interior box structure to the plastic housing. Also, don’t try to remove the large screw which appears to hold the dial in place. The threaded part is not a large diameter and is apparently easy to break. Uhmmm . . . yeah. I broke it on the junk unit. Oh, well.
It turns out that a bead of silicone adhesive is all that is used to keep the two halves together. All I needed to do was push up on the dial-meter support while pulling the opposite way with my index finger and the two halves came apart easily.
Using eyeglass cleaner, a cotton swab and lens cleaning cloth, I removed the film from the surfaces of the mirrors and lenses. This made quite an improvement.
I put the two halves of the unit back together and am temporarily holding them together with Velcro straps. This is partially because I want to make sure everything is working well, and partially because the mount for the image adjustment screw seems weak and I want to see if it is getting worse. When I’m ready I’ll run a bead of silicone around the mating surface and permanently glue them back together.
Testing – I’ve been experimenting with it a little bit. I calibrated the range to 50 yards by using a string marked off at 25 yards, and beginning at a stop sign measured to 25 yards, marked the ground with some chalk and then measured off another 25 yards. After calibrating it, both my son and I were able to measure the stop sign at 50 yards multiple times. This week I plan to do more testing. Used in conjunction with a transit, I should be able to calculate the height of trees, buildings, bridges, cliffs, etc.
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