true—In surveying, astronomy, and engineering, the word “true” is used to define exact, correct, or actual values as differentiated from relative measured, or fictitious values, e.g., true North vs. magnetic north, true sun vs. fictitious sun, true altitude vs. measured altitude. Depending on the use, “true” can have an exact meaning. Its use in descriptions to define direction, i.e., “thence true North,” is not exact and has been subject to many interpretations. The generally accepted interpretation is that true North means a meridian determined with respect to the Earth’s axis by astronomic methods. Since this meridian can be better define by the terms “astronomic north” or “geodetic north,” the use of the won “true” is discouraged in this sense.

true line—See line, true [USPLS].

true North—See North, true.

true to scale—Having the property that measurements on a map convert exactly to measurements between corresponding points on the ellipsoid when using the scale of the map. Because no map projection can keep a constant ratio between distances on the map and corresponding distances on the ellipsoid, no map is true to scale at all points.

true value—That value of quantity which is completely free from blunders and errors. Since the errors to which physical measurements are subject cannot be known exactly, it follows that the true value of a quantity cannot be known with exactness. In survey work, the most probable value is used as best representing the true value of the quantity.

–

Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

Part of LearnCST’s exam text bundle.