Custom horizon file builder.

What a horizon file is

It is a list of compass bearings with the height of the skyline at each one. Planning software draws it on the altitude chart and uses it to decide whether a target is really up. The N.I.N.A. documentation describes the format as “a simple style of azimuth altitude pairs,” says gaps between azimuths are interpolated, and says a minimum of two pairs is needed. Its example starts with a comment line, “# Az Alt,” and runs from 0 to 360 degrees, finishing on the same height it started on. Telescopius says to use the same Az and Alt layout as N.I.N.A.

What this page fixes

• A header such as “Az, El,” which the Telescopius developer says caused an invalid-format error when copied from a phone camera tool, is dropped.
• Commas and semicolons become spaces, since Telescopius needs the commas removed.
• Rows are sorted by azimuth. A repeated azimuth keeps the last reading and the page counts how many it replaced.
• If there is no point at 0 or 360 degrees, one is added by interpolating across north, so the circle closes smoothly.
• Altitudes outside 0 to 90 and azimuths outside 0 to 360 are rejected with the line number, not guessed at.

Which way is zero

The N.I.N.A. page does not say where azimuth 0 points or which way it increases. This page assumes the usual compass bearing: north is 0, east is 90, south is 180 and west is 270. Check your own program’s convention before relying on the file, and if it differs, convert your readings before pasting them here.

Taking the readings

Stand where the telescope will sit, point a compass or phone camera at the highest part of the skyline every 5 to 15 degrees of bearing, and write down the altitude. Put extra points where the skyline changes quickly, such as either side of a chimney or a tree. Because gaps are interpolated in a straight line, a rooftop between two readings is rounded off, so the more points the better near obstacles.

Worked example

The sample readings are the numbers from the example horizon file in the N.I.N.A. documentation, retyped with commas and a “Az, El” header, as a phone app might give them, and without the final 360-degree line. The page drops the header, removes the commas, and adds “360 14” because the file starts at 14 degrees at north. That gives 17 pairs. For a target at azimuth 100 degrees and altitude 40 degrees: the readings either side are 90 degrees at 70 degrees and 105 degrees at 35 degrees, so the horizon at 100 is 70 − 35 × (10 ÷ 15) = 46.7 degrees, and the target at 40 degrees is blocked.