Drawing a local horizon file

A horizon file records how high the ground, trees and buildings rise around your site, so the calculators can tell you when something is blocked by terrain rather than by your own observatory. It is a plain text file you can write in any editor.

The format

One line per point, two numbers separated by a space: azimuth then altitude, both in degrees. Azimuth runs clockwise from due north — 0° north, 90° east, 180° south, 270° west. Altitude is how far above level the obstruction rises at that bearing.

# CygnusWave sample horizon profile
# azimuth(deg) altitude(deg) - one pair per line, 0 = due north, increasing eastward
0 12.2
22 17.6
37 32
62 39.6
84 24.8
88 22.9
94 19.8
104 15.8
126 15.5
138 16.5
150 17.5
165 16.5
…

Points do not need even spacing. Put them where the skyline changes — a few across a flat field, many across a ragged treeline. Anything between two points is interpolated in a straight line, so a sharp edge needs a point on each side of it.

Rule	Detail
Azimuth range	0 to 360. 0 and 360 are the same bearing
Order	Increasing azimuth. Sort before saving if you measured out of order
Altitude	Degrees above level. 0 is a clear flat horizon
Separator	A space. Commas and tabs also read correctly
Comments	Lines beginning `#` are ignored
Extension	`.hrz` by convention, but any text file works

Measuring your horizon

Find north. Use the polar-alignment mark on your pier if you have one, or a compass corrected for local magnetic declination.
Stand where the telescope will be, at roughly the height of the optical axis. A horizon measured from ground level will read too high.
Work round in steps — every 10° or 15° is enough for open ground. Add extra points either side of anything with a sharp edge: a roofline, a mast, the corner of a building.
Measure the altitude of the skyline at each bearing. A phone clinometer app is accurate enough; a smartphone level app held against a straight edge also works.
Record the highest point at each bearing, not the average. It is what actually blocks you.

Trees grow and lose leaves. A profile measured against bare winter branches will not describe the same sky in July, so it is worth re-measuring a tall treeline every few seasons.

A worked example

This is the sample profile plotted the way the calculators read it. The shaded area is sky you cannot reach; the dots are the measured points, joined by straight interpolation.

29 points. The ridge near 310° rises to 53.2° — over half the sky lost to the north-west — while the eastern side stays near 15°.

Download the sample file

Using it

In either calculator, open Local Horizon, switch from Flat to Custom, and paste the contents into the box. The profile is combined with your walls and roof: whichever is higher at a given bearing is what blocks you, and the tool names which one it was.

You can also go the other way. Once your building is described, the roll-off-roof tool will export the combined limit — walls, parked roof and terrain together — as a horizon file for your planning software.

A pointing below your horizon is reported, not corrected. The tool tells you it is blocked and leaves your figures alone, so you can see how far below the skyline a target sits.

Common mistakes

Magnetic north instead of true north. Declination reaches 20° in parts of the world, which rotates the whole profile.
Measuring from the ground. Stand at the height of the optical axis.
Too few points across a sharp edge. Straight-line interpolation will cut a corner off a building and quietly hand you sky you do not have.
Counting your own walls twice. The horizon file is for terrain and neighbouring structures. Your own walls and roof are described in the building card and added separately.