A position circle is a

circle A circle is a shape consisting of all points in a plane that are at a given distance from a given point, the centre. Equivalently, it is the curve traced out by a point that moves in a plane so that its distance from a given point is const ...

that can be measured both from a

chart A chart (sometimes known as a graph) is a graphical representation for data visualization, in which "the data is represented by symbols, such as bars in a bar chart, lines in a line chart, or slices in a pie chart". A chart can represent tabu ...

and from the surface of the

earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

for the purpose of

position fixing Geopositioning, also known as geotracking, geolocalization, geolocating, geolocation, or geoposition fixing, is the process of determining or estimating the geographic position of an object. Geopositioning yields a set of geographic coordinates ...

. For the purposes of land or coastal navigation, a position circle can be generated by making a horizontal angle measurement between two landmarks using a

sextant A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of celes ...

. Two overlapping position circles, or one position circle and one or more other observations can be used to give a position fix. When a horizontal angle measurement is made between two known points on land, the observer will be located at the apex of a triangle, with the other two corners of this triangle consisting of the landmark pair. The observer will also be sitting on a set of points that fall along a large circle. The diameter of the circle will be dependent upon the distance between the landmark pair and the range of the observer to the landmark pair. At each point along this circle, the angular separation between the landmark pairs will be the same. Two such circles exist, one in front of the landmark pair, and one on the backside. It is usually straightforward to tell which circle is relevant. After the observation is made, the angular reading can be transferred onto a nautical chart (or map) by means of a

protractor A protractor is a measuring instrument, typically made of transparent plastic or glass, for measuring angles. Some protractors are simple half-discs or full circles. More advanced protractors, such as the bevel protractor, have one or two sw ...

instrument. The Practical Pilot: Coastal Navigation by Eye, Intuition, and Common Sense Leonard A. Eyges The

origin Origin(s) or The Origin may refer to: Arts, entertainment, and media Comics and manga * Origin (comics), ''Origin'' (comics), a Wolverine comic book mini-series published by Marvel Comics in 2002 * The Origin (Buffy comic), ''The Origin'' (Bu ...

of the circle can be found as follows. Draw a baseline connecting the two landmarks. Bisect the line using a

compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...

. Construct a line

90 degrees 9 (nine) is the natural number following and preceding . Evolution of the Arabic digit In the beginning, various Indians wrote a digit 9 similar in shape to the modern closing question mark without the bottom dot. The Kshatrapa, Andhra and ...

to the baseline using a compass. Using a protractor, draw a line from either landmark to intersect the 90 degree line off the baseline at the observed angle. This point is the origin of the circle of position. Using a compass again, the entire circle can be drawn.

References

Navigation Curves {{Cartography-stub

See also

References