Terrain Clearance

Terrain elevation comes from the Copernicus DEM (90 m resolution) through the Open-Meteo Elevation API. PlaneWX samples elevation along your filed route, not from obstacle databases.

• Samples are spaced about every 5 NM along the route polyline (at least two points).
• Requests batch up to 100 coordinate pairs; longer routes split across batches.
• Each sample stores lat/lon, elevation in feet MSL, and distance along the route.
• Clearance at a sample is cruise altitude minus terrain elevation at that point (feet AGL along the profile).

When a full server pre-flight runs and a terrain profile is available, PlaneWX finds the lowest clearance along the route and classifies it:

• Under 1,000 ft AGL at the tightest sample: Terrain Clearance exceedance (the pre-flight check fails).
• 1,000 ft up to under 2,000 ft AGL: Terrain Clearance caution (the check still passes, with a WX Score penalty).
• 2,000 ft AGL or more: no Terrain Clearance caution from this check.

The caution message names the clearance in feet AGL, the distance along the route where it is tightest, the route terrain class (flatland, hilly, mountainous, or high-mountain), and the highest terrain on the profile. You will see it in the pre-flight / caution strip on the briefing.

• Marginal clearance (1,000 to under 2,000 ft AGL): yes. The server adds a caution penalty of about 15 to 25 points (higher when closer to 1,000 ft) to the pre-flight caution total, which is subtracted from the WX Score. All caution penalties together are capped at 45 points.
• Insufficient clearance (under 1,000 ft AGL): the pre-flight check fails. A failed hard pre-flight sets the WX Score to 0.
• When the flight is more than 12 hours out and the full pre-flight check has not run, the server adds no terrain clearance caution or penalty. The AI analysis gets the terrain profile and may deduct for terrain itself.
• When the server has evaluated terrain clearance, matching AI terrain clearance deductions are removed so the same issue is not counted twice.

Separately from the caution bands above, PlaneWX computes a minimum safe altitude (MSA) from the same terrain profile. The briefing uses it as the floor for its altitude suggestions, and the trip form shows it next to the Cruise Altitude picker with a warning when you pick an altitude below it. The MSA uses 14 CFR 91.177 clearance numbers:

• If route relief (highest minus lowest sample) is greater than 2,000 ft, clearance above the highest terrain is 2,000 ft (treated as mountainous for this heuristic).
• Otherwise clearance above the highest terrain is 1,000 ft.
• The raw MSA (highest terrain + clearance) is rounded up to the next 500 ft.
• Briefing altitude suggestions never go below 2,000 ft, and never below the computed MSA when a profile exists.
• If the terrain profile is missing, briefing altitude suggestions use a conservative 5,000 ft floor and show a Verify terrain clearance note.

• Man-made obstacles and wires are not included. Towers, masts, power lines, and similar obstacles are outside the DEM profile.
• No profile means no check. Routes shorter than 5 NM skip terrain, the terrain request gives up after 8 seconds, and a failed request leaves no profile. In those cases the briefing runs no Terrain Clearance check: no caution and no penalty.
• Clearance is checked only at the sample points, about every 5 NM. Terrain between samples can be higher than either neighbor.
• Empty or failed terrain profiles skip MSA from DEM data; altitude suggestions then use the conservative floor above.
• This is decision support for your planned cruise altitude along the sampled route. It is not a substitute for charts, obstacle data, or your own terrain awareness.

• Altitude Advisor (winds, clouds and turbulence by altitude)
• WX Score Explained
• Understanding Your Briefing