Scan the Horizon

What does Scan the Horizon do?

It draws the view around you the way the sign at a lookout does: the hills, mountains and coast in grey, with the towns, peaks, islands, lakes and landmarks named and how far away each one is. Hold your phone up and the drawing turns as you turn. Tap a name for its height or population and a link to Wikipedia; the Detail slider chooses how many names to show.

How does it know what is on the horizon?

It downloads elevation data for the land around you, out to 150 km (250 km near high mountains), and works out, bearing by bearing, which hills and ridges stand against the sky. It allows for the curve of the Earth and for refraction, the slight bending of light by the air that lets you see a little further than a straight line would. Each named place is then checked against the ground in front of it: a town behind a hill is left off, and a big one gets an arrow over the hill that hides it.

Why isn't a hill I can see named, and what about trees and buildings?

Names come from GeoNames, an open database of places. A hill that isn't in it isn't named, and a small one may only be named with the Detail slider turned up; the List shows every name the slider allows. The elevation data is the shape of the land rather than what stands on it (bare ground where there are detailed national surveys, as in Australia and Switzerland), so the view is drawn as if trees and buildings were glass: a town hidden from you only by buildings or a forest is still shown.

Does it work indoors, or without a compass?

Yes. The drawing is made from data, not from the camera, so it works indoors and at night. With a compass the drawing turns as you turn; steel and electronics indoors can throw a compass off, so drag the drawing to line it up. Without a compass, on a computer for example, drag to look around and scroll or pinch to zoom.

How accurate is it?

A phone compass is good to a few degrees, so line the drawing up with something you can see by dragging it. The elevation data is detailed to a few metres in parts of Australia and Switzerland and to about 30 m elsewhere, so a sharp peak can come out a little low; where a peak's official height is known, it is drawn to that height. Whether something far away just shows over a ridge can change with the weather, because the air bends light more on some days than others.

What does it send, and where?

Your exact position stays on your phone. To draw a view it downloads elevation tiles for the area around you from Mapterhorn (or from Amazon Web Services if Mapterhorn doesn't answer) and place names from this site, so those servers can tell roughly where you are and see your IP address. A place you search for is looked up by Photon (komoot) or Open-Meteo. A link you copy carries the place rounded to about 1 km, unless you choose the exact one. There are no cookies or accounts; your settings are kept in this browser only.

Does it work on iPhone, Android and computers?

Yes, in Safari, Chrome, Firefox or Edge. On a phone, tap Use my location and allow location and motion when asked, and the drawing turns with you; on an iPhone, hold the phone upright facing the view, or flat, for a moment so the compass can settle. Camera shows the drawing over the live picture. On a computer, drag the drawing to look around and scroll to zoom.

Does it work offline?

Partly. Once it has been opened with a connection, the page works without one, and views you have already opened are kept on your phone for a while, so they can be drawn again without signal. Looking from somewhere new needs a connection, for its elevation data and place names, and so does the place search. To keep it handy, add it to your home screen: in Safari tap Share, then Add to Home Screen; in Chrome, choose Install app or Add to Home screen from the menu.