Image files contain no geographic data as to how the areas correspond to locations on the Earth’s surface. The image below shows an image file with no geographic data in an ArcGIS Online web map.
Note that the image is not projected to its exact location on the Earth’s surface because images containing no geographic data are projected on-the-fly. Refer to ArcGIS Blog: Projection on the fly and geographic transformations for more information.
By georeferencing, the image file is overlaid with real-world vector data wherein areas on the map correspond to locations on the Earth’s surface, thus providing valuable information for analysis.
To add an image file as a georeferenced image to an ArcGIS Online web map, the image must be georeferenced in ArcGIS Pro, and published as a hosted tile layer in ArcGIS Online. To do so, follow the steps below.
Note: The Unknown Coordinate System pop-up is displayed at the top-right corner after adding the image to the map as the image contains no geographic data.
The image below shows the image file georeferenced in ArcGIS Pro.
The image below shows the georeferenced image projected in an ArcGIS Online web map as a hosted tile layer.