Learn what DME distance on an aviation chart measures, why it differs from GPS distance, and how to identify and use the correct station.
On an aviation chart, a DME distance is the aircraft’s slant-range distance, in nautical miles, from the specific DME ground station named for that fix or procedure. It is not normally distance to the airport, runway threshold or next waypoint, and it is not exact horizontal ground distance.
In our Aviation & Real-World Flying coverage, we apply one practical rule: identify the DME source before interpreting the number. DME supplies distance only; it does not tell you which direction to fly. Our comparison of DME distance, VOR bearing and NDB direction explains how those functions differ.
How is DME distance shown on a chart?
A charted DME fix couples a distance with a named or procedure-defined DME station. Exact notation varies between chart publishers, but typical examples include:
ABC D8.0or8.0 DME ABC: the fix occurs when the DME indication from station ABC is 8.0 NM.12 DME ARC: maintain approximately 12 NM DME from the specified station while flying the arc.DME required: the procedure depends on DME information or an authorised substitute.
The source may be printed beside the fix, identified in a chart note such as DME distances from ABC, or established by the procedure’s navaid information. Never assume the nearest VOR supplies the distance; a course can come from one facility while DME comes from another.
Not every distance printed on an approach or en-route chart is DME. Numbers between waypoints may represent along-track mileage. Treat a number as DME only when its notation, associated station or chart legend identifies it that way.
Why can DME differ from GPS or map distance?
DME measures the direct three-dimensional distance between the aircraft and the DME antenna. This is called slant range and can be approximated by DME = √(horizontal distance² + vertical separation²).
| Aircraft position | Horizontal distance | Height above antenna | Approximate DME |
|---|---|---|---|
| Far from station | 20 NM | 1 NM | 20.02 NM |
| High and close | 1 NM | 1 NM | 1.41 NM |
| Directly overhead | 0 NM | 1 NM | 1.00 NM |
The vertical figure is height above the DME antenna, not simply altitude above sea level. Slant-range error is negligible when the aircraft is far from the station, but it becomes significant when high and close. That is why a DME display may not reach zero while passing overhead.
How should pilots use a charted DME fix?
A charted DME fix is used by reading the correct station’s distance while following the procedure’s separate lateral guidance.
- Find the source. Read the procedure title, navaid box, fix label and chart notes to establish which DME facility supplies the distance.
- Select the correct equipment source. Tune the published facility where manual tuning is required. On an ILS/DME, selecting the localiser frequency commonly pairs the associated DME channel automatically, although avionics design varies.
- Verify the facility. Check the displayed identifier and any required station identification rather than trusting the frequency alone. Our guide to tuning, identifying and using a DME receiver covers the instrument workflow.
- Follow the associated course. One DME value describes a range around the station, not a unique position. The chart normally combines it with a localiser, radial, bearing, track or other defined path.
- Act at the published value. At the stated DME, cross the fix, begin the turn, apply the altitude restriction or start the descent exactly as the procedure requires.
For an arc, make small heading corrections to keep the indicated distance near the published value. Simmers can use our walkthrough for practising a charted DME arc in a flight simulator rather than duplicating the full technique here.
Does DME show distance to the runway?
DME shows distance to its antenna, not automatically to the runway threshold or touchdown zone. An ILS DME antenna may be offset from the threshold, so the indication at touchdown can be above zero. Use charted threshold-crossing or runway distances only when the chart explicitly defines them.
Can GPS replace charted DME?
Not automatically. In real-world operations, approved area-navigation equipment may be authorised as a DME substitute, but that depends on the regulator, aircraft approval, operating rules and any restrictions printed on the procedure. A tablet moving map or consumer GPS is not an approved substitute.
In a simulator, GPS distance to the DME station is a useful cross-check, but it may show horizontal map distance rather than DME slant range. Select the actual published station—not the airport reference point—and expect the largest disagreement when high and close to it.
Which DME chart-reading mistakes cause problems?
The most common errors come from using the right number with the wrong reference.
| Mistake | Correct interpretation |
|---|---|
| Reading distance to the airport | Use distance from the chart’s named DME antenna. |
| Assuming every profile distance is DME | Check whether it is labelled as DME or merely along-track mileage. |
| Using the navaid that supplies the course | Confirm whether a different facility supplies the DME. |
| Expecting zero directly overhead | Allow for vertical separation and slant range. |
| Treating a DME value as a complete fix | Combine the distance with the charted course, radial, bearing or track. |