If a person is interested in doing a real analysis of the "FBI" flightpath plot, he or she needs to analyze the actual thing from the FBI - not the "red ball, blue ball" file that totally obscures the red crosses that mark the positions on the actual FBI file. Also, a person needs to understand about how positions are plotted on such charts by people who know how to do it. (Another useful area of knowledge I'll address in a later post if anyone cares.)
The attached figure is a low quality copy of part of the "FBI" flightpath plot. Forget about the positions plotted on it (and the broad felt-tip line) for now, and just think of the figure as a chart on which you will plot positions. Refer to the actual "FBI" plot if necessary to see details referenced in the discussion.
In the figure, four scales are identified with distinctive labels. The two latitude scales are the same. The two longitude scales look similar but are not exactly the same. The short marks along each scale mark full minutes of latitude or longitude. Every fifth mark along each scale is longer than the others. Every tenth mark is longer and extends equally on both sides of the scale line.
Note that no numbers appear by the minute marks. When using such a chart it is wise to carefully annotate the number of minutes for at least every 10-minute mark. Otherwise, you are likely to make mistakes such as occurred in plotting points to the west of PDX on the "FBI" flightpath plot.
This is important partly because you will find that some of the minute marks have been overprinted by some map features, and there are some map features that look much like minute marks but are not in the right places to be minute marks.
Where such a situation occurs, it's a good idea to mark a minute in the right place on the scale if you might be plotting something around missing coordinate mark. The spacing of the ones you add can be determined by measuring the spacing between other nearby minute marks. [On the "FBI" plot, for example, 5' of latitude equals 136 pixels.] Take care not to use spacing from one longitude scale on another longitude scale. The distance corresponding to a minute of longitude varies with latitude. A minute of longitude is a smaller distance the higher you go on the map. (For example, a minute of longitude is a distance of zero at the north and south poles.)
The marks are more readily recognized on the original flightpath plots than on this low-res copy I made, but you need to be able to magnify areas somewhat to do so.
On a complete aero chart such as the "FBI" plot chart from which I cut the figure, a few latitudes and longitudes are pre-printed. These are the latitudes and longitudes on which the scales are drawn. The longitude scale at the bottom of the figure is on latitude 45°30'N. The one at the top is on latitude 46°N. The latitude scale on the right is on longitude 122°30'W, and the one on the left is on longitude 123°W. The values are outside the extents of the figure, but they're marked along the scale lines.
To plot a point on the chart, you need to know the latitude and longitude of the point. You will draw a left-right line at the latitude and an up-down (away and back, N-S) line at the longitude. These two lines will intersect to make a cross marking the point to be plotted.
You don't want these lines to extend all the way across the chart if you're going to want to also plot other points because doing so would result in an unintelligable grid rather than a group of separate plotted points. So, when you start to draw a line for a latitude, you want it to extend only a bit each side of the longitude where you will draw the intersecting line.
Since it will sometimes be a little hard to estimate where the intersecting (second) line will be, especially if the plot cross is going to be far from a scale, the first line of a cross will typically be a bit longer than the second one. Such crosses (one line considerably longer than the other) are an indicator that the plotting was done by someone with some experience or instruction. Lines with mis-aligned sections are an indicator of an amateur that didn't make the initial lines long enough.
On a printed version of a chart a person can position and orient the lines using a "drafting machine," or by using just a straight edge and the scales preprinted on the chart. If a drafting machine is used, the machine and chart must be aligned before drawing the lines.
We'll use just the straight edge. On a digital file, the equivalent method is to use a tool that draws a thin straight line between the desired latitude on the left and right scales and another between the desired longitude on the upper and lower scales.
Drawing the lines that will intersect at the desired latitude and longitude is not as simple as you might think. How it is done will depend upon the skill level of the plotter and upon the perceived need for precision and accuracy.
If you place that straight edge exactly on the desired coordinate and then draw a line along that straight edge, you will find that the line is offset from that exact coordinate. This is because the drawing tool, such as a pencil, doesn't put the line exactly on the edge. A tool with a sharp tip will help. But the tip will have to be run close up against the edge. With an ordinary pencil, for example, tilt the pencil so that the needle-sharp tip is right up against the edge while you draw the line. With such a sharp tip, the drawn line will be very thin. If the pencil has a lead soft enough to make the line dark, the tip won't remain so sharp through more than a few plotted points.
Drawing tools such as mechanical pencils require a different technique for precision and accuracy. Place the center of the lead on the scale at the desired coordinate. Move the straight edge up against the drawing tool tip. Look closely at the offset between the edge and the lead center. Hold the straight edge down tight where you have just positioned it while you slide/rotate the other end of the straight edge to place the other end on the desired coordinate while visually adding an offset equal to the one you noted at the first setting. Then hold the straight edge at the last position and place the pencil there to ensure that the offset you added is correct. Adjust the offset if needed. Then hold this position while you check, and reposition if needed, the alignment on the first position. What you have done is position and align the straight edge so that the line drawn will be closely on the desired coordinate.
Different methods can be used to locate the positions on the scales, depending upon the precision of the data and the desired precision/accuracy of the finished plot. Take a latitude of X°30.7' for example. You could look at the scale and visually estimate a position 7/10 of the way from the 30' mark to the 31' mark. This is an easy way that would give results about as precise/accurate as the chart itself in the case of the aero charts used for the "FBI" flightpath plot. Because it is easy, this method would be used by a knowledgeable plotter if the source data (latitude and longitude vs. time) has a precision of less than a minute of latitude and longitude.
Or, you could use a caliper, proportional divider, or scale and divider to accurately mark the 7/10 point with a "needle-point" hole in the paper. This tiny hole gives a precise way of positioning the drawing tool. The pencil lead (or whatever) sinks into the hole, so the hole centers the drawing tool tip. The hole will also help keep the tool tip from moving when you move the straight edge up against the tool tip.
Or, you could use a scale on the straight edge of a piece of paper or plastic showing tenths of minutes. If plotting on a digital copy of a chart, you can make digital versions of such scales.
Finally, if you're trying to plot precisely, use a thin lead or something even finer.
