- Georeference an image
- Create a world file
TERMS TO KNOW
- control points
- target data
- world file
- polynomial transformation
- First order (affine) transformation
- Second order polynomial transformation
- third order polynomial transformation
In this exercise, we will be georeferencing a scanned map to real-world coordinates. In order to do this, we need some data to 'reference' the scanned map to, this is referred to as the target data. In this case, we will use shapefiles in the State Plane Coordinate system to reference the map with.
In order for an image to be displayed in real-world coordinates, there must be a world-file stored in the same directory/folder as the image. The world-file also must be named the same thing with a different file extension. For instance, you will have an image called BentCreekTrailMap.jpg and you will create a world file that will be called BentCreekTrailMap.jpw. This will be accomplished using the Georeferencing Toolbar in ArcMap.
READING - Read the following links from the ESRI WebHelp 9.2 - you are responsible for this information!
- Create a folder to do this work. Call the folder BentCreek (or something else that makes sense to you).
- Download and unzip the BentCreekTrailMap.zip into your folder for this exercise.
- Download some shapefiles for the Bent Creek Experimental Forest from the USFS on the internet ==> http://www.srs.fs.usda.gov/bentcreek/GIS/bent_creek_gis_data.htm. Particulary, you will use the boundary and roads shapefles for the referencing but get them all for your map. Put these files in your folder for this exercise and uzip them.
- Add the layers residing in map coordinates (boundary and roads) and the raster dataset (BentCreekTrailMap.jpg) you want to georeference.
- In the table of contents, right-click a target layer (the referenced dataset) and click Zoom to Layer.
- From the Georeferencing toolbar, click the Layer drop-down arrow and click the raster layer you want to georeference. View the Georeferencing toolbar -Tip- To display the Georeferencing toolbar, click the View menu, point to Toolbars, and click Georeferencing.
- Click Georeferencing and click Fit To Display.This will display the raster dataset in the same area as the target layers. You can also use the Shift and Rotate tools to move the raster dataset as needed. To see all the datasets, you may have to adjust their order in the table of contents.
- Click the Add Control Points tool to add control points.
- To add a link, click the mouse pointer on a known location on the raster dataset, then on a known location on the data in map coordinates (the referenced data).You may find it useful to add your links in a Magnification window. When working with two raster datasets, you may want to open the Effects toolbar and adjust the transparency or turn layers on and off in the table of contents to view each image as you add your links.
- Add enough links for the type of transformation.You need a minimum of three links for a spline or 1st-order polynomial (affine), six links for a 2nd-order polynomial, and ten links for an affine or 3rd-order polynomial.
- Click View Link Table to evaluate the transformation.You can examine the residual error for each link and the RMS error (if you did more than 3 links). If you’re satisfied with the registration, you can stop entering links.
- 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.xml file extension. It also creates a world file for some of the file formats, including .tif and .img files.
- As always, export your work from ArcMap to an image and post to your blog.
- You could look for road intersections, land features, building corners, or other objects that you can identify and match in your raster dataset and aligned datasets.
- You can delete an unwanted link from the Link Table dialog box. Press Esc to remove a link while you’re in the middle of creating it.
- The Rotate and Shift tools are no longer available after you place the first link.
- You can permanently transform your raster dataset after georeferencing by using the Rectify command; click Georeferencing and click Rectify or use the Warp tool.
- To align your image in the same space as the data in the data frame, click the Georeferencing drop-down menu and click Fit to Display.