How to Georeference in Global Mapper
While most data files used in a GIS setting have internal or associated external coordinate reference information, this is not true for all data you may encounter. If you have ever tried to load a solo simple image file, such as one downloaded from Google Images, into Global Mapper, you have likely experienced this lack of full reference information firsthand. In order to display unreferenced data, the user has to define the file’s geospatial location manually. This process is referred to as georeferencing or rectification. Often used with raster data, but available with any data type in Global Mapper, georeferencing is the process through which a user places control points to tie unreferenced data to a coordinate space in order to display the data at the correct location on the map.
The most common use case for georeferencing in Global Mapper is imagery. While there are many GIS image formats that can store the projection and reference information for the raster data within the file, imagery, scanned printed maps, and many other images commonly do not contain georeference information. To display the image on the map in Global Mapper, the information must be added.
Before beginning the georeferencing or rectification process with the image you would like to load, reference data needs to be imported. Reference data allows features to be identified in the unreferenced image, and on a referenced map, in order to place control points connecting a pixel location on the image with a point in coordinate space. Global Mapper’s Online Data tool is an incredible resource for streaming imagery or street map data into a workspace to use as reference data. Whether using streamed data and/or referenced vector and raster layers, it is very helpful to have reference data that clearly displays features identifiable in the unreferenced image.
When loading an unreferenced image through the Open Data option found in Global Mapper’s File toolbar or File menu, you will be met with a dialog asking if you would like to manually rectify the image. The other options offered in this dialog include faking the position of the image by placing it at an arbitrary location for viewing or using the image as a picture point feature, which will not display the image projected in Global Mapper.
After selecting to manually rectify the image from the prompt in Global Mapper, the rectification window will open. With three data view windows, the rectification window can be resized or maximized for best use. Working from left to right, the three data viewers in the rectification window are the unreferenced data overview, the zoomed view of unreferenced data, and the reference data view. Both the zoomed view of unreferenced data and the reference data view can be zoomed and panned within and will be used for the placing of control points.
Below the zoomed view of unreferenced data is the rectification projection. This projection is inherited from the open workspace containing the reference data, but it can be changed through the rectification window Set Projection button. The projection displayed in the rectification window will be the projection in which the control points are recorded.
To georeference the image, control points need to be added. These control points will be recorded as coordinates in the current projection and pixel coordinates in the unreferenced image. The easiest way to place a control point is by clicking to place a point in both the zoomed view of unreferenced data and the reference data view. Clicking at the point location in each of these views will populate the point coordinate values shown below the data views. If you are happy with this first control point, click the Add Point button to add the control point to the list. Repeat this process to add multiple control points spread throughout the unreferenced image.
Through the Options menu in the Rectification window, the specific rectification method can be selected. Different methods require a different minimum number of control points, but even the simplest method requires at least two control points. Often the selection of the Automatic option is best as it allows Global Mapper to determine the best method based on the added control points.
Once all control points have been added, click OK in the bottom right of the rectification window to import the rectified image into the Global Mapper workspace.
The now-georeferenced image displayed in Global Mapper can be saved in the workspace, but similar to any other data editing in the program, manually rectifying the image does not save that information to the source image file. To create a fully georeferenced image file outside of Global Mapper the georeferenced image needs to be exported to a format that will save the full reference information in or with the image file.
At any point, if the position of a rectified data layer needs to be adjusted, or the control points used for rectification need to be viewed or saved, the rectification window in Global Mapper can be opened with a right-click on the layer in the Control Center. In the right-click context menu the Rectify option will launch the rectification window again allowing you to add or remove control points, change the rectification projection, or save the control points to a file with an option in the rectification window File menu.
Georeferencing data is an incredibly useful tool in Global Mapper as it 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! If you have any questions, please contact us.