Q&A: Measuring the Height of a Flagpole

Question: The guy across the street just installed a huge flagpole in his front yard. I’m not sure it’s legal because it looks way taller than others I’ve seen on private property. We’re not in an HOA but there must be some limits in the building code. So what I want to know is, short of climbing the pole with a tape measure, how can I measure the height of that thing. Thanks. — GA, San Antonio, TX

Answer: Here in Maricopa County (AZ) the ordinance limit is 80 feet, but different places have different codes so I can’t help you there. Check with your local planning and zoning commission. But I can help with how to measure the height of that flagpole. There’s a cheap easy way that can get you within a couple percent of the actual height, and there’s an expensive but highly accurate way. We’ll start with cheap and easy.

The slideshow shows a height measurement trick I learned in Boy Scouts. It’s called the “tree felling method” and versions of it can still be found in the official BSA Handbook and Merit Badge requirements. It works using just your thumb, but holding a stylus of some sort will improve accuracy. Here’s how it works:

• Position yourself far enough from the target that its top is in the same field of view when you look at its base.
• Hold the stylus so that its top is at the same height as the top of the target.
• Slide your thumb along the stylus until it’s at the same height as the target’s base.
• Rotate the stylus 90° while keeping the tip of your thumb at the base of the target.
• Observe where the tip of the stylus points. Look for a marking or object at close to the tip. [This is where an assistant can be useful for marking a point.]
• Use a tape measure (or paces) to find the distance from that point to the base of the target. That distance will be (close to) the same as the target’s height.

For the experiment in the slideshow I used a length of PVC pipe sticking up near the edge of my yard. I got lucky and the stylus matched up with a spot of white quartz on that green rock. I measured the distance from the base of the PVC to the quartz and got 29 inches. When I measured the PVC directly I got 29.5 inches. That’s an error of only +1.7%. Good enough for casual measurements.

Of course, you don’t want to trespass on your neighbor’s property. So do the tree felling from across the street in your own yard. Note where the tip of the stylus points relative to a public sidewalk, curb, or road on your neighbor’s side. You can then measure that distance with a tape or by paces.

If you need greater accuracy you can rent a surveyor transit (or build a theodolite). You need to know the distance (D) to the target, so if you don’t want to trespass you can first use the device to triangulate D from your side of the street. Then you can measure the sight line angle θ and calculate the total height of the target using trigonometry as shown:

People skilled in the use of these devices could measure the height to within fractions of an inch (in the case of a neighbor’s flagpole), but that would be overkill for your purposes. Try the tree felling method, and consider that most flagpoles are sold in increments of 5 feet, so if you round your result to the nearest multiple of 5 you’ll probably have the true height.

Next Week in Sky Lights ⇒ Adaptation or Coincidence?