How To: Georeference an image to align with other data
Instructions provided describe how to georeference raster data to align correctly with other data.
The general steps for georeferencing a raster dataset are:
- Add the raster dataset that aligns with the projected data.
- Add control points that link known raster dataset positions to known positions in map coordinates.
- Save the georeferencing information when the registration is satisfactory.
For most rasters, the georeferencing information is stored in a separate file with the same name as the raster but with an .aux file extension. Permanently transform the raster by using the Rectify command on the georeferencing toolbar. The coordinate system assigned to the raster is the same as the coordinate system defined on the data frame the raster is part of.
- Add the layers residing in the map coordinates and the raster dataset to be georeferenced.
- Right-click the Table of Contents, a target layer (the referenced dataset) and click Zoom to Layer.
- On the Georeferencing toolbar, click the Layer drop-down arrow and click the raster layer to be georeferenced.
- Click Georeferencing > Fit To Display. This displays the raster dataset in the same area as the reference layer.
- Click the Control Points tool to add control points.
- To add a link, click on a known location on the raster dataset, then click on a known location on the referenced data.
Note: It may be useful to use a Magnification window to add the links. When working with two raster datasets, open the Effects toolbar and adjust the transparency or turn layers on and off in the Table of Contents to view each image as links are added.
- Notice the image shift with each control point added. Add as many controls as needed until the data are aligned correctly.
- Click Georeferencing > Update Georeferencing to save the transformation information with the raster dataset. This creates a new file with the same name as the raster dataset, but with an .aux file extension. It also creates a world file for .tif and .img files.
- To display the Georeferencing toolbar, right-click the Tools menu, point to Toolbars, and click Georeferencing.
- Look for road intersections, land features, building corners, or other objects that can be identified and matched in the raster dataset and aligned datasets.
- Permanently transform the raster dataset after georeferencing by using the Rectify command. Click Georeferencing > Rectify.