Introduction

There is a great deal of geographic data available in formats that can not be immediately integrated with other GIS data. In order to use these types of data in GIS it is necessary to align it with existing geographically referenced data, this process is also called georeferencing. Georeferencing is also a necessary step in the digitizing process. Digitizing in GIS is the process of “tracing”, in a geographically correct way, information from images/maps. The process of georeferencing relies on the coordination of points on the scanned image (data to be georeferenced) with points on a geographically referenced data (data to which the image will be georeferenced). By “linking” points on the image with those same locations in the geographically referenced data you will create a polynomial transformation that converts the location of the entire image to the correct geographic location. We can call the linked points on each data layer control points.

The selection of control points is important. Some guidelines:

• They should be easy to confirm as representing the same geographic location (street intersection, political boundary, landmark, etc.).
• They should be spread across the image to be registered; one suggestion is to select a control point near each of the corners of the image and few throughout the interior will often work well.
• Good overlap between the two dataset is also important.
• Make sure you are clicking as close as possible to the same geographic location, zooming in can help in this process.

As you add points the complexity of the transformation that is possible increases. This is not always a good thing, 1st, 2nd, and 3rd order polynomials can be calculated, usually a 1st or 2nd order transformation is all that is necessary. In order to complete a 1st order transformation you need a minimum of three control points, for a 2nd order you need a minimum of six control points, and for a 3rd order you will need a minimum of ten points. A 1st order transformation will shift, scale, and rotate your image while 2nd and 3rd order transformations will bend and curve straight lines, which might be necessary, but is rare.

Data:

Three file will be used in this training module, NYStreet.shp, ManhattenCensus.shp, and Manhattan.sid. The latter is a raster image of the same type as jpeg, gif, tiff, etc.; it’s just a digital picture at this point. The first file is a shapefile that contains the street map for the entire state of NY; the second is a layer that contains only the census tracts and extent for Manhattan, which will be useful for zooming to and initially coordinating the image. The first thing you are going to do is “Clip” the part of the street map that intersects Manhattan. The street layer will be used for setting control points.

Creating a New Layer by Clipping

Open Both layers (NYStreet and ManhattanCensus).

Open ArcToolBox (red toolbox on menu) and open Analysis Tools, then Extract. Choose the clip tool.

The input feature is the street map.

The clip feature is the census tracts of Manhattan.

The output class should go somewhere you can find it and be named something you will recognize (ManhattanStreet.shp as an example).

Don’t worry about cluster tolerances. Click Ok.

Add the new layer and remove the original street layer.

Georeferencing/Registering an Image

Tools:

The primary tool we will be using is the georeferencing toolbar

The georeferencing toolbar is probably not visible when you first open ArcGIS, you can open it be clicking on view, selecting toolbars, then activating the georeferencing toolbar option.

Add the manhattan.sid file.

Right click the shapefile (not image) and “Zoom to Layer,” this is a nice feature you might have used before that let us get to the geographic extent of a specific data layer quickly.

From the georeferencing toolbar, click the “Layer” dropdown arrow and select the manhattan.sid image name.

Click “Georeference” and select “Fit to Display” from the dropdown.

This will display the image in the same general space as the manhattan census tracts layer.

Before adding control points you should move the image around a bit to get it closer to the orientation and position of the GIS layers you are using.

Use the Rotate and Shift tools for this.

Now you are read to add control points. Click on the control points tool.

You should be using the clipped data for linking, and use the labels too. (Right click the layer and select label features).

To add a link, click the mouse on the image first, then on a known location on the GIS database. You can use the magnify tools to get a close look at subsets of the data.

You can also turn on the effect toolbar to change the transparency to see both layers at once if you like, or you can turn the layers off and one as you select points from each.

Once you’ve added a few links you can open the link table to see how each has worked.

View Link Table is the final icon on the right of the georeferencing toolbar.

In the link table you can view the Total RMS Error as well as the residual for each link that contributes to the overall Total. The errors are associated with the amount of disagreement between the two control points for each link once the transformation is set (once you’ve added more than three links). You have to be sure that the two points that are linked are the same place in world (same street intersection for instance), as you could have a very low error and residuals but not have an accurately registered image.

Once you are satisfied with the registration, and you can delete links or start all over by deleting them all, you can Update Georeferencing, this will save your transformation. This process will add a .aux file that contains information about the transformation that is necessary to view this file in the future with other data.

Don’t forget to save your work, save your map as “xxx”.mxd.