True north, magnetic north and what the line-of-sight check means

A bearing is a compass direction measured clockwise from north: 0° is north, 90° east, 180° south, 270° west. The tool shows the bearing from your antenna to the transmitter two ways because there are two norths.

True north is the direction to the geographic North Pole, the north that maps, satellite imagery and GPS use. Magnetic north is where a compass needle points, which is the Earth's magnetic pole, and that sits some distance from the geographic pole. The angle between the two at your location is called magnetic declination.

In Australia the difference is large enough to matter. It is about 12 to 13 degrees east on the Sydney and Brisbane coasts, about 12 degrees in Melbourne, about 8 degrees in Adelaide, about 15 degrees in Hobart, and close to zero in Perth, where it is slightly west. Point an antenna in Sydney using a compass but a true bearing and you will be about 13 degrees off, enough to lose signal on a long or marginal path.

The tool calculates the declination for your exact location and today's date from the World Magnetic Model, then shows the true bearing and the magnetic bearing side by side. They describe the same direction; they just assume different norths.

Use the magnetic bearing with anything that has a needle or a magnetometer: a hand compass, or a phone compass app, or the compass mode built into the tool.

Use the true bearing with anything that is a map: Google Maps or Apple Maps in satellite view, a printed map, or a landmark you can identify on a map. A simple method is to open satellite view centred on your roof, find a landmark that sits on the true bearing (a tower, a hilltop, a road junction), then aim the antenna at that landmark from the roof.

If you are using the tool's compass mode, hold the phone flat, well away from the antenna mast, gutters, a metal roof, solar panel frames and the car. Steel and electrical equipment pull the needle several degrees. Calibrate the compass first by moving the phone in a figure of eight, and check the reading from two spots a few metres apart; if they disagree by more than a few degrees, the surroundings are interfering and the map method is more reliable.

The tool draws a straight line from your antenna to the transmitter's antenna and checks whether the ground rises above that line anywhere along the way. It uses a terrain model with a resolution of about 30 metres, the transmitter's height above ground from the ACMA licence record, the antenna height you selected, and an allowance for the curvature of the Earth.

Clear means no ground crosses the line. Partly obstructed means a ridge rises above the line by up to about ten metres. Obstructed means terrain crosses the line by more than that, and the tool tells you how far away the highest point is and by how much it intrudes.

An obstructed path does not mean no television. TV signals bend over ridges and scatter off terrain, and many homes behind a hill receive well; what the verdict tells you is that the signal reaching you is weaker than it would be with a clear path, so the direction and the height of the antenna matter more. The reception map's class rating for your address is the better guide to expected signal quality; the verdict explains why that rating is what it is.

The model has limits worth knowing. It uses bare ground: buildings, trees and your neighbour's roof are not included, and in a built-up area they are often the real obstruction. From a suburb page the path starts at the centre of the suburb, not your house; enter your street address in the tool to move the start point to you. The first 250 metres from your antenna are ignored on purpose, because the terrain model is too coarse to say anything useful about the ground next to your house.

The chart shows the ground between your antenna and the transmitter. Distance runs left to right, starting at your antenna and ending at the transmitter; height above sea level runs bottom to top. The shaded shape is the ground, the straight line is the line of sight from your antenna to the transmitter's antenna, and the band around the line is the zone the signal mostly travels through, which is wider in the middle of the path than at the ends.

Where the ground pokes above the line, the path is obstructed there, and the marker shows the worst point. Where the ground stays under the line but inside the band, the signal still gets through with some loss. The chart is redrawn when you change the antenna height, so you can see whether a higher mount would lift the line clear of a ridge; it will not, on its own, tell you whether that height is practical on your roof.

Most suburbs can receive more than one transmitter: the main site for the area and one or more smaller translators that fill in shadowed districts. The tool lists them with their own bearing, distance and verdict, stated as facts. Translators often use different channel frequencies and sometimes a different polarisation from the main site, so an antenna aimed at one will not simply pick up the other; the channels page for your suburb shows what each carries.

Which site an installer chooses depends on a signal measurement at the antenna, not on the map, which is why the tool does not make that call for you.