How to Georeference in ArcGIS Pro

ArcGIS Pro has many powerful GIS tools for mapping and digitizing and georeferencing expands the resources available for you to use. You can add any imagery you want to your mapping project including scans of paper maps or aerial photographs. Georeferencing aligns these images with your choice of reference imagery and saves how it is projected so that you can share your work across other projects.

Once you have some familiarity with using ArcGIS software, it is easy to add any imagery you need and georeference it so that it can be correctly overlayed with other images, used for digitizing new features, or reprojected into different coordinate systems.

Requirements:

A computer

A working copy and license of ArcGIS Pro from Esri

Any maps or imagery which you want to georeference

Open a map in an ArcGIS project folder and add any files which you want to georeference or use as a reference aid. You can directly add files by dragging and dropping them into the map frame, using the "Add Data" button under the layer tab on the Map ribbon, or by adding a folder connection in the catalog by right clicking "Folders" and browsing to the folder containing the data you want to add.

When you add a file which has not been georeferenced, you will be prompted to build pyramids and calculate statistics. These are optional in most cases and will take additional time to complete if you choose to do them. You will then receive a notification saying "Unknown Coordinate System". This means that the file has no information attached to it yet saying where it should be projected. As a result, the default location at 0,0 latitude and longitude off the coast of Africa will be used until you specify otherwise. To see the image at its current location, you can right click on it in the contents panel and select "Zoom To Layer". Once you are ready to begin georeferencing, pan and zoom the map to the location which your imagery should be located.

Select the Image you want to georeference in the contents pane and then click the "Georeferencing" button in the "Imagery" ribbon. This will open a new ribbon of tools which you can use, start by clicking the "Fit to Display" button so that your image will move to your work area. You can pan and zoom your map and use the "Fit to Display" tool to get the image close to the correct location and then use the "Move", "Scale", and "Rotate" tools to further adjust your image.

Next you can begin adding control points to finalize the alignment of your image. These control points will attempt to move the image so that the source imagery that you are georeferencing will allign with whatever target reference imagery you decide to use. Click the "Add Control Points" button on the ribbon and then select a point on the source image you are georeferencing with a red X and then select a point on the target reference imagery which matches to the same location with a green X.

There are eight different transformations which you can apply to your image that will shift or stretch your image in different ways in order to make the control points match up. Once you add three control points, try using different transformations to see what works best for you. Sometimes one transformation will overly distort your image and you will have to try using a different one.

After adding several control points, you can open the Control Point Table from the ribbon to see how far off the control points are from each other. You want their residual numbers to be as close to zero as possible but at least staying below one or two is best depending on the difficulty of matching locations. If a control point has too high of a residual, try deleting it by selecting it and clicking the delete button. You can then try replacing the control point to see if it improves your residual error.

Once you are satisfied with your results, make sure to click the save button and then click the "Close Georeference" button on the ribbon. Your image should now be aligned to the location which you want it and you can continue georeferencing further images or perform other GIS tasks.

You now have the ability to use any unreferenced imagery you find in ArcGIS Pro. Aerial imagery can be a great reference material for your work and scanned or photographed maps can be used in numerous ways. For example, in the photographs included in here, I georeferenced a Sanborn fire insurance map from 1897 and then digitized the roads, property lines, and building footprints. Georeferencing is often a nescesity in historical GIS work when you need to use old maps and photographs.