as "Newtonian noise", since Newtonian gravity calculations are sufficient to characterize many of the local (earth-based) signals. The term absolute gravimeter has most often been used to label gravimeters which report the local vertical acceleration due to the earth. Relative gravimeter usually refer to differential comparisons of gravity from one place to another. They are designed to subtract the average vertical gravity automatically. They can be calibrated at a location where the gravity is known accurately, and then transported to the location where the gravity is to be measured. Or they can calibrated in absolute units at their operating location. There are many methods for displaying acceleration fields, also called gravity fields. This includes traditional 2D maps, but increasingly 3D video. Since gravity and acceleration are the same, "acceleration field" might be preferable, since "gravity" is an oft misused prefix. Commercial absolute gravimeters Gravimeters for measuring the earth's gravity as precisely as possible are getting smaller and more portable. A common type measures the acceleration of small masses free falling in a vacuum, when the accelerometer is firmly attached to the ground. The mass includes a retroreflector and terminates one arm of a Michelson interferometer. By counting and timing the interference fringes, the acceleration of the mass can be measured. A more recent development is a "rise and fall" version that tosses the mass upward and measures both upward and downward motion. This allows cancellation of some measurement errors; however, "rise and fall" gravimeters are not yet in common use. Absolute gravimeters are used in the calibration of relative gravimeters, surveying for gravity anomalies (voids), and for establishing the vertical control network. Atom interferometric and atomic fountain methods are used for precise measurement of the earth's gravity, and atomic clocks and purpose-built instruments can use time dilation (also called general relativistic) measurements to track changes in the gravitational potential and gravitational acceleration on the earth. The term "absolute" does not convey the instrument's stability, sensitivity, accuracy, ease of use, and bandwidth. So it and "relative" should not be used when more specific characteristics can be given. Relative gravimeters The most common gravimeters are spring-based. They are used in gravity surveys over large areas for establishing the figure of the geoid over those areas. They are basically a weight on a spring, and by measuring the amount by which the weight stretches the spring, local gravity can be measured. However, the