ARP Centroid Calculator — Tool Guide

Who It's For & When to Use It

Airport planners siting a proposed Airport Reference Point for a new airport, a relocated or extended runway, or a master-plan alternative where the published FAA ARP no longer reflects the runway layout. Use it when you need a defensible center-of-runways coordinate for an ALP, a planning study, or a design report — when comparing how an added or lengthened runway shifts the ARP, or when you have runway-end coordinates in hand and need the length-weighted centroid without building a spreadsheet. For an existing airport's official ARP, reach for the ARP Lookup instead.

How to Use It

1. Import runway ends from an airport (optional). Enter an FAA LID or ICAO identifier in the airport bar and the tool pulls each runway end's coordinates from FAA NASR, prefilling the rows. If only the non-authoritative OurAirports source covers the field, a continue/stop banner appears before any data is used.
2. Enter each runway's two end coordinates. For every usable runway, type the latitude and longitude of End 1 and End 2 in decimal degrees, with an optional designation like 17L/35R that labels the runway in the result.
3. Confirm the coordinate format. The inputs take signed decimal degrees — negative for south latitude and west longitude. If your survey is in degrees-minutes-seconds, convert to decimal first; the tool reports the result in both decimal degrees and DMS.
4. Add or remove runway rows. Use + Add Runway for every runway that should contribute to the centroid, and remove any row you don't need. Rows left completely blank are ignored at compute time, and the two ends of a runway must be distinct coordinates.
5. Compute the ARP. The tool finds each runway's centerline midpoint (the arithmetic mean of its two ends), weights the midpoints by runway length — haversine great-circle length on a spherical-earth model, reported in US survey feet — and averages them. Because the weights are relative, the sphere radius cancels out of the result.
6. Read the result. The computed ARP latitude and longitude appear in decimal degrees and DMS. The Decision-Engine block below lists each contributing runway with its length and relative weight, the total weighted length, the controlling standard, the assumptions, and the verify-before-use warning.
7. Copy or hand off. The copy buttons put either just the coordinates or the full block — method, per-runway weights, data-vintage note, and disclaimer — on the clipboard. Cross-tool links carry the ARP into the State Plane Zone Lookup, or into the ARP Lookup to compare against the published value.

Key Features

• Length-weighted centroid, not a plain average. Midpoints are weighted by runway length so a long primary runway pulls the ARP more than a short crosswind runway — the geometric center of all usable runways the FAA method intends, not an unweighted mean of endpoints.
• NASR runway import. An optional airport bar prefills the runway rows from FAA NASR runway-end coordinates by LID/ICAO, with a data-source badge and cycle date, so you don't have to transcribe coordinates by hand.
• OurAirports fallback banner. When NASR doesn't cover a facility, the tool surfaces the mandatory non-authoritative-source warning with continue/stop, never silently using community data.
• Decision-Engine result block. Controlling standard, assumptions, per-runway logic (length, weight, midpoint), warnings, and data vintage are laid out beneath the coordinates — ready-to-cite language for a narrative.
• Dual coordinate output. The ARP is reported in signed decimal degrees and in degrees-minutes-seconds, both copy-ready.
• Cross-tool hand-off. One-click links carry the computed ARP into the State Plane Zone Lookup (to detect the SPCS83 zone) and the ARP Lookup (to compare against the official published value).

FAA References

• AC 150/5300-13B Chg 1, Airport Design — governing standard; defines the Airport Reference Point as the approximate geometric center of all usable runway surfaces (¶1.8.1).
• AC 150/5300-13A ¶107 — legacy computation method carried forward: the ARP is the length-weighted average of runway centerline midpoints.
• Runway lengths are computed by the haversine great-circle formula on a spherical-earth model (R = 6,371 km) and reported in US survey feet; because the midpoint weights are relative, the sphere radius cancels out of the result.

Limitations & Disclaimers

AvPlot is technical planning production support — accurate enough for design reports, planning studies, and ALP narratives. It is not a replacement for stamped engineering or construction documents. As the tool itself states: this is a computed planning estimate, not a substitute for a stamped survey or the published FAA ARP — verify against survey-grade data before official or regulatory use.

• The result is only as good as the runway-end coordinates entered; imported NASR coordinates carry their cycle date, and OurAirports fallbacks are non-authoritative.
• The spherical-earth haversine model serves planning precision; it is not a geodetic-grade ellipsoidal computation.
• The tool computes the centroid of the runways you supply — it does not decide which runways are usable, nor does it reconcile the result against an officially published ARP.

Related Tools

• ARP Lookup — the official FAA-published Airport Reference Point for an existing airport, to compare against the computed centroid (guide).
• State Plane Zone Lookup — detect the SPCS83 zone for the computed ARP coordinates (guide).
• Runway Linework Generator — draw the runway and its safety surfaces as CAD-ready linework around the layout (guide).
• Unit Converter — convert coordinates, distances, and other planning units when prepping inputs (guide).

Tips & Best Practices

• Import from NASR when you can — it transcribes both ends of every runway with their cycle date, removing a common source of coordinate-entry error.
• Include every usable runway, not just the primary: the centroid is length-weighted, so leaving out a crosswind runway moves the ARP.
• Convert DMS survey coordinates to signed decimal degrees before entry, and keep south latitude and west longitude negative.
• Compare the computed centroid against the published ARP — a large gap usually means a runway changed since the last FAA publication, which is exactly what you'd document in the narrative.
• Copy the full block (method + disclaimer) into your project notes; it records the contributing runways, their weights, and the citation behind the coordinate.