August 03, 2003
Dave Slater v. the Silva Compass
Sixty dollars of sad
By most literature you come to believe that Silva makes the best compass and the best book to learn compass navigation is "Be expert with map and compass, Bjorn Kjellstrom". Both are quite lacking.
The book speaks in generalities and is really meant to get you to become a sport orienteer. The compass is precision injection molded, but does not sight bearings very well, and in my case was off 6 inches in 22 feet. I really expected more for $60.
A much better book for couch potato navigation is "The Essential
Wilderness Navigator, David Seidman" having better compass instructions that
Silva's manual, and covers the full scope of navigation from improving your sense of direction, maps, compass, navigation, bearings, fix, transit, running fix, distance off, practical field navigation, sources of error, natural clues, extreme environs, and finally 1 page on orienteering. I was quite impressed and think I might be able to navigate with map and compass. It was so interesting I actually went outdoors to see if I could read a map, find a known bearing, verify local declination and compare with my gps.
Like all things become after leaving the couch, it was much more difficult than I expected. Tried to find a high ridge out in the open on Browntown road with hopes of seeing Pittsbrugh and surrounds far off with some points to match my 1:150,000 topos which would allow me to measure map bearings to verify local magnetic declination and verify my gps compass also.
Could not find an open ridge line and was chased down the road by two very arge dogs, shortly thereafter the road was closed and we had to return past the ogs. We put up the top and closed the windows and made it past the moonshine guarding dogs sweating profusely. Proceeded to Veltries overlooking the Springdale Valley. Could see about 2 miles so the map scale as too large and no bearings could be measured. Didn't matter much since the power lines and giant guard rails totally screwed up the compass and gps. Took advantage of the hazy view to glean what we could and was able to see riddle road, boat launches, 28, and several unnamed streets that made map sense.
Headed for Northmoreland Park to compare compass and gps in an open field with no power lines hoping to beat the squall line coming in from northern Ohio. My look at the weather radar revealed it was mainly going east but it swung south and time got late so we caught some showers on the way. Disgustedly compared the compass to gps and they got closer than in my back yard the other day. The gps is super sensitive to "levelness" and thinks the declination is 9 degrees. I don't know if it has values stored or what. Will check what it says for other areas of the country sometime.
Returned home and made a setup rig to verify the sitting of the compass. ran a string across the dining room table off into the living room and hung a plumb line at the far end of the room. Carefully aligned the compass to the string and taped it down. Carefully aligned my eye to see center of compass and lower index mark in alignment with mirror center line, then noted the v notch site was 6 inches off to the left of the plumb line. Mom verified. Investigated compass parts alignment, angles, mirror assembly, center line.... all seemed to be in order. Must be me.... pissed around another 2 hours trying to figure out what I was doing wrong or what was wrong with the compass. Only thing is see is the index mark crossest to you is slightly off. Rotate West to the top index mark and expect East to be matched up with the opposite index mark, but no. Instead of being centered on the mark it aligns with the left edge. Ok so I aligned the compass with the left edge of the nearest index mark and the far index mark centered. Even the v notch aligns when the lid is folded 180 out. That's as good as I can align it. Still the vnotch aims 6 inches left, I think that is 1.3 degrees off.
I tried slightly twisting the mirror etc to bring it into alignment with the sight but nothing worked. Finally I decided to mark a new line on the mirror that does align with the compass dial and index marks when the v notch is on the plumb line. It was easy to do and was about 1/32 to the left at the bottom of the mirror and about 3/64 to the left at the top ofthe mirror. Once that new line was marked with a sharpie the aiming was dead on. Mom verified that it was easy to take a bearing now since the new line matches up with the index marks better also.
I would like to understand where the error creeps in. I sure don't see where. Before you buy a compass you better do some high grade looking at it and be very particular about how it seems to align the index marks with the center line. All 3 should align, the top and bottom index mark, and the center of the compass needle. I don't know if that will ensure the compass will take good bearings but it sure wont if they don't.
--Dave