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How accurate must measurements be in high-order geodetic surveying?

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surveying Figure 1: Photogrammetric photographs from two short, overlapping flight strips arranged for supplying mapping details. Photo-control points are shown on only one photograph; shading indicates a typical terrain feature such as a lake (see text).

In geodetic surveying, operations are classified into four "orders" based on their level of accuracy, with first-order surveys requiring the smallest permissible error.1

For high-order geodetic instruments, such as modern theodolites equipped with five-inch glass circles, the accuracy is approximately one second of arc, or 1/3,600 of a degree. This level of precision allows a surveyor to detect a target's sideways movement of one centimeter at a distance of two kilometers. Furthermore, in geodetic surveying, measurements of all three angles of a triangle are expected to result in a sum of 180 degrees within one second of arc.2

These measurements are performed under rigid specifications to ensure the highest standards of accuracy, as they serve as the foundational framework for mapping and engineering projects. Because increasing accuracy involves significantly higher costs, these precise control surveys are used to establish a network of points from which less accurate, shorter measurements can be made for specific project details.12