using the plotter...

[bigulua]

Hi everyone,
I'm currently studying for my PPL written exam and have a question about using the plotter to determine course. I know that the course can be found by placing the straight edge of the plotter along the course line, with the grommet (hole in the middle of the plotter) at the intersection of the course line and a meridian (vertical line); No problem. MY QUESTION IS, how do you measure a course when it is nearly vertical (i.e., close to 180 degrees) and/or does not cross a meridian line? I've had some trouble with this and would greatly appreciate any help you can offer. Thanks!
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One example where my question comes into play is the following question:

Refer to figure 23 of the FAA PPL Written Test Packet

What is the estimated time en route from Sandwich Airport (area 1) to St. Maries Airport (area 4)? The wind is from 215 degrees at 25 knots, and the true airspeed is 135 knots.

(In order to calculate this problem, you must determine that the true course is 181 degrees; the final answer is 34 minutes.)

 

[PropsForward]

I attempted to look up the question and this is what I came up with.

If you look in the middle of the figure, you will see the horizontal line with the 48° under it. You can align your plotter with this to get your true course.

Also, the Q I found offers answers of 30 34 & 38. I would say that 1° of error will not net a 4 minutes change even though it is amost a direct crosswind.

 

[bigulua]

Thanks for your reply, PropsForward.

Ok, so what I did was place the straight edge of the plotter along the course line, with the grommet on the intersection of the course line and the horizontal line of longitude; the line of longitude ran through the top part of the plotter (protractor) and I determined the course to be 181 degrees using the inner scale(offering readings between 30 and 210 degrees). DOES THIS SOUND RIGHT?

 

[midlifeflyer]

Bulls-eye.