The easiest way to explain distance and direction measuring is by drawing a line between any two distinct points on your chart and practice with the instructions below. So take your 1:500 000 aeronautical chart and pick any two landmarks to draw a line between. To make the exercise a bit easier, pick two points at least 30cm apart and roughly east/west from each other.
The easiest way to measure distance on an aeronautical chart is to use a ruler with a scale in nautical miles. These are available from most pilot supply shops.
These rulers usually have several scales on them, so be sure to use the right one (i.e. the 1:500 000 scale when using a 1:500 000 chart or the 1:250 000 scale when using a 1:250 000 chart).
The direction of a straight line drawn on a chart is measured in reference to a line of longitude. As our 1:500 000 aeronautical chart is a lambert conformal conic chart, the direction measured will vary slightly depending upon where along the chart you measure.
This difference will be mostly negligible in typical PPL flying, but best practice says you should measure the direction halfway along the line.
Draw a line between two points and place your protractor half way along it, with the protractor’s north (360°) aligned with true north (remember: the black meridians of longitude on the chart all point to true north).
The true direction (255°T) of your line can be read along the edge of the protractor.
Since we use a magnetic compass in our aircraft, we need to convert the true direction (255°T) into a magnetic direction (°M). To do this, we need to find the nearest Isogonal to our track.
An Isogonal is a line that joins all the points of equal magnetic variation. The nearest isogonal to this flight is the 1.5°W Isogonal, marked on the chart with a dashed line.
If your line falls between two isogonals, then use the average of the two as your magnetic variation.
To know whether to add or subtract the variations, use the saying:
So for 1.5°W variation, we add 1.5° to our true direction.
True Direction Measured on the Chart = 255°T
Magnetic Variation from the Chart = 1.5°W
Magnetic Direction = 255° + 1.5° = 256.5°T
If the variation was east, we would subtract the variation from our true direction to get the magnetic direction.
Lastly, check the compass deviation card in the aircraft to see what deviation needs to be applied.
In this case, we have:
For 240°, Steer 242° (Deviation of +2°)
For 270°, Steer 269° (Deviation of -1°)
Our magnetic direction of 256.5° is halfway between 240° and 270°, so we use an average of the two deviations.
The deviation we need to apply will be +1°, making our Compass Direction 257.5°.
We can round this to 258°.