Tutorial 13 Georeferencing Material for Use in ArcGIS

8/8/2019 Tutorial 13 Georeferencing Material for Use in ArcGIS

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Tutorial: Georeferencing Material for use in ArcGIS (such as scanned plans, maps and sketches, or aerialimages)

Module content:

This tutorial will cover how to define co-ordinates for an unreferenced file so that it can be used within a .mxdproject, also known as georeferencing. This example will use an image which has been scanned from a book and

saved as an .jpeg. The same procedure can be used to georeference other material, such as scans of historicmaps or plans or hand drawn sketch plans or even images from sources such as GoogleEarth.

The image that we are going to georeference is called‘Loxley_mills2’ and is saved in the ‘Resources’ folder in the ‘GISTutorial Data Folder’.

Source: Crossley, D. 1989. Water

Power on the Sheffield Rivers .Sheffield Trades Historical

Association and the University ofSheffield. Page 77.



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Open your tutorial .mxd project and add the .jpeg fileLoxley_mills2.

The file is not georeferenced, so a warning stating ‘Unknown SpatialReference’ will come up on screen. Click OK.

You need to add the ‘Georeferencing’ toolbar to your project window. Right click in an area of grey at the top ofthe ArcMap screen and select Georeferencing from the drop down list.

The ‘Georeferencing’ toolbar will be added.

Select the target layer from the drop down list. In thiscase the ‘Layer:’ text box is already set to the layer that we wantto georeference: Loxley_mills2.jpeg.

Click Georeferencing and select Fit To Diplay, so that theimage is moved to the same area as the target layers.

The image will be moved so that it appears in your data view.

OS map data © Crown Copyright/database right 2009. An Ordnance Survey/EDINA supplied service.

OS map data © Crown Copyright/database right 2009. AnOrdnance Survey/EDINA supplied service.



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Select the ‘Add Control Points’ tool.

The ‘Add Control Points’ tool allows you to define the correct geographical co-ordinates for the .jpeg image, usingcorresponding known features on already georeferenced layers within the .mxd. You need to study image andthe .mxd to select out features which occur on both.

It is best to try and spread the control points over the entire image, rather than just concentrating them in onespecific area. Ideally, try to have control points near each corner of the image and several spread throughout. Aminimum of 3 control points are necessary to georeference an image in ArcMap. Selecting more control pointsdoes not necessarily improve accuracy.

The first click is used to define the source coordinates (on the image file). The second click is used to define thedestination coordinate. The source control point is defined with a green cross hair, the destination control pointwith a red cross hair.

Select your first control point on theimage. You will probably need to zoom infor accuracy.

First source control point




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Next we need to select the same point on one of the georeferenced layers, such as ‘Topo_Line’, which is an OSsurvey layer.

Select the same point on the ‘Topo_Line’ layer. You will probably need to use the pan and zoom tools. Click toadd a destination control point. The .jpeg will be moved to line up the source and destination control points.

We need to add a second set of control points. Select

another point on the .jpeg.

Use the zoom and pan tools to find the same featureon the ‘Topo_Line’ layer and click to add thedestination control point.

The location of the image will update. The image is better aligned, but we still need to add at least one more setof control points.

Add a third set of control points.

In this case, the image is still not aligned. Tryadding some more control points to improvethe alignment.


OS map data © Crown Copyright/database right 2009. An OrdnanceSurvey/EDINA supplied service.



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You can check the alignment between each of your source and destination points in the ‘Link Table’. Click on the‘View Link Table’ icon in the ‘Georeferencing’ toolbar.

The Link Table dialog box provides a way to view the source anddestination coordinates in a tabular format. Each row in the ‘Link

Table’ represents one set of control points.

The ‘Residual’ column shows the ‘error’ between the source anddestination control points. The higher the number the greater theerror or discrepancy. If the value is particularly high, you mightwant to delete that set of control points to adjust the error.

To delete a set of points, cl ick the row so that it is selected in blue and then click on the cross icon.

In this case you will find that the hand drawn image is not accurate enough to perfectly georeference it. But try

experimenting with control points to get the best match.

Saving the Georeferencing

Once you are satisfied with the georeferencing, you need to save the information. To save the georeferencinginformation, click Georeferencing in the georeferencing toolbar and click Update Georeferencing. This createsa new file with the same name as the image, but with an .aux.xml file extension. For some file formats (including.tiff files) the save also creates a world file.

Google Earth Imagery

You could try using control points to georeference an aerial photographs from Google Earth. Google Earthimagery is available for personal use. The source should be acknowledged and the Google logo retained on theimage. For more information on the use of Google Earth imagery use the following link:http://earth.google.com/support/bin/answer.py?answer=21422&ctx=sibling


Tutorial 13 Georeferencing Material for Use in ArcGIS

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