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Parallax in the context of surveying refers to the apparent displacement of a point as a result of an observer's change in position. This phenomenon occurs when the line of sight to the point being observed changes, which can lead to inaccuracies if not accounted for. For instance, when making measurements with tools such as leveling instruments or theodolites, if the observer's eye is not aligned with the instrument's reticle or line of sight, the point may appear to shift. Therefore, understanding and correcting for parallax is essential in ensuring accurate measurements in surveying practices.

While the other options mention various concepts related to surveying and measurement, they do not accurately describe parallax. A constant error in instruments can affect measurements but is not referred to as parallax. Measurement of angles on a photograph relates to photogrammetry rather than displacement due to observation. Lastly, the calculation of GPS coordinates involves different principles entirely and does not directly address the effects of observation position on perceived points.