5.5.3 Combining Angles and Distances to Determine Positions In modern theodolites, some mechanical parts are replaced with electronics.Ĭredit: Łukasz Fus. A theodolite is essentially the same instrument, except that it is somewhat more complex and capable of higher precision. A transit (Figure 5.11) consists of a telescope for sighting distant target objects, two measurement wheels that work like protractors for reading horizontal and vertical angles, and bubble levels to ensure that the angles are true. For these reasons, land surveyors rely on transits (or their more modern equivalents, called theodolites) to measure angles. Moreover, some rocks can become magnetized and introduce subtle local anomalies when using compass. Let us first take a look at angles and how they apply to surveying.Īlthough a standard compass can give you a rough estimate of angles, the Earth’s magnetic field is not constant and the magnetic poles, which slowly move over time, do not perfectly align with the planet’s axis of rotation as a result of the latter, true (geographic) north and magnetic north are different. We will briefly discuss this equipment and their methodology. What the surveyors and their equipment are doing is carefully measuring angles and distances, from which positions and elevation can be calculated. Often one surveyor operates equipment on a tripod while another holds up a rod some distance away. You probably have seen surveyors working outside, e.g., when highways are being realigned or new housing developments are being constructed. Once a control network is established, however, surveyors produce positions using instruments that measure angles and distances between locations on the Earth's surface.Ĭredit: Thompson, 1988.
Nowadays, geodesists produce extremely precise positional data by analyzing radio waves emitted by distant stars.
Before reliable timepieces were available, astronomers were able to determine longitude only by careful observation of recurring celestial events, such as eclipses of the moons of Jupiter. If positions are measured relative to other positions, what is the first position measured relative to? The answer is: the stars. You might wonder how a control network gets started. Vertical control points however, were established by the technique of spirit leveling which is more suited to being conducted along gradual slopes such as roads and railways that seldom scale mountain tops. horizontal stations were often located on high mountains or hilltops to decrease the need to construct observation towers usually required to provide line-of-sight for triangulation, traverse and trilateration measurements. In the U.S., the National Geodetic Survey (NGS) maintains a National Spatial Reference System (NSRS) that consists of approximately 300,000 horizontal and 600,000 vertical control stations (Doyle,1994).ĭoyle (1994) points out that horizontal and vertical reference systems coincide by less than ten percent. In 1988 NGS established four orders of control point accuracy, ranging in maximum base error from 3mm to 5cm. Land surveyors measure horizontal positions in geographic or plane coordinate systems relative to previously surveyed positions called control points, most of which are indicated physically in the world with a metal “benchmark” that fixes the location and, as shown here, may also indicate elevation about mean sea level (Figure 5.10). Positions on the globe, for instance, may be specified in terms of angles relative to the center of the Earth, the equator, and the prime meridian. Geographic positions are specified relative to a fixed reference. The techniques and tools of conventional surveying are still in use and, as you will see, are based on the very same concepts that underpin even the most advanced satellite-based positioning. Yet, none of the power or capabilities of GPS would have been possible without traditional surveyors paving the way. Ease, accuracy, and worldwide availability have made ‘GPS’ a household term.
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