Visual Systems Lab · Calgary Vision Centre

Longest Sightline Finder

Standing on flat ground, you can see about 4.7 km before the curve of the Earth gets in the way. From the top of a 236 m skyscraper, like Calgary’s Bow Tower, it’s about 54 km. Stand on a mountain and look at another mountain, and it gets a whole lot further. Type a place, tap the map or use your location, and the finder heads for the highest spot nearby and works out how far you could see from there on a clear day.

Or tap anywhere on the map. Coordinates and Google Maps links work too.

Off: only places open to the public are starting points, such as the ground, lookouts and observation decks. On: building roofs count too, for a view most people can’t actually get.

More options

Height is in metres: 1.7 is standing on the ground. Add more for a balcony or tower, or choose “Stay exactly here” to use only your spot.

The longest view

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Or try nearby
The tallest building and highest peak near your place will appear here.
Where the view starts Line of sight Farthest ground visible in each direction

Side view

Your eye is on the left. The red line is your line of sight skimming over the hills, and the dashed line is sea level curving away with the Earth.

How far you can see in each direction

  • Appears after a search.
How it works

The four ingredients. As the article explains, the longest views need a tall place to stand, a tall target, the right air and a clear line of sight. The finder climbs to the highest spot nearby and checks every peak in range for the first two. “Air bending light” covers the third: cold air near the ground bends light around the curve, which is why the 443 km Pyrenees-to-Alps photo needed better than normal conditions. The haze check covers the fourth.

Finding the starting point. Around the place you choose (10 km unless you change it), the finder looks for the highest spot open to the public: the top of the ground, checked point by point, or a public observation deck. Tall buildings still block the view. With “Imagine I could stand on any rooftop” on, the roof of any building 60 m or taller counts too (where a mapped height runs to the tip of a spire, as it does for One World Trade Center, the roof is used). For 53 decks in 42 big cities (from One World Observatory and the CN Tower’s SkyPod to the Burj Khalifa and Tokyo Skytree), the height of the highest public level comes from published sources and is built in; elsewhere a deck is taken as 85% of its tower’s height. All of these come from OpenStreetMap and free elevation data. Hilltops covered in houses are skipped, and so is a starting point in the middle of town, because nearby buildings would block it at street level.

Working out the view. From that spot it looks out along 1,440 compass directions. Along each one it checks whether each piece of ground rises above everything in front of it. The farthest piece that does is how far you can see that way, and the longest of all 1,440 is the answer.

The curve of the Earth. Distant ground sinks below a level line from your eye by about d²/2R, where d is the distance and R is the Earth’s radius. That is why far mountains disappear from the bottom up. Air bends light slightly downward, which lets you see a little farther, more so over cold ground.

What’s counted. Hills, mountains and buildings 60 m or taller are counted. Trees and ordinary buildings are not, so check that nothing close by is in the way.

Seeing it with your eyes. The answer assumes clear air. Haze fades distant things: contrast drops as e−3.912·d/V (Koschmieder), where V is how far you can see that day, and the eye needs about 2% contrast. “How much shows” compares the visible part of the far point with the 1 minute of arc that 20/20 vision resolves.

Data. Elevation: Terrain Tiles on AWS Open Data (SRTM, Canada’s CDEM, the US NED and others), about 20–50 m detail nearby and a few hundred metres far away. Places, peaks and buildings: OpenStreetMap. Town names: Open-Meteo’s geocoder (GeoNames) and OpenStreetMap Nominatim. Sea-floor depths count as sea level. The elevation data has a few single-pixel faults, spikes hundreds of metres above everything around them; each one is replaced by the height of the ground beside it.

Finder version 2026-10-12.3