12:02 a.m. on November 9, 2010 (EST)
Bill S said:
Ed,
That's fantastic accuracy, considering ...
My results are based on triangulating known benchmarks, using distant bench marked landmarks, the further away, all the better. BTW that is +- 15 yards. Having a steady hand helps, a good eye helps more, but taking multiple readings, eliminating the outliers, and averaging the rest is the biggest help. One can also sight off of more than two reference points, to quadangulate, pentangulate… Another trick is eliminating the handshakes altogether, resting the compass on a steady object. An error I’ve seen people make is using their compass when it is not plumb, especially if an embolism has formed in the liquid at altitude. Something people also overlook is a good reading is compromised if you attempt to chart on a map that is not laying flat. Lastly you may have to estimate how magnetic declination affects your specific sighting, since the correction listed on the map varies from the actual value at a specific point of longitude, but this is really splitting hairs.
It helps to have a correctly calibrated compass. I found even good compasses are off a bit, fresh from the factory. The current compass I use is accurate when lining up the left edge of the rear sight with just inside the left edge of the cross hair, assuming the center of the bezel tic marks as the dead-on increment point.
You bring up an interesting point, comparing my performance against USGS product accuracy. My results were based on experiences back in the 70s, before satellites revolutionized cartography, WHEN I still had good eyes. (That should elicit a whole bunch of tangential comments.) Thus your observations have me scratching my own head, making a skeptic of me too. I would doubt my own assertions, but we diddled with this field exercise several times in different locations, all with similar results. Perhaps my results are partly because I used technology similar to what they used back in the day, thus similar results. One of the bench marks I triangulated was new at that time, but the rest were from the 30s and 40s. The question beckons: Are the practices currently used to survey, benchmark, and map significantly more accurate than those practiced fifty years ago, enough so as to have context in this topic?
Having good compass skills doesn’t preclude some silly and frustrating errors. The fact non-surveyed map points can be in error has caused problems for us in the past. I remember a navigation error caused by taking a bearing off the edge of an incorrectly drawn lake, resulting in us going down the wrong side of a buttress ridge, stranding us above a sheer wall, whereas the correct route choice would have delivered us to the lower valley. We lost most of a day’s effort, being forced to re-climb most of the ridge the next day to get back on course. Frustration due to bad weather has also caused me to learn some exotic orienting techniques, such as utilizing trig to attempt orienting off a single bearing point. Yea, I admit it. I have packed a slide rule and inclinometer into the Alaskan wilderness, in addition to compass and altimeter. (And to think I thought orienting competitions were for geeks!) Fortunately most backcountry navigation doesn't require such regimen, or even a compass for that matter.
Ed