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Georeferencing is the process of aligning spatial data (layers that are shape files: polygons, points, etc.) to an image file such as an historical map, satellite image, or aerial photograph. This document describes the basic steps for Georeferencing an image using ArcGIS.*
Add a Basemap and Image to the Map Project
- Click on View → Toolbars → Georeferencing
- Add a Georeferenced Basemap to your new Map Project. We recommend adding the ESRI Imagery World 2D Map or ESRI Street Map World 2D from the ESRI servers.
- Add the image you want to Georeference to your map project.
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- From this view, click on the Up One Level button Up One Level Button .
- You will see the file name of the image again. Simply highlight the file name by clicking on it once.
- Click Add to add the file to your Map Project.
Control the Image Size and Position in the Map Project
The Scale Tool
Many times, images added to a Map Project appear so large in the Data Frame (the main viewing area) of ArcMap that they cover the entire viewing surface. You can re-size the image by using the Scale Tool in the Georeferencing Toolbar.
Scale Tool
Make an Image Smaller
- Select the Scale Tool from the Georeferencing Toolbar.
- Move the cursor to the corner of the image.
- Click and drag from the corner to the center of the image.
- Release the mouse. The image will resize.
Make an Image Larger
- Select the Scale Tool from the Georeferencing Toolbar.
- Move the cursor to the center of the image.
- Click and drag from the ccenter to beyond one of the outside edges. The further outside the edge of the image you drag, the larger the image will become.
- Release the mouse. The image will resize.
The Shift Tool
You can move an image file around the Data Frame using the Shift Tool to reposition it for the purposes of viewing Control Points on your image and Basemap simultaneously.
Scale Shift Tool
- Select the Shift Tool from the Georeferencing Toolbar.
- Click and hold the mouse on part of the image. Drag the cursor to the desired area in the Data Frame.
- When you release the mouse button, the portion of the image on which you clicked and dragged will move to the area where the cursor was when you released the mouse.
The Rotate Tool
To orient an image, use the Rotate Tool to turn the image to match the Basemap.
Scale Rotate Tool
- Select the RotateTool from the Georeferencing Toolbar.
- Click and drag the image to rotate it in the Data Frame.
- When you release the mouse button, the image will rotate.
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Add Control Points to Your Image
The Add Control Points tool is the tool that allows you to georeference the image. This is the process of matching points on the image layer to corresponding points on the shape layers
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Add Control Points icon
This tool allows you to georeference the image:
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.
Add Control Points Using Visual Cues
Control points can be added using visual cues. These usually take the form of visible landmarks, like the corner of a sharply defined agricultural plot of land, or the bend of a river.
Note: it is a good idea to zoom in on your image when adding control points for better accuracy.*
- Select the Add Control Points button
Add Control Points Button - Left click on a known point on the image. This will place a cross mark on that location.
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- Left click on the matching control point in
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- your Basemap. This will 'move' the image and better
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- align the control points.
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- Repeat this step with each control point
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- .
Add Control Points Using Latitude and Longitude
You can enter x (longitude) and y (latitude) coordinates as Control Points rather than use If you would like to input x and y coordinates as an alternative to the method mentioned above, after placing . To do this,
- Left click on a known point on the image. This will place a cross mark on
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- that location
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- .
- Right click on the point you just created. A menu will appear.
- Choose Input X and Y. The Enter Coordinates dialog box will appear.
- Enter the coordinates in the appropriate boxes
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- .
For every set of control points you create, an entry is created in a table that records the original coordinates, the control point coordinates, and the residual error. Access the table by choosing the View Link Table icon from the Georeferencing toolbar
Entries in this table can be deleted one at a time (highlight the entry in the table and click the delete icon) thus making corrections is easy.
Adjusting the Final ProductResidual error is the measure of the fit between the true locations and the transformed locations of the output control points. If there is a link with a high level of error, consider deleting it by highlighting the point in the link table and clicking the delete icon.
Depending on the number of control points you have, from the Georeferencing table you can perform either a 1st, 2nd, or 3rd order transformation. The transformations compare the coordinates of the source image with the control points creating two least-square-fit equations to translate the image coordinates into map coordinates.
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Most of the time either a 1st or 2nd order will suffice, try all three and choose the one that works best
Saving changes
When you are satisfied with the georeferencing process, three options for saving changes are offered:
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