Trace your horizon with your phone
Sweep your phone's camera once round your trees and rooftops. On a computer you can draw it on the chart or type the directions that are blocked. Download a horizon file for N.I.N.A., or see your skyline against the seven-night forecast. No account needed.
The tracer needs JavaScript and a phone with a camera and a compass.
What a horizon file is
A horizon file lists how high your skyline reaches in each compass direction, one azimuth and altitude per line. In N.I.N.A., choose it under Options, General, Custom Horizon: your skyline then appears on every altitude chart, and the advanced sequencer can wait for a target to clear it. A wall or a roof corner stays a straight edge in the file instead of becoming a slope. You can trace it with your phone, draw it on the chart or type the directions that are blocked, and the file comes out the same way.
How accurate a phone compass is
Your phone measures height well, to about a degree. The compass is the weak part: next to a car, a shed or anything made of iron it can be several degrees out without showing any sign of it. So once you have traced, the page asks you to aim at the Sun or the Moon. Their positions are known exactly, so the difference is your compass error, and your skyline is turned to remove it.
What happens to your location
Your location turns the compass from magnetic north to true north, worked out on your phone. To find the Sun or the Moon, the page sends your position rounded to about 10 km, and nothing about your trace is stored on our side. The trace stays on your phone until you start again or save it to an account. The privacy notice has the details. This page counts visits with Google Analytics; it never sends your location or your trace to Google.
Using it with the forecast
Save your skyline to an account and The Quiet Sky counts only the hours each target is really in view: in night scores, usable windows, target suggestions and night plans. A 2° safety margin is added there, for not setting up in exactly the same place every night. Open the forecast.