Baseline

As a base line (Engl. Line of position , LOP ) is in the classic navigation an approximately straight line referred to the earth's surface, on which, according to a measurement of the position of the observer must be located.

The general case of a baseline is a geometric location , i.e. a curve on which all points lie for which the measured value is fulfilled. On the globe these are mainly great circles , small circles or hyperbolic curves, on an aerial or sea ​​map approximate straight lines , and depth lines in the case of plumbing .

The simplest case of a baseline determination is the bearing , i.e. a magnetic or geographic direction measurement . It belongs to the method group of terrestrial navigation and is mostly graphical - e.g. B. on the nautical chart - evaluated.

Direction or bearing line

If a target point on the nautical chart (a so-called landmark ) is sighted - for example at an angle (southeast) - the direction or bearing line is obtained by drawing a straight line from the target point in the opposite direction, i.e. (northwest). If the measurement and the compass are correct, your own location must be on this line. If the compass is misaligned, a correction ( loading ) must be attached to the measured value. The bearing of a second landmark ( cross bearing ) results in your own position in the intersection of the two base lines. ${\displaystyle P=135^{\circ }}$ ${\displaystyle P+180^{\circ }=315^{\circ }}$

Bearings can relate to magnetic north ( magnetic north , mwN ) or astronomical north, i.e. the true meridian (true north, rwN ). In English it is called true north (TN) and the bearing related to it is true bearing (TB). In contrast, the driven or flown is course true course (TC) called.

In addition to the compass or the bearing disc , the determination of a directional baseline can also be done by so-called deck bearings (often at port entrances) or by radio direction finding from a nautical transmitter ( radio beacon line ). The position line can then also be transferred to the map or compared directly with the target course of the ship. The location of aircraft can be determined in a similar way , but this is usually already automated (see VOR or TACAN ).

Aviation Q key

The correct direction from the point of interest to the aircraft or ship corresponds to the QTE in the international Q code . For the calculation of the LOP (QTE) has the True bearing (TB on German true bearing ) must be known:

LOP = TB + 180 ° or
LOP = TB - 180 ° if the total is more than 360 °.

In contrast to this classical bearing almost all courses and bearings refer the radio and air navigation to miss True North (Engl. Magnetic north, MN ) Accordingly, the radio aids - notwithstanding the above definition - as QDR coded.

Other forms of baseline

In general, a baseline (LOP) can be defined as the entirety of all points ("geographical location") on which the observer can be based on his measurement:

1. When bearing on a straight line in the opposite direction
2. when measuring a distance on an arc around the target point
3. at the elevation angle of a mountain, lighthouse etc. also on a circle around the target point
4. with a difference in distance on a hyperbola-like curve (see hyperbola navigation )
5. when measuring the height of a star on a large, circle-like astro base line
6. when plumbing the sea ​​depth along a depth line on the nautical chart.

The above cases are strictly only valid on the flat surface of the earth or on the sea. With measured inclined distances , the circular arc (2 and 3) becomes a segment of a sphere, and with three-dimensional locating further geometric locations arise in space, for example

• when measuring the air pressure the so-called PLOP (pressure line of position)
• in aviation, a lateral displacement due to drift - see also Single Heading Flight
• For measurements to GPS or other satellites, a system of distance spheres around the respective earth satellites .

In general, a baseline (a single measurement) is not enough to determine the location, because the location can be anywhere on the LOP. At least two LOPs are required for this. Only the intersection of two base lines (or of three in three-dimensional space) gives the exact location ( position or fix ).

In terms of elementary geometry , base lines are geometric locations . The astronomical baseline method was discovered in 1837 by the Boston captain Thomas Sumner by a fortunate circumstance and used for the first time. According to him, such base lines are sometimes referred to as the Sumner line .

See also

• Angle measurement , backward cut , dangerous place
• Danger line
• Astronavigation , equal altitude method

literature

• Albrecht-Vierow: Textbook of Navigation . 11th edition (reworked by B. Soeken and H. Hansen), 430 pages and 6 plates, Decker's Verlag, Berlin 1925
• Wolfgang Kühr: The private pilot . Flight navigation, Friedrich Schiffmann Verlag, Bergisch Gladbach 1981, ISBN 3-921270-05-7
• Jürgen Mies: Radio navigation . Motorbuch Verlag, Stuttgart 1999, ISBN 3-613-01648-6
• Peter Dogan: The Instrument Flight Training Manual . 1999, ISBN 0-916413-26-8
• Walter Air: CVFR textbook Mariensiel 2001
• Jeppesen Sanderson: Private Pilot Study Guide . Englewood 2000, ISBN 0-88487-265-3
• Jeppesen Sanderson: Private Pilot Manual . Englewood 2001, ISBN 0-88487-238-6