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  Plotting 101


You may think you know all you need to know about plotting, if so skip to the section that interests you. But before you do, a quiz. When is plotting required? If you said when a route segment "between the operational service volume of ICAO standard ground-based navigational aids exceed 725 nm," then you probably do know it all. Otherwise, you might want to study what follows:


Plotting Myths Busted

Myth: You don't need to plot over fixed track systems, like the ones between Hawaii and California


Figure: Pacific Organized Track System (PACOTS), (Jeppesen En Route Charts)

Just because the route doesn't change on a daily basis doesn't mean you or your FMS will not make mistakes. The distance between ground-based navigation aids far exceeds those required by FAA Order 8900.1, Volume 4, Chapter 1, Paragraph 4-80.A, you must plot. More information: When is Plotting Required?.

A quick look at a few pilot blogs and postings confirms that few airlines require plotting over fixed track systems. Listening to the HF I've heard one or two chewed out for getting it wrong and I've heard from a POI or two that they get it wrong more often than we in the corporate world. (They get fewer violations because their companies pay most of the cost of operating the system.) But in this day and age, how are these navigation errors possible?

I think the leading cause of these navigation errors in complacency. More about this: Pilot Psychology / Complacency. But you could find yourself in trouble from another source. Chances are you are getting your oceanic waypoints via an uplink and those waypoints are coming from a computer. The computer got them from someone. If that someone made a mistake, chances are it will have been caught by another airplane before you. But what if you are the first after a database update? In the years since I've flown with uplinked waypoints I've caught three database errors, though none were oceanic. In my view, taking a few minutes to plot is a small price to pay to keep from hitting another airplane.

Myth: As long as you have a navaid tuned, you don't need to plot.


Figure: Standard High Altitude Service Volumes (Aeronautical Information Manual, Figure 1-1-1.)

You can have a navaid tuned and identified far outside its service volume, which means it doesn't count when making the plot / don't plot decision. Typical service volumes from [Aeronautical Information Manual, ¶1-1-8:

  • Standard High Altitude Service Volume between 18,000 and 45,000 ft: 130 nm
  • Standard Low Altitude Service Volume between 1,000 and 18,000 ft: 40 nm
  • Standard Terminal Service Volume between 1,000 and 12,000 ft: 25 nm
  • NDB HH Service Volume: 75 nm
  • NDB MH Service Volume: 25 nm

Myth: You don't have to plot in Class I airspace.

Plotting requirements have never been tied to Class I or Class II airspace, but the issue is doubly moot now that Class I airspace is virtually worldwide for aircraft with GPS. More about this: Class I vs Class II.

When is Plotting Required?

[FAA Order 8900.1 Volume 4, Chapter 1, Paragraph 4-80.A]

  • Plotting Procedures. Plotting procedures have had a significant impact on the reduction of gross navigational errors. There is a requirement to plot the route of flight on a plotting chart and to plot the computer position, approximately 10 minutes after waypoint passage. Plotting may or may not be required, depending upon the distance between the standard ICAO ground-based NAVAIDs.
    • Plotting procedures are required for all turbojet operations where the route segment between the operational service volume of ICAO standard ground-based navigational aids exceeds 725 NM.
    • Plotting procedures are required for all turboprop operations where the route segment between the operational service volume of ICAO standard ground-based navigational aids exceeds 450 NM.
    • The Administrator requires plotting procedures for routes of shorter duration that transit airspace where special conditions exist, such as reduced lateral and vertical separation standards, high density traffic, or proximity to potentially hostile border areas.
    • Any existing approvals that differ from the plotting requirements in this paragraph and Class II navigation procedures should be reviewed and revised as necessary. Direction and guidance is available from the navigation specialists in coordination with AFS-400.

[Advisory Circular 91-70A, ¶3-6.a.]

(2) Turbojet Operations. All turbojet operations, where the route segment between the operational service volume of ICAO standard ground-based NAVAIDs exceeds 725 NM, require plotting procedures.

(3) Turboprop Operations. All turboprop operations, where the route segment between the operational service volume of ICAO standard ground-based NAVAIDs exceeds 450 NM, require plotting procedures.

(4) The Administrator requires plotting procedures for routes of shorter duration that transit airspace where special conditions exist, such as reduced lateral and vertical separation standards, high density traffic, proximity, or potentially hostile border areas.

Where did these numbers come from? They appear to be arbitrary. The original version of this advisory circular came out September 6, 1994 and has the same 725 and 450 nautical mile requirements. There does not appear to be an ICAO specification, most international regulations recommend plotting procedures and North Atlantic regulations appear to assume they are being used. My earliest navigation manuals recommend plotting every 30 minutes or after a significant change in heading, speed, or altitude. My last few jobs where navigators were involved required a position plot every hour. As I continue to search for the evolution of the 725 number I remind myself that I don't know everything and if the book tells me I must plot beyond 725 nautical miles from the service volume of an ICAO standard ground-based navaid, that's what I do.

Why is Plotting Required?


Figure: One-degree error, (Eddie's notes)

[Advisory Circular 91-70A, ¶3-6.a.(6)] The FAA requires crews to use a plotting chart to provide themselves with a visual presentation of the intended route. Regardless of the type of LRNS in use, operators must use plotting charts. Plotting the route will increase SA and reveal errors or discrepancies in the navigational coordinates that flight crews can correct before such errors can cause a deviation from the ATC cleared route. As the flight progresses, plotting the position approximately 10 minutes after passing each waypoint helps confirm that the flight is on course. If the plotted position indicates off track, the flight may have deviated unintentionally and the flight crew should investigate at once.

In the example chart, the crew has made a one-degree error in the FMS. After ten minutes the plotted position clearly shows something is wrong, allowing the crew to fix things before they stray too far off course and possible in another aircraft's path.

These errors are caught . . . here is a two degree error by a GV:

Date: 10/01/01
Error: Gross Navigation Error
Narrative: GLF5 W/B RAN F500 cleared GOMUP 62/20 64/30 observed by radar 6105N022217W (90 NM off track.) A/c reported 62/20 as cleared when actually at 60/20. No L-o-s in Reykjavik OCA*. *Checking separations in Shanwick OCA. Follow-up with operator.

Figure: Gross navigational error report example, from an Eddie source.

How is a common plotting chart laid out?


Figure: Plotting chart layout, (Eddie's notes)

[ICAO Annex 4]

  • 2.1.7 Recommendation.— The charts should be True North orientated.
  • 2.15.1 True North and magnetic variation shall be indicated. The order of resolution of magnetic variation shall be that as specified for a particular chart.
  • 2.15.2 Recommendation.— When magnetic variation is shown on a chart, the values shown should be those for the year nearest to the date of publication that is divisible by 5, i.e. 1980, 1985, etc.
  • World Geodetic System — 1984 (WGS-84) shall be used as the horizontal (geodetic) reference system. Published aeronautical geographical coordinates (indicating latitude and longitude) shall be expressed in terms of the WGS-84 geodetic reference datum.
  • Mean sea level (MSL) datum, which gives the relationship of gravity-related height (elevation) to a surface known as the geoid, shall be used as the vertical reference system.

Apart from saying the chart should be north up, must be based on WGS-84 and mean sea level, the chart printers are pretty much given discretion on how to print their plotting charts. There are some basic guidelines you should know for the chart you are using:

  • The horizontal lines are "parallels of latitude," usually a line for every degree with a major line every five degrees.
  • The vertical lines are "meridians of longitude," with a line for every degree with a major line every five degrees.
  • Longitude and latitude are subdivided into 60 parts known as minutes and labeled with a single quote mark (').
  • Minutes are further subdivided into 60 parts known as seconds and labeled with double quote marks (").
  • Bedford (KBED) airport, for example is at 42°28'11.8" N, 71°17'20.4" W; which is pronounced "forty-two degrees, twenty-eight minutes, eleven point eight seconds north, seventy-one degrees, seventeen minutes, twenty point four seconds west.

More about this: Navigation: Coordinates.

How to Plot a Position.


Figure: Position plotting example, (Eddie's notes)

There are many techniques on how to do this correctly, here is mine:

  • Ensure you are in the correct quadrant: In the north the latitudes increase as you go up, in the south they increase as you go down. In the west the longitudes go up as you head west, in the east the go up as you head further east. Locate the nearest five degree line in the general area of your point, in the example 30° N.
  • Locate the five degree line just above your latitude, in our example 35° N.
  • If your chart has one degree markings locate the nearest degree below your point, otherwise count the degree lines. In our example 33° N.
  • Count the tick marks between degree lines on the chart you are using. In our example there are six so we conclude each tick mark represents 10 minutes of latitude. Counting up four tick marks we identify 30°40' N latitude.
  • Notice that a line connecting 33°40' N 150° W and 33°40' N 160° W does not cross 33°40' N 155° W, it runs high. That's because the chart is a Lambert Conformal Projection which bends toward the poles. To get a more accurate position, we need to find the 33°40' point on the scale closest to our position.

Figure: Position plotting example, (Eddie's notes)

  • When dealing with longitude, we look for the nearest five degree line under our desired position, in our example 155° W.
  • Then we look for the nearest five degree line greater than our desired position, in our example 160° W.
  • We can now locate the nearest degree under our position, in our example 156° W.
  • Counting the tick marks, we see there are six so 30' of longitude will be three greater.
  • Unlike latitude, lines of longitude appear parallel between lines of latitude, so we can draw a line at 156°30' W between tick marks and it will remain accurate.
  • We then transpose our mark identifying 33°40' N to this line of longitude and viola, we have our position.

How to Plot a True Course.

For a tutorial on headings, courses, variation, and the like, see: Navigation - Direction.

Measuring Course


Figure: Jeppesen Plotter, (Eddie's collection)

A plotter is nothing more than a circular instrument designed to give angular differences between lines. A navigation plotter, such as the Jeppesen model shown, will typically have a hole in the center of a compass rose. To use:

  • Decide if you want the start, mid, or ending course. Most pilots will need the start course. For more about this, see: Start / Min / End Point Differences.
  • Place the hole in the center of the compass rose over a line of longitude near the desired point, in our example we've used 40°W which is also the longitude of our start point.
  • Align the line along the 0° mark to your desired course. You may find it helpful to insert your pencil or pen point in the hole while rotating the plotter to line up with your course.
  • Read the true course along the line of selected longitude, 085° in our example.
  • Note: You will have two choices aligned with the line of longitude, 085° and 275° in our example. In most cases it will be the number on top, but when dealing with courses near vertical it can be confusing. Always remember to give your answer a common sense check. In our example, we are headed to Europe and the answer should be generally easterly.

Measuring Distance


Figure: Dividers, step 1, (Eddie's collection)

[AFM 51-40, Page 5-8.]

  • One of the disadvantages of the Lambert Conformal chart is the lack of a constant scale. If the two points between which the distance is to be measured are approximately in a north-south direction and the total distance between them can be spanned, the distance can be measure on the latitude scale opposite the midpoint. However, the total distance between any two points that do not lie approximately north or south of each other should be be spanned unless the distance is short. All distances should be measured as near the mid latitude as possible.
  • In the measurement of long distances, select a mid latitude lying approximately half-way between the latitudes of the two points. By using dividers set to a convenient, reasonably short distance, such as 60 nautical miles picked off at the mid latitude scale, you may determine an approximate distance by marking off units along the line to be measures as shown [in the figure].
  • The scale at mid latitude is accurate enough if the course line does not cover more than 5 degrees of latitude (somewhat less at high latitudes). If the course line exceeds this amount or if it is crosses the equator, divide it into two or more legs and measure the length of each leg with the scale of its own mid latitude

If you have a set of dividers and a flat surface you aren't afraid to scratch, find the distance between waypoints is quite easy. First, place the points of the dividers on the start and end waypoints.


Figure: Dividers, step 2, (Eddie' collection)

Next, find a line of longitude near the course line. It is important to use a line of longitude about the same latitude as the course, since these will change over great distances. Each degree of latitude equals 60 nautical miles.


Figure: Post It Measuring Tool, (Eddie's notes)

If you don't have a set of dividers — do you really want to have such a sharp instrument in the cockpit? — you can construct your own with a PostIt note or other straight-edged paper. You will be accurate within a nautical mile if you do it this way:

  • Place the straight-edged paper along side the course. In our example, the PostIt note isn't long enough so we've used two, overlapping, notes.
  • Place a tick mark at the starting and ending waypoints.
  • Move the straight-edged paper to the nearest line of longitude at about the same latitude. Place one tick mark over a convenient line of latitude, 49°N in our example.
  • Read the distance from the ending tick mark, using 60 nautical miles per degree. In our example the distance covers at least 6° of latitude, which comes to 360 nautical miles. The mark goes further by a tenth of a degree, meaning another 10 nautical miles. We conclude the distance between waypoints is therefore 370 nautical miles.

Note: You can also determine the distance using a set of "10 Degree Tables," where we would see the exact distance is actually 369 nautical miles. (Our PostIt note was off by 1 nautical mile, or had an error rate of 0.27 percent, not bad.

More about this: 10 Degree Tables.

Start / Mid / End Point Differences


Figure: Starting vs Mid vs Ending Points, (Eddie's notes)

Your plotting chart is based on a Lambert Conformal projection, the lines of longitude converge near the poles. Except for the equator, the lines of latitude are not straight, they curve toward the equator. The measurement of your true course depends on where you place the center of your plotter and it does make a difference. In the figure shown, flying from 33°N 160°W to 33°N 150°W should, intuitively, require a 090° true course. The actual course, however, depends on what you want: the starting, mid, or ending course.

Most flight planning services offer either the starting or midpoint courses. Some pilots want to know what their initial course will be, others want the average course on the entire leg. It is a matter of personal preference.

I prefer using the starting course, since that is what the FMS will be showing prior to crossing the waypoint. (Following waypoint passage the course will update to reflect the "current" great circle route to the next waypoint. This course will constantly update as you progress, only reaching the midpoint course when you are actually at the midpoint.)

More about this: Navigation: Initial vs. Midpoint Course.

How to Determine a Magnetic Course.


Figure: Variation Example, (Eddie's notes)

To determine the magnetic course, you will need to add or subtract the variation:

  • Look for the nearest lines of magnetic variation on the chart before and after the midpoint and interpolate if necessary.
  • If the variation is West, add this value to the true course to determine magnetic course. If the variation is East, subtract this value from the true course to determine magnetic course:
  • TC + West Variation = MC
    TC - East Variation = MC

In our example, it appears our midpoint is very close to the "16°W" line of variation and no interpolation is necessary. Based on this we add 16 to our 270 true course and verify that our FMS and flight plan show a 286° magnetic course plus or minus a few degrees.

For a tutorial on magnetic variation, see: Navigation - Direction.

Note: It is not uncommon to find differences of 2 or 3 degrees in the magnetic course determined from a plotting chart and that reported in a computer flight plan. The magnetic variation changes over the years and your chart may be dated or hasn't been updated with the correct variation.

How to determine the position of a VOR radial/DME.


Figure: VOR/DME Plotting, (Eddie's notes)

There are many ways to turn a VOR radial/DME into a latitude and longitude, the best of these may very well be inside your FMS. If you need to do this on a plotting chart, this method works well:

  • Place the plotter hole over the VOR in question and your pencil point in the hole to hold the plotter centered over the VOR. (CON in our example.)
  • Find the nearest line of variation. (6° West in our example.)
  • Rotate the edge of the plotter to 360* in the northern hemisphere, 180* in the southern hemisphere, and then rotate toward the magnetic pole by the amount of the variation. (Rotate 6* to the west, in our example.)
  • Move your pencil from the center hole to the straight line of the plotter that describes the line to the magnetic pole, move the plotter so the straight edge connects that point to the VOR. Draw a line with a flag on it from the VOR in the direction of the pole. Label this line "360° Mag" if you like. (In our example the flag points slightly left.
  • Now return the plotter center hole to the VOR and rotate it in relation to your 360° Mag line by the number of degrees in the VOR radial. Draw a line as you did earlier by placing a tick mark, moving the plotter, and drawing the line. (In our example the line is 15° clockwise from the Magnetic north line.)
  • Determine the distance using the Distance Measuring techniques shown above. Place a tick mark on the VOR line. (015°/85 DME in our example.)
  • The latitude and longitude can be read directly from the chart using the Plot a Position techniques shown above. (55°15'N 8°25'W in our example.)

If you are doing this to check the accuracy of your FMS, you might have easier tools at your disposal. More about this: Navigation Accuracy Check.


Advisory Circular 91-70A, Oceanic and International Operations, 8/12/10, U.S. Department of Transportation

Aeronautical Information Manual

Air Force Manual (AFM) 51-40, Air Navigation, Flying Training, 1 July 1973

FAA Orders 8400 and 8900

ICAO Annex 4 - Aeronautical Charts, International Standards and Recommended Practices, Annex 4 to the Convention on International Civil Aviation, July 2009

Revision: 20141225