National Spatial Reference System (Federal Geodetic Control Committee, 1988).ĭoyle (1994) points out that horizontal and vertical reference systems coincide by less than ten percent.
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In a Chapter 2 activity, you may have retrieved one of the datasheets that NGS maintains for every NSRS control point, along with more than a million other points submitted by professional surveyors.Ĭontrol network accuracy standards used for U.S. Coordinates associated with horizontal control points are referenced to NAD 83 elevations are relative to NAVD 88. 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). Land surveyors measure horizontal positions in geographic or plane coordinate systems relative to previously surveyed positions called control points. Elevations are expressed as distances above or below a vertical datum such as mean sea level, or an ellipsoid such as GRS 80 or WGS 84, or a geoid. Positions in plane coordinate grids are specified as distances from the origin of the coordinate system. 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 pattern at left represents a higher-resolution instrument. The two grid patterns represent the smallest objects that can be detected by the instruments. That limit is called resolution.įigure 5.4.1, below, shows the same position (the point in the center of the bullseye) measured by two instruments. No matter what instrument, there is always a limit to how small a difference is detectable. Measurements, however, can be only so precise. There is no limit to how precisely a position can be specified. Instrument errors follow from the fact that space is continuous. Judgment becomes a factor when the phenomenon that is being measured is not directly observable (like an aquifer), or has ambiguous boundaries (like a soil unit).Įnvironmental characteristics, such as variations in temperature, gravity, and magnetic declination, also result in measurement errors. Human errors include mistakes, such as reading an instrument incorrectly, and judgments. In general, there are three sources of error in measurement: human beings, the environment in which they work, and the measurement instruments they use. Errors are also introduced when second- and third-generation data is produced, say, by scanning or digitizing a paper map.
Errors are introduced in the original act of measuring locations on the Earth surface. All measurements contain some degree of error. Positions are the products of measurements. These are fun, thought-provoking exercises to help you better understand the ideas presented in the chapter. Take a minute to complete any of the Try This activities that you encounter throughout the chapter.
This chapter explores the technologies and procedures used to produce positional data, and the factors that determine its quality. Resolution and precision are two aspects of data quality. In practice, positions occupy 2- or 3-dimensional areas as a result of the limited resolution of measurement technologies and the limited precision of location coordinates. In theory, a single position is a "0-dimensional" feature: an infinitesimally small point from which 1-dimensional, 2-dimensional, and 3-dimensional features (lines, areas, and volumes) are formed. A property boundary, for example, is made up of a set of positions connected by line segments. Like the letters that make up these words, positions are the building blocks from which features are constructed. Positions, then, are a fundamental element of geographic data. We defined "feature" as a set of positions that specifies the location and extent of an entity.
As you recall from Chapter 1, geographic data represent spatial locations and non-spatial attributes measured at certain times.
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