Georeferencing is the process of assigning x, y coordinates to a raster file, such as an aerial photograph, scanned historical map, etc. so GIS software can place the resulting georeferenced file in its specified location on a map. The georeferencing process involves assigning control points to the raster image and matching them to control points on an existing georeferenced data set.
Georeferenced fire insurance map of Harvard Square (1885). Map from Insurance Maps of Boston, Vol. 4.
For our georeferencing workshop, we'll be using a 2003 map of the HMS Colossus. The CISMAS researchers provided UTM coordinates on the map, so we'll be utilizing one common georeferencing approach: mapping by corners. We'll start with a rough overlay of our map image and then we'll use the georeferencing tools to add control points based on the UTM coordinates for the four corners of the map.
More info about Universal Transverse Mercator (UTM) is posted under foundational concepts.
When you add the image of the HMS Colossus wreck into ArcGIS Pro, a pop-up window will appear stating that there's an unknown coordinate system. This is a standard message that appears when you add data into ArcGIS Pro that's missing spatial reference information.
Since the software doesn't know "where" to place our map, it will be placed at the default location (0,0), the intersection of the Equator and the Prime Meridian. Georeferencing is one of the processes that can be used to help supply spatial reference information (e.g., coordinates, datum, projection, etc.) to our map file so it overlays in the correct location on the map.
Until we complete this process, our map will be hanging out on null island:
The control point table is an essential component of the georeferencing process. You can use the control point table to edit and delete the control points you've added while georeferencing your map. You can also use the control point table as a workaround if you experienced issues right-clicking to add target coordinates to a control point. If you were one of the students who experienced this issue, you can use this workflow:
Select Data > Export Raster to open the Export Raster pane.
In the Export Raster pane, make sure that the Output Raster Dataset is saving to your project folder or the Home Folder if you're using Amazon AppStream. Confirm that the Coordinate System is set to WGS_1984_UTM_Zone_30N.
Set the NoData value drop-down menu to 0. If you don’t populate this field, a black border will display around your map.
Choose TIFF as your output format. Change the Compression Type to LZW. This will decrease the file size of your exported file.
Click Export to export your raster. The export process will create a GeoTIFF of your map. A GeoTIFF is a geospatial image file format that has coordinate data embedded into the file. If you added your GeoTIFF to a new map project, it would overlay in the correct location on the map.