A global navigation satellite system (GNSS), composed of user receivers, satellites, and ground control stations, is designed to provide accurate positions in 3-dimensional space. The Global Positioning System (GPS) is one of four GNSS freely available today. The fundamental GNSS approach is widely used today in cellphones, automobiles, autonomous vehicles, uncrewed aerial vehicles, and even as tagged luggage or bicycles. Significant improvements in the GNSS approach is available to the educated mapping scientist with an appropriate GNSS receiver/antenna, the appropriate environmental context, and processing of the satellite signals. This chapter provides the reader the fundamental concepts involved in the GNSS approach. With such knowledge the reader can make informed decisions about receivers, their use in the environment, and processing approaches for their unique applications.
Georeferencing is the recording of the absolute location of a data point or data points. Georectification refers to the removal of geometric distortions between sets of data points, most often the removal of terrain, platform, and sensor induced distortions from remote sensing imagery. Georeferencing is a requisite task for all spatial data, as spatial data cannot be positioned in space or evaluated with respect to other data that are without being assigned a spatial coordinate within a defined coordinate system. Many data are implicitly georeferenced (i.e., are labeled with spatial reference information), such as points collected from a global navigation satellite system (GNSS). Data that are not labeled with spatial reference information can be georeferenced using a number of approaches, the most commonly applied of which are described in this article. The majority of approaches employ known reference locations (i.e., Ground Control Points) drawn from a reliable source (e.g., GNSS, orthophotography) to calibrate georeferencing models. Regardless of georeferencing approach, positional error is present. The accuracy of georeferencing (i.e., amount of positional error) should be quantified, typically by the root mean squared error between ground control points from a reference source and the georeferenced data product.