Compass Use

Taking an accurate bearing on smoke is the most important part of the role of a Smokespotter.

 The bearing compasses we use for smokespotting are highly accurate, but only if used properly.  Mounted on a tripod (there's a tripod screw socket on the bottom) they can achieve an accuracy of 0.5 degrees, but using a tripod would generally be quite inconvenient.  When hand-held their accuracy can be within one to two degrees which is adequate for sighting over several kilometres.

Unlike an orienteering compass, a bearing compass is designed so that you can look "through" it to see the bearing to the object you are sighting on.  In fact, you don't look through the compass, but by holding the compass to one eye and keeping the other eye open you can see the sight-line in the compass superimposed on the distant object.
 

The fact that you need to sight with both eyes means that the way you hold the compass is critical.  If you are holding the compass to your left eye, you must grip it by the edge with your left hand, as in the image above.  Gripping it with your right hand would mean that your hand would block the view of the object from your right eye.

Do NOT try to use one eye to simultaneously look over the top of the compass to the distant object and look into the viewing port on the compass while keeping the other eye closed.  You cannot take an accurate bearing in this way.  You must use both eyes.  This can seem quite strange at first, and you might feel that you will never be able to focus one eye on the close-up compass bearing and the other on a distant object.  With a little persistence it becomes quite easy.  It will come more quickly if you figure out which of your eyes is the more short-sighted (better able to focus on close objects) and use that one for looking into the compass viewing port.

What you see in the viewing port of the compass is the vertical sight-line and the bearing.  There are two figures given for bearings: the larger one is the bearing from you to the object - this is the bearing you need to report; the smaller figure is the back-bearing - i.e. the bearing from the sighted object back to you.  The two bearings are 180 degrees different. (There are two mistakes in the figure above, which I copied from the Silva user's manual.  The smaller figure above 360 should be 180, not 160, and the values of the bearing increase from right to left, so that 360 degrees is actually to the left of 350 degrees on the compass).

The bearing to the sighted object in the above figure is 356 degrees.

It is important not to "approximate" when reporting the bearing to the Smokespotter Coordinator. For example, the above bearing should never be "rounded" to 355 degrees.

Another important aspect of using any compass is to hold it level.  Unless it is level the card may not rotate freely, giving an inaccurate reading, sometimes wildly so.  Holding it level from left to right is reasonably easy because the sight line will not be vertical if the compass is not level.  Ensuring the compass is level from "front to back" is more difficult, especially as there may be a tendency to point it somewhat up or down, depending on whether the object being sighted is higher or lower than you are.

The bearing you get using your compass is a "magnetic" bearing - there is a magnet under the compass card.  So the above bearing is expressed as "356 degrees magnetic".  This is the direction in relation to the North Magnetic Pole.  Unfortunately the top of a Google Earth map is pointing to "True North" (the Pole where all the lines of longitude come together), so the Coordinator will need to apply a correction to your magnetic bearing  to make it a "True" bearing.

The use of magnets in compass cards means that the accuracy of a compass is affected by any nearby steel material (or electrical currents).  DO NOT TAKE BEARINGS FROM INSIDE A BUILDING (EVEN IF IT IS A BRICK OR TIMBER BUILDING), OR WHEN YOU ARE STANDING NEAR A METAL WATER TANK, SHED OR VEHICLE.  Such bearings can be very inaccurate.  You need to be outdoors and several metres away from any large metal objects to take an accurate compass bearing.

Using the compass on a GPS, or a smart phone with GPS, to take a bearing is not likely to be as accurate as you might think.  First, these units do not have a viewing port and sight line, so they are already less accurate because of that.  Second, some of them will show a bearing on the screen, but will only begin to give an "accurate" bearing when you are moving.  Take a few steps toward the object and you might see the "bearing" change.  If you don't have a bearing compass, a bearing from one of these units might be useful to get a first approximation of the location of a fire - but you must tell the Coordinator what you are using - not only because of the questionable accuracy, but also because unless the GPS includes an electronic compass which has been set to provide magnetic bearings, it will be giving True bearings which don't need correction before plotting on Google Earth

Similarly, orienteering compasses can be used to get a bearing, but it will not be sufficiently accurate to precisely locate a fire unless it is quite close to you.  Always tell the Coordinator if you are not using the Silva Sightmaster Bearing Compass issued to Smokespotters.

when it comes to bushfires, we are all neighbours