October 21, 2025
Georeferencing in GIS
A coordinate reference system (CRS) is a set of parameters that distinguish the geographic coordinates of where things are in the world. In other words, a CRS defines how your data lines up with real-world locations so everything appears where it should on the map. When viewing, analyzing, or editing GIS data, having the correct CRS is essential for maintaining spatial accuracy. Global Mapper® enables users to effortlessly georeference their imagery.
While some file types, like GeoTIFFs and GeoJSONs, come with this information built in, others—such as plain imagery or ASCII text files—don’t. Without this geospatial information, an image can’t be displayed correctly in Global Mapper. Georeferencing lets you assign real-world coordinates to an image by placing control points, helping your data land precisely where it belongs on the map.
How To Georeference Imagery
One of the most common use cases for georeferencing is to assign coordinates to pixel values in imagery. An advanced type of georeferencing is rectification, which involves correcting distortion and resampling the image as needed. Some raster formats—like GeoTIFFs and JP2 files—already include CRS information within their headers. However, historical images, scanned paper maps, and types of aerial imagery typically do not contain this geospatial metadata. In order to display these ungeoreferenced images on a map in Global Mapper, coordinates must be assigned.
1. Start with Reference Data
Before you can add coordinates to an image, you’ll need some reference data as a starting point. Reference data helps you identify recognizable features on the unreferenced image so you can place control points that connect a pixel location to a known point in coordinate space.
Global Mapper’s Online Data tool is a great way to bring in high-quality imagery or street map data directly into your workspace for this purpose. Whether you’re using streamed data or existing vector and raster layers, having clear, detailed reference data makes it much easier to line up your image accurately. If your image happens to have a graticule, grid, or other coordinates labeled on the map, you can use these coordinates as your control points if you are confident of the accuracy of these positions (or check them against other reference data as well).
2. Load an Unreferenced Image
When loading an unreferenced image into Global Mapper—whether through the File menu, the dockable File Browser, or simply by dragging and dropping—you will be prompted to choose how the image should be positioned. You’ll see a few options:
- Manually Rectify Image, which allows you to georeference it by assigning control points.
- Fake Coordinate to Allow Viewing, which places the image at an arbitrary location for quick viewing (without true spatial accuracy).
Load as a “Picture Point”, which displays the image as an attached point feature rather than a projected raster.
To georeference your image, select Manually Rectify Image. This opens the rectification dialog, which contains three map windows arranged from left to right:
- Unreferenced Data Overview – A full view of the unreferenced image.
- Zoomed Unreferenced Data View – A detailed, zoomable area of the same image.
- Reference Data View – A view of your reference layer (such as imagery or map data) used to place matching control points.
Both the zoomed unreferenced data and reference data views can be panned and zoomed independently, helping you precisely line up matching features between the two.
Below the zoomed unreferenced data window, you’ll find the Rectification Projection. This projection is inherited from your current workspace containing the reference data but can be changed using the Set Projection button. The projection displayed here determines the coordinate system in which your control points are recorded.
New IIQ Format Support:
While the rectification of historical maps is one of the most common georeferencing workflows, another popular application involves IIQ (Intelligent Image Quality) files—the RAW image format produced by Phase One’s high-resolution cameras. These cameras are engineered to capture exceptional dynamic range and image fidelity using a lossless compression format. With the release of Global Mapper v26.2, users can now directly import IIQ files without needing to convert them to TIFFs first. This enhancement streamlines the workflow for processing and georeferencing high-quality aerial imagery, allowing users to maintain the full range and clarity of their original files while saving valuable time.
3. Add Control Points
A key part of georeferencing an image is identifying matching features on both the unreferenced image and the reference map. These alignment locations are called control points, storing the pixel coordinates on the unreferenced image and the real-world coordinates based on the current projection.
The easiest way to place a control point is to find a recognizable feature—such as a road intersection or building corner—that appears in both views. Then, click that spot in both the zoomed view of unreferenced data and the reference data view. The coordinate values for the selected points will automatically populate in the fields below the data windows. If the placement looks correct, click Add Point to add it to your list of control points. Repeat this process to add several control points evenly distributed across the entire image for the best alignment results.
There are several rectification methods that can be selected in the Options menu. Each method requires a minimum number of control points. The most basic georeferencing requires two control points to place the image on the map at an appropriate scale and correct rotation. With the addition of more control points, more advanced methods can be used to calculate a transformation to rectify lens distortion or account for differences in projection. For most workflows, the Automatic option is recommended – it allows Global Mapper to determine the best method based on your control points.
Once you’ve finished adding control points, click OK in the bottom-right corner of the rectification window to import your newly rectified image into the Global Mapper workspace.
4. Revise Image Rectification
If you ever need to adjust the position of a rectified image layer—or review and save the control points used during rectification—Global Mapper makes it easy. Simply right-click the layer in the Control Center and select Rectify from the context menu. You can correct the position of any layer using this option. This will reopen the rectification window, where you can add or remove control points, change the rectification projection, or save your control points to a file using the File menu.
Lock In Your Image’s Location: Save a Georeferenced Image
The rectified image now displayed in Global Mapper can be saved within your workspace; however, just like other data edits, saving the workspace does not embed the CRS information into the source image file. To create a fully georeferenced image, you’ll need to export the rectified layer to a format that supports spatial reference data, such as GeoTIFF or JP2.
To do this, right-click the georeferenced image layer, select Export, and choose your preferred file format. The exported file will include the geospatial information, allowing it to be accurately positioned in your next project.
Georeferencing in Global Mapper allows users to add necessary geospatial information to unreferenced data to be correctly displayed with additional GIS layers, and saved for use in future projects. With this ability to manually align unreferenced images with map data, the range of files that can be used in Global Mapper for display and analysis is expanded. To try georeferencing data in Global Mapper, download a 14-day free trial today!
