Wednesday, July 28, 2010

plotting a bearing

We have talked about picking a point and navigating to it. And we have talked about picking a compass bearing and paddling toward it, but how about plotting a bearing on a chart, and figuring out what the compass bearing to follow would be? It is exceptionally easy. But first we need a plotting tool. I surfed the web, and found one, but really wanted direction on how to make one. I couldn't find any, but I knew it wasn't that hard. So this is what I did.

In Adobe Photoshop I created a compass rose with an inner and outer ring. The outer compass rose indicates true north, and the inner compass rose is adjustable to indicate magnetic north.  I then spun the smaller, inner rose to the declination for Alaska, flattened the image, and too it to kinko's - which is a US based photocopy shop. I had them print it on clear acetate. I cut it into a smaller square. Punched a hole in the middle and added a string. Cost? 81 cents. I need a better round string, and I would like the acetate to be a bit thicker. But it works just fine.

So once you have a plotter, this is how you use it. Place the center of the plotter over your location, or in the case of the video, a future location. In this case using a ruler I drew a potential route on my chart, and now I am going back to add bearings and distances. Align the Star on top with vertical running lines on the chart that represent North. Take the end of the string and lay it over the line you previously drew as a potential course. You now have a choice. You can read the outer ring to get a true bearing, or read the inner ring to get a magnetic bearing. In this case I did true, and labeled the written bearing with a 'T'. But I could have just as easily read the magnetic ring and labeled it 'M'.

To get distance is even easier. Take the string from the plotter and use it to measure the distance of the leg you have just labeled. You could then take that distance to the scale on the bottom of the chart, but it is easier - particularly on these big charts - to take it to the side, near where I am working. One minute (there are 60 minutes of latitude in 1 degree of latitude) of latitude is one nautical mile. label the distance under the line with an 'NM' for nautical mile, which is 1.8522 kilometers.

This is how you plan multi day trips. plotting out bearings with distances. You can then see how long each leg will be, to give you an idea of paddling distances. 

plotting from Paddling Otaku on Vimeo.

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