MAP, DATA & GIS LIBRARY
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Georeferencing a Map Image in ArcMap
Introduction: Aligning geographic data to a known coordinate system so it can be viewed, queried, and
analyzed with other geographic data.
Section 1 – Preparation
1. Run ArcMap.
2. To add a basemap in which to georeference to, click the down arrow beside the
Add Data button
(from the main toolbar) and choose Add Basemap...
3. From the ‘Add Basemap’ window, select Imagery. Click Add. The Imagery basemap uses the ‘Web
Mercator Auxiliary Sphere (WKID 102100)’ coordinate system. This may open a ‘Hardware Acceleration’
window. Click Yes.
4. Use the Zoom
and Pan
tools to view the extent of the desired area.
5. Add the map you want to georeference using the Add Data button
. This will open the ‘Add Data’
window. Click the Connect Folder icon
and navigate to the folder housing the image and click OK.
Select the image you wish to georeference and click Add. You will be prompted by 1 or 2 windows:
 The first window is ‘Create Pyramids for…’. Click Yes. (This will increase image loading performance.)
 The second window is the ‘Unknown Spatial Reference’ dialogue box. Simply click the OK button.
6. To display the Georeferencing toolbar, click the Customize menu, and click Toolbars > Georeferencing.
7. From the Georeferencing toolbar, click the Layer drop-down arrow and click the map you would like to
georeference.
8. Click the Viewer button
on the Georeferencing toolbar. This will open up a new Image Viewer window
that displays the raster that you are georeferencing. It may not appear anything in this window, click the
Full Extent button .
9. In the Image Viewer window, click the Reposition
button to view both windows simultaneously.
10. In the Table of Contents, uncheck the map layer you are georeferencing to turn if off (so you can see the
imagery behind it).
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Section 2 – Georeferencing Procedure
1. From the Georeferencing toolbar, click the Add Control Points button
to add control points. Control
points are points that are known to be common in both the image that already resides in map coordinates
and the image that you are georeferencing. The best control points are features that do not change over
time such as intersections and railroads.
2. To begin, click on a common point on the image in the Image Viewer window, and then select the same
point on the previously georeferenced image (Basemap).
3. These connections are known as links. A table of all the links made can be seen by selecting the View Link
Table button
. This is where you can examine the residual error of each link, as well as delete links that
appear to be warping the image.
4. Repeat steps 1 & 2 for a minimum of 3 control points. These points should be spread across the image,
preferably in a triangular formation.
5. NOTE: It is possible to save your Control Points to prevent data loss or in the event that you want to look
at the details at a later date. From the Georeferencing toolbar, click the View Link Table button
, then
click the Save button
. In the ‘Save As’ window, navigate to your file directory, name the file
appropriately and click Save. Your Control Points are saved. If you would like to recover your saved Control
Points, click the Open button
, select your file (Text Document) and click Open.
6. When completed, click the Georeferencing drop-down arrow on the Georeferencing
toolbar and click Update Display. This shows you what the image will look like once it is
saved with geographic coordinates. If a warning appears, “The control points are
collinear or not well distributed. This will affect the warp result.”, replace a few
control points with some that are better dispersed across the map.
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7. In the Table of Contents, check the map layer you are georeferencing to turn it back on. The map will
appear ‘pinned’ to the basemap.
8. To transform your georeferenced image, click the Georeferencing button on the Georeferencing toolbar
and select Rectify. This saves the image with the map coordinates to a “new dataset”, or file, without
altering the original scanned image.
9. In the Save As window, accept the default for Cell Size, NoData as, and Resample Type. Click the Browse
button to save the georeferenced image to a specified location, and Name the file appropriately. The
default TIFF is the suggested file format with no Compression Type. Click Save.
June 2013
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