# What is the spacing of obstacle lights on towers?

Radio antennas and other tall buildings have lights so pilots don't fly into them, especially at night. Is there a fixed spacing so you can determine the height of the tower by counting the lights?

In theory you might be able to make an estimate, but practically speaking it's impossible and a lot easier to just look at a sectional chart.

The FAA has a detailed guide on obstacle lighting standards, which usually applies to structures over 200ft in height but there's a lot of possible variation:

Recommendations on marking and/or lighting structures can vary depending on terrain features, weather patterns, geographic location, and in the case of wind turbines, number of structures and overall layout of design.

The actual number of lights and their placement depends on the type and size of the structure. Appendix 1 of the FAA guide has examples for water towers, radio masts, cooling stacks, bridges, chimneys etc. The vertical light spacing seems to vary from 250-375ft, depending on the structure type, but I didn't go through it systematically.

So if you have plenty of time you might be able to determine the approximate height of a structure by comparing its lighting layout to the guide and estimating based on that, but that's very unlikely to give you an accurate number. For a pilot in flight or planning a flight, that isn't safe or practical and the simple solution is to just look at the VFR sectional chart to get the height. For example, this tower is 1094ft high and the top is at 1994ft above sea level (so the ground level is 900ft above sea level at that location):

• It wasn't clear from my question, but as a non-pilot, I was hoping to be able to determine the height of a mast by counting lights & multiplying. I guess I could at least get a rough estimate by assuming 250ft spacing? – FreeMan Dec 11 '14 at 15:36
• @FreeMan You could do that but the margin of error would be very large. A completely different idea is to use a smartphone app. – Pondlife Dec 11 '14 at 15:45
• Great tip! Hard to do, though, while driving, especially at night. I guess I'll just count and multiply, since precision isn't that critical from the car. It'll just be a good guideline. If I get really picky, I'll be sure to look it up later. – FreeMan Dec 11 '14 at 17:12
• If you want to go look them up later, skyvector.com has sectional charts that you can zoom in and out of similarly to Google Maps. – reirab Dec 11 '14 at 22:15

There are regulations on the color, brightness, and location of lights, but counting them doesn't tell you how high it is. You can tell the height by looking on a sectional, which lists the heights of obstacles next to the depiction.

• Thanks, quite disappointing for a non-pilot wondering how tall that antenna really is. – FreeMan Dec 11 '14 at 14:15
• ok, i didn't know you weren't a pilot, otherwise I would have given you @pondlife's answer :) – rbp Dec 11 '14 at 15:47
• My inner 6-year-old still flies F-16s, does that count? ;) – FreeMan Dec 11 '14 at 17:12
• @FreeMan You can easily obtain a reasonably accurate height measurement for any given tower by the stick method. – Stefan Mesken Jul 30 '17 at 13:56