ATCT Line-of-Sight — Tool Guide

Who It's For & When to Use It

Airport planners running tower-siting studies and feasibility screens. Use it when comparing candidate ATCT sites, when documenting the visibility case for a preferred site in a siting study or planning report, or when testing how a proposed cab height performs against the full set of movement-area targets — including how nearby structures or terrain points shadow individual sight lines. It is most useful at the screening and narrative stages, where you need a defensible, citation-trailed visibility result without standing up a CAD sight-line model.

How to Use It

1. Identify the airport (optional but recommended). In the Airport (for zone detection & CSV export) bar, type a 3–5 character FAA LID or ICAO (e.g. KDEN) and press Look Up. AvPlot pulls the airport from FAA NASR, shows its name, ident, and elevation, and detects the State Plane zone from the ARP — auto-filling the zone field for lat/lon input. If only non-authoritative OurAirports data is available, a banner appears; choose Continue with OurAirports data or Stop and enter a zone code by hand.
2. Define the candidate tower site (Step 1). Pick the Coordinate system — Northing / Easting (ft) or Latitude / Longitude — then enter the tower position in the matching fields. Add Tower ground elev (ft MSL) and Eye height AGL (ft) (the candidate cab eye level). Optionally enter Eye above cab floor (ft) so the solver can report a cab-floor height, and set the Shadow corridor half-width (ft) (default 50 — an analysis parameter, not a regulatory value). When you choose lat/lon, a State Plane zone field appears; it is auto-filled from the airport ARP when available, or enter it manually (e.g. CO-C).
3. Enter targets and obstacles (Step 2). Two paths share the same engine, selected by tab:
   • Enter a few targets — add rows in the Targets grid (ID, description, position, elevation MSL; up to 200), the optional Obstacles grid for shadowing checks (ID, description, position, top elevation MSL), and a Lateral-discrimination pairs box (one pair per line, e.g. T1, T2) for the two-point check.
   • Upload survey CSV — download the template, then choose a flexible-header CSV (or tab-delimited file). Recognized columns: ID, Type (target/obstacle), Lat+Lon or Northing+Easting, Elev, and Description. This path requires an airport (for zone auto-detect) or a State Plane zone code, plus the tower from Step 1; pick an Export format and Project name.
4. Run the analysis. In the manual path, press Analyze line-of-sight. In the CSV path, press Process & download to generate and download the chosen export directly.
5. Read the verdict and per-target results. The Result Block leads with the site verdict — Incidence PASS/FAIL, Lateral discrimination PASS/FAIL/n-a, Min required eye AGL, and Required cab floor AGL when a cab-floor offset was given. Below it: a site summary, the controlling target that drives the minimum cab height, the controlling standard, assumptions, any warnings, the data vintage, and a scope note. A per-target line-of-sight table lists distance, incidence angle, the minimum, pass/fail, required eye AGL, and shadowing per target; a lateral discrimination table follows when pairs were supplied.
6. Export and capture. Use Copy results to put a tab-delimited summary on the clipboard, or use the CSV path's export to download a results CSV, a geo-referenced DXF, or a KMZ. Save Project stores the inputs to your account; work also autosaves as you type.

Key Features

• Airport Identity Bar. One NASR lookup populates the airport name, ident, and elevation and auto-detects the State Plane zone from the ARP, so lat/lon coordinates convert transparently. An explicit zone code can be entered to skip the lookup, and the OurAirports fallback banner gives a continue/stop choice when only non-authoritative data exists.
• Two entry paths, one engine. Type a few critical targets by hand, or upload a full-survey CSV with flexible headers (comma- or tab-delimited; lat/lon or northing/easting, mixed freely). Both run the same Order 6480.4C analysis pipeline.
• LOS angle of incidence from the cab eye to each target, checked against the Order 6480.4C minimum of 0.80°, with the citation carried on the result.
• Two-point lateral discrimination between planner-selected critical pairs, against the 0.13° minimum.
• Minimum cab eye-height solver — the lowest eye elevation that satisfies the incidence criterion at every target, with an optional eye-above-cab-floor offset to express the result as a cab-floor height, and the controlling target named in the output.
• Obstacle shadowing — point obstacles within the plan-view corridor of a sight line are flagged where they shadow it. The corridor half-width is an analysis parameter (default 50 ft), echoed in the assumptions, not a regulatory value.
• Decision-Engine output. Every run returns a structured Result Block — Result (verdict), Controlling standard, Assumptions, Logic (controlling target), Warnings, and Data vintage — so the visibility case is defensible and traceable.
• Exports. A results CSV that mirrors the input IDs with result columns appended; a geo-referenced DXF on dedicated layers (ATCT_TOWER, LOS_LINES, OBSTRUCTION_POINTS, SHADOW_LINES); or a KMZ with the same four groups as folders for Google Earth. The obstruction and shadow layers appear only when obstacles are present.
• Project save & autosave. Inputs persist to your account by project name and autosave continuously while you work.

Object discrimination per Order 6480.4C Table 4-1 is intentionally not computed — that determination is deferred to the FAA Tower Visibility Toolkit.

FAA References

• FAA Order 6480.4C, Airport Traffic Control Tower Siting Process — governing standard for the line-of-sight criteria: angle of incidence (0.80° minimum), two-point lateral discrimination (0.13° minimum), minimum cab height solve, and obstacle shadowing.
• FAA Tower Visibility Toolkit — the FAA companion analysis for object discrimination (Table 4-1), which this tool intentionally does not compute.
• FAA NASR 28-day subscription — source of the airport reference point used to derive the State Plane zone for coordinate conversion, with the active cycle carried on the result.

Limitations & Disclaimers

AvPlot is technical planning production support — accurate enough for siting studies, planning reports, and ALP narratives. It is not a replacement for a stamped ATCT siting study or for the FAA's own siting determination. Verify all results against current FAA publications before use in official submittals.

• Single candidate tower site per analysis — geometric visibility criteria only, not the full Order 6480.4C siting process (cost, operational, and environmental factors are out of scope).
• Object discrimination (Table 4-1) is not evaluated — use the FAA Tower Visibility Toolkit for that criterion.
• Obstacle shadowing uses point obstacles within a planner-set corridor width; it is a screening check, not a terrain or 3D obstruction model.
• Coordinate conversion depends on the correct State Plane zone — confirm the auto-detected zone, or supply one, before relying on lat/lon results.

Related Tools

• Obstruction Analysis — evaluate obstacles against Part 77 and approach surfaces; uses the same shared obstacle schema.
• ARP Lookup — pull the airport reference point and elevation from NASR before siting work (guide).
• State Plane Zone Lookup — identify or verify the State Plane zone the analysis will convert into.

Tips & Best Practices

• Look up the airport first. A single NASR lookup auto-detects the State Plane zone, so lat/lon targets convert without manual zone entry — and the airport ident drives the CSV georeferencing.
• Keep coordinate systems consistent. The tower and grids share the selected coordinate system; for lat/lon, confirm the zone field is populated before analyzing.
• Use the controlling-target line to iterate. The Logic section names the target that drives the minimum cab height — the fastest place to look when a candidate site fails or the required height is higher than expected.
• Tune the shadow corridor deliberately. The half-width is a screening parameter, not a regulatory value; widen it to be conservative, and remember it is echoed in the assumptions for your record.
• Prototype manually, then scale with CSV. Test a few critical points by hand to confirm the setup, then upload the full survey CSV and export DXF or KMZ for the report.
• Pair the export with the Tower Visibility Toolkit. Because object discrimination is out of scope here, run that criterion in the FAA toolkit and cite both results together.