Friday, May 18, 2012

Review: compass

For many years growing up, I would respond to a call of "Get lost!" with the smart-ass reply "I've got a compass", which as you might imagine always made me friends. However, it was true, and a compass has been part of my collection of kit for as long as I can remember. I probably had a Swiss Army Knife first, but a compass was right up there. When I lived in Calgary, and did "Outdoor Ed" at Dalhousie Junior High I picked up this Silva base plate compass, which has been with me since. We did orienteering and map reading, navigation and the like. No one ever really pays attention to those things at school, but some of it stuck with me, and I soaked that class up eagerly. Being evacuated from Dubai at the lead up to Desert Storm, fairly unnecessarily, it was fairly daunting for a young teenager to face the prospect of navigating the badlands and desert of the Arabian Peninsula without an adequate means of navigation. I was taught to drive the 4wd in the desert, in order to get away in case of invading Iraqi hordes, and was probably one of the key events in my desire for preparedness.  The lensatic compass came years later, a Christmas present.

  First the base-plate compass. The hard clear plastic base features three rulers, in mm and two in common map scales, for ease of estimating distances as well as a "direction of travel" indicator arrow to take bearings off. The fluid filled needle chamber is patterned on its base with a series of guide-lines, to assist with aligning to grid-lines om a map when relating true-north to magnetic north when taking readings. The bezel is stiff enough to not slip, but moves smoothly. Neither the magnetic needle or the numbers are luminescent but they are clear and easily viewed in dim lighting.

Here is my lensatic compass, it is a knock-off of a US Army M-1950 model and for what it is, is a pretty good tool.  One of the features of this kind of compass a powerful tool is that they are powerful means of taking a bearing on a distant point. It accomplishes this by means of a sighting wire, and a sighting notch, much like the iron-sights of a gun. You align the wire in the notch on a far away point, and you get a very accurate line to that point. The lens in the sighting armature which allows the user not only to see the measurements from the internal protractor without moving the sighting arms from the bearing, but also enable a much finer markings to be read, again increasing the accuracy of the reading.  Tips on how to do this can be found on the Lensatic Compass Guide or from old army manuals such as here easily enough.

Being a fairly cheap knock off, my lensatic compass lacks the a tritium dial or markings, but the four points of the compass are luminescent. The making lines and the bezel are functional enough, and there are 120 "notches" on the bezel, giving a 3 degree "click" for each turn. A second lens over the long line allows for close inspection of the bearing.

The one problem I have found after quite a few years of owning it is that the fluid in the chamber has leaked, and a large collection of bubbles have entered the chamber, and press down on the plastic disk of the protractor. This causes the needle to be pushed off line, with a real risk of erroneous readings. I'm going to try to repair this, but I'd say that this is the risk you take using cheap knock-off pieces of measurement equipment. Good to practice with, or dangle off my gear when I am doing Stargate lasertag LRP, but not something I would stake my life on in a survival situation. 

Get quality compasses. They needn't be expensive military grade pieces, as my Silva shows, but they need to work dependably.

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