N
Nomad
Guest
I've been using a Plastimo Iris 50 marine hand bearing compass to take bearings on distant objects to estimate how far away they are (using triangulation). While the compass certainly feels easier to use than a protractor type in terms of getting accurate readings, I wasn't overly happy with the readings I was getting. It's not quite a slick to use as one might like, and it does suffer from some parallax error.
The method is to point it such that the index line is coincident with the target, and then rotate it until a couple of raised features either side of the 'viewfinder' are the same distance from the index line - get the index line in the middle of the raised features, in other words. The problem with this is that the compass is held very close to the eye and the raised features are very blurred. Your eye also has to flick left and right to look at the raised bits, which makes it hard to see if they are equidistant from the index line. Furthermore, if you do start with the line in the middle, and then move your eye left and right orthogonally, the index line moves away from the reading - at the extremes (by which time, the alignment is obviously off centre), the reading changes by about +/- 2 degrees. Overall, getting an accurate reading is a bit of a fiddle, and quite difficult to do quickly. The basic issue is that the 'rifle sights' don't work very well and care is needed in eye positioning to eliminate the parallax error between the index line and the card.
I tried an experiment with a fine black marker - I put a little line in the middle of the 'viewfinder', thinking that, if I line that up with the red index line, I'd be able to get better alignment. Didn't work - it was far too blurred due to being so close to the eye.
So I pulled the compass apart, which was easy - just ease back the rubbery housing and pull out the disk at the base, followed by the sealed compass cell. This revealed the prism that's at the other end of the optical part - the viewfinder is a magnifier that looks into the prism, and then down onto the card. The red index line is on the horizontal surface of the prism, just above the card.
I reckoned a vertical line on the sloping part of the prism might work, so I tried various ideas. First was a line with a black marker, and this was sort of okay, but quite hard to see. I then tried a thin sliver of white tape, but that was almost impossible to see. Next was two wide bits of white tape with a bit of black tape over the gap. Not very good at all. Finally, I tried a thin sliver of black tape, and got a result. Due to the focus point of the optical system, the black tape isn't rendered sharply, but is clear enough to be able to see its position relative to the red index line.
Here's a view of the compass cell with the thin bit of black tape...
And a closer view of the prism area...
The tape is about 1mm wide, maybe a little wider at the top (it was cut freehand). Once I was happy that just a bit of black tape would be okay, I jiggled its position around a bit with the tips of some fine tweezers. After that, I put some thin card over it and burnished the card to get the tape into good contact with the prism.
Here's the view through the 'viewfinder' with the eye over to the left...
...and over to the right...
You can clearly see that the black line is displaced from the red line.
Here it is with the eye in the middle (ie, between the hard-to-see raised features that form the backsight of the rifle sights)...
It's quite easy to get the two lines to coincide, and very quick. Although the black line isn't quite as sharp as the red one, they do merge into one when they are aligned, and it's quite clear where they lie with respect to the degree graduations on the card. In the photo above, they are between 132 and 132.5 - pretty close to 132.25 degrees.
The method is to point it such that the index line is coincident with the target, and then rotate it until a couple of raised features either side of the 'viewfinder' are the same distance from the index line - get the index line in the middle of the raised features, in other words. The problem with this is that the compass is held very close to the eye and the raised features are very blurred. Your eye also has to flick left and right to look at the raised bits, which makes it hard to see if they are equidistant from the index line. Furthermore, if you do start with the line in the middle, and then move your eye left and right orthogonally, the index line moves away from the reading - at the extremes (by which time, the alignment is obviously off centre), the reading changes by about +/- 2 degrees. Overall, getting an accurate reading is a bit of a fiddle, and quite difficult to do quickly. The basic issue is that the 'rifle sights' don't work very well and care is needed in eye positioning to eliminate the parallax error between the index line and the card.
I tried an experiment with a fine black marker - I put a little line in the middle of the 'viewfinder', thinking that, if I line that up with the red index line, I'd be able to get better alignment. Didn't work - it was far too blurred due to being so close to the eye.
So I pulled the compass apart, which was easy - just ease back the rubbery housing and pull out the disk at the base, followed by the sealed compass cell. This revealed the prism that's at the other end of the optical part - the viewfinder is a magnifier that looks into the prism, and then down onto the card. The red index line is on the horizontal surface of the prism, just above the card.
I reckoned a vertical line on the sloping part of the prism might work, so I tried various ideas. First was a line with a black marker, and this was sort of okay, but quite hard to see. I then tried a thin sliver of white tape, but that was almost impossible to see. Next was two wide bits of white tape with a bit of black tape over the gap. Not very good at all. Finally, I tried a thin sliver of black tape, and got a result. Due to the focus point of the optical system, the black tape isn't rendered sharply, but is clear enough to be able to see its position relative to the red index line.
Here's a view of the compass cell with the thin bit of black tape...
And a closer view of the prism area...
The tape is about 1mm wide, maybe a little wider at the top (it was cut freehand). Once I was happy that just a bit of black tape would be okay, I jiggled its position around a bit with the tips of some fine tweezers. After that, I put some thin card over it and burnished the card to get the tape into good contact with the prism.
Here's the view through the 'viewfinder' with the eye over to the left...
...and over to the right...
You can clearly see that the black line is displaced from the red line.
Here it is with the eye in the middle (ie, between the hard-to-see raised features that form the backsight of the rifle sights)...
It's quite easy to get the two lines to coincide, and very quick. Although the black line isn't quite as sharp as the red one, they do merge into one when they are aligned, and it's quite clear where they lie with respect to the degree graduations on the card. In the photo above, they are between 132 and 132.5 - pretty close to 132.25 degrees.
