Windy Wednesday: The distance to the horizon for Dummies

This is a mini-post ahead of Saturday’s. The distance to the horizon varies from where you are standing on land. There are any number of ways to estimate distance to horizon.

  • If you’re standing at sea level, divide your height in half. So if you’re 5’6″ (5.5 feet) tall, the distance to the horizon is only about 2.75 miles.
  • Use an online distance to horizon calculator (link below)
  • Use the AIS Marine Traffic website which will give the position of most vessels you can see out on the water.
  • Use geographical features e.g. far off headlands, a map and a piece of string

I whale watch, most often from Dunabrattin at Boatstand, from the first car park going west. This is 22 metres (72 foot) above sea level. (I can find elevation using free online elevation finders. There’s a link to one below). Helvick head is 20 km from Dunabrattin as the crow flies on a map. So I can see at roughly 20km from 22 metres above sea level at Dunabrattin.

For more accuracy we can use formulas. This is the most simple one:

Multiply your height in metres above the ground by 13**, and take the square root of that.

So for Dunabrattin…

  • Height above sea level = 22 metres
  • Add my height up to to eye level = 23.6 metres
  • Multiply by 13 metres** (1.5 foot if using imperial) = 306.8
  • Calculate the square root (a calculator or Google does this for you) = 17.5157072366

According to this the horizon from straight out from Dunabrattin is 17km distant.

From Gallwey’s Hill in Tramore the horizon is 19km distant

From the Tramore Racecourse roundabout the horizon is 23km distant

But… that’s just the horizon. At 190-260 metres above sea level, the height of the proposed turbines is no small consideration. It seems to me that even a windfarm at 22km will be well visible from much our coast. And at 5 km or 10 km distance?Yikes.

Many of you will also be aware that it’s possible to see things over the horizon due to effects of the light. That is why one far off container ship may seem tiny while the upper decks of another one even further over the horizon is five times larger.

See you on Saturday when I’ll try and describe the proposal for the Waterford coast in 500 words or less. Any corrections, – especially to my equation! – comments etc. can be sent via the contact form on this blog.

**I am not sure where the 13 comes from. I think its something to do with triangles and the radius of the earth but as I have an allergy to sums my head imploded before I could read much more.


Elevation Finder

Marine Traffic

Distance to Horizon Calculator