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 map coordinates and the raster dataset to be georeferenced.
Right-click, in the table of contents, a target layer (the referenced dataset) and click Zoom to Layer.
Click, from the Georeferencing toolbar, the Layer dropdown arrow and click the raster layer to be georeferenced.
Click 'Georeferencing' and click '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.
Click, to add a link, the mouse pointer on a known location on the raster dataset, then on a known location on the the referenced data.
Note: It may be useful to use a Magnification window to add in 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 control as needed until the data is aligned correctly.
Click 'Georeferencing' and click '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 match in the raster dataset and aligned datasets.
- Permanently transform the raster dataset after georeferencing by using the Rectify command. Click Georeferencing and click Rectify.