Horizontal plane

A spirit level on a shelf

In geometry, physics, astronomy, geography, and related sciences, a plane is said to be horizontal at a given point if it is perpendicular to the gradient of the gravity field at that point in other words, if apparent gravity makes a plumb bob hang perpendicular to the plane at that point. Alternatively, a spirit level, which exploits the buoyancy of a bubble, can be used to determine if the plane is horizontal.

In radio science, horizontal plane is used to plot an antenna's relative field strength in relation to the ground (which directly affects a station's coverage area) on a polar graph. Normally the maximum of 1.000 or 0 dB is at the top, which is labeled 0o, running clockwise back around to the top at 360°. Other field strengths are expressed as a decimal less than 1.000, a percentage less than 100%, or decibels less than 0 dB. If the graph is of an actual or proposed installation, rotation is applied so that the top is 0o true north. See also the perpendicular vertical plane.

In general, something that is horizontal can be drawn from left to right (or right to left), such as the x-axis in the Cartesian coordinate system.

Discussion

Although the word horizontal is commonly used in daily life and language (see below), it is subject to many misconceptions.

Verticals at two separate points are not parallel. The same holds for their associated horizontal planes
field lines for a non homogeneous knobbly planet in motion may be curved. The white, red and hlue bits illustrate the planet's heterogeneity.

Practical use in daily life

The concept of a horizontal plane is thus anything but simple, although, in practice, most of these effects and variations are rather small: they are measurable and can be predicted with great accuracy, but they may not greatly affect our daily life.

This dichotomy between the apparent simplicity of a concept and an actual complexity of defining (and measuring) it in scientific terms arises from the fact that the typical linear scales and dimensions of relevance in daily life are 3 orders of magnitude (or more) smaller than the size of the Earth. Hence, the world appears to be flat locally, and horizontal planes in nearby locations appear to be parallel. Such statements are nevertheless approximations; whether they are acceptable in any particular context or application depends on the applicable requirements, in particular in terms of accuracy.

The y-axis on the wall is vertical but the one on the table is horizontal.

In graphical contexts, such as drawing and drafting and Co-ordinate geometry on rectangular paper, it is very common to associate one of the dimensions of the paper with a horizontal, even though the entire sheet of paper is standing on a flat horizontal (or slanted) table. In this case, the horizontal direction is typically from the left side of the paper to the right side. This is purely conventional (although it is somehow 'natural' when drawing a natural scene as it is seen in reality), and may lead to misunderstandings or misconceptions, especially in an educational context.

See also

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