Georeferencing transforms images / maps from ..... Coordinate system to .... Coordinate system
Answers
Data usually comprises an array of numbers. Spatial data is similar, but it also carries numerical information that allows you to position it somewhere on earth. These numbers are part of a coordinate system that gives you a frame of reference for your data, to locate features on the surface of the earth, to align your data relative to other data, to perform spatially accurate analysis, and to make maps.
All spatial data is created in some coordinate system, whether it is points, lines, polygons, rasters, or annotation. The coordinates themselves can be specified in many different ways, such as decimal degrees, feet, meters, or kilometers—in fact, any form of measurement can be used as a coordinate system. Identifying this measurement system is the first step to choosing a coordinate system that displays your data in its correct position in ArcGIS Pro, in relation to your other data.
Coordinate systems
Data is defined in both horizontal and vertical coordinate systems. Horizontal coordinate systems locate data across the surface of the earth, and vertical coordinate systems locate the relative height or depth of data.
Horizontal coordinate systems can be of three types: geographic, projected, and local. You can find out what coordinate system your data is in by examining the layer's properties. Geographic coordinate systems (GCS) most commonly have units in decimal degrees measuring degrees of longitude (x-coordinates) and degrees of latitude (y-coordinates). The location of data is expressed as positive or negative numbers: positive x- and y-values for north of the equator and east of the prime meridian, and negative values for south of the equator and west of the prime meridian.