index.md

About

Last Updated [06/06/2019]
Created by OSU Maps and Spatial Data

Table of Contents

• Introduction
• McCasland Maps
• • Has this map already been georeferenced?
• • Locating the File
• Georeferencing with ArcGIS Pro
• • Starting a New Project
• • Adding Files
• • Georeferencing
• • • Adding Latitude and Longitude
• • • Adding Control Points
• • • Transforming the Image
• • • Deleting Control Points
• • Saving the Project
• • Ensuring Proper File Location
• Conclusion
• Further Reading/Resources

Introduction

Georeferencing is the process of adding geographic information to a raster image (i.e. maps, satelite images and aerial photographs) so mapping software can place the image in its real world location. This is done by assigning geographic coordinates to the raster's pixels.

McCasland Maps

There are many steps to locating and selecting a McCasland map for georeferencing.

1. Open a web browser and visit the OSU Library McCasland Maps Collection.
2. Browse the maps collection until you find a map with latitude and longitude lines.
3. Click on the map and scroll to the bottom of the page where you will find the file name.

Has this map already been georeferenced?

1. To locate this map on the desktop, click Digitization (\ulib.okstate.edu)(T:) aka the T File.
2. Click the Maps folder.
3. Click MPSI_Aerials.
4. Select the McCasland_Georeferencing folder and search for the map's file name.

Note: If the file name is in this folder, the map has already been georeferenced.

Locating the File

1. To locate this map on the desktop, click Digitization (\ulib.okstate.edu)(T:) aka the T File.
2. Click the Maps folder.
3. Click Maps_00.
4. Select the McCalsand folder. Here you will see folders sorted by equal intervals.

5. Select the folder that contains the file name of the map you chose and locate the correspondig TIF File.

6. Open the file in Adobe Photoshop and click File, Save As.

7. Make sure to name this new file exactly as the TIF is named and change the Save as Type to JPEG. Then click Save.

8. A pop up should appear. Do not change any of the settings unless instructed to do so. Click OK and exit Photoshop.

Georeferencing with ArcGIS Pro

Starting a New Project

1. To begin a new project, open ArcGIS Pro.
2. Click Map under New, Blank Templates.

3. Name the project and choose a file location that will be easy to access. Then click OK. A world map screen should open.

Adding Files

Now that a new project has been created, a folder connection must be added to import data.

1. To do this, click Add Folder under the Insert tab of the toolbar to create a folder connection.

2. Select the desired folder and click OK.
3. The folder connection should appear under Folders in the Catalog pane.

4. Locate desired data. Right click the file and click Add to Current Map. The selected file is now added to the project and should appear in the Contents pane. It is okay if the raster is not displayed on the map as long as the file is visible in the Contents pane.

Note: For georeferencing in ArcGIS Pro, JPGs are the preferred file type.

Georeferencing

Once a file has been added to the project, the georeferencing process can begin.

1. In order to start georeferencing, the desired file must be selected in the contents pane.
2. Click Georeference under the Imagery tab of the toolbar. A new Georeference tab should appear on the toolbar.

3. Click Fit to Display under the Georeference tab. If the raster image was not visible before, it should now appear on the map. The size of the image can be adjusted by zooming in or out and clicking Fit to Display as needed, or by using the Move, Scale or Rotate tools. The image does not need to be the exact size of the geographic area it covers. This will be corrected during the georeferencing process.

Adding Latitude and Longitude

Georeferencing is made possible by adding latitude and longitude lines to the basemap.

Adding Control Points

Control points are used to align pixels on the raster image with real life coordinates.

1. To add control points, click Add Control Points in the Adjust column of the Georeference tab.

2. For maps, locate and select the point of an intersecting latitude and longitude line on the raster image. A red box should appear, indicating the selection on the map.

3. Turn off the raster layer by clicking the check box next to its name in the Contents pane.
4. Find and select the corresponding coordinates on the original world map. Be as precise as possible to ensure minimal error. Once this is done, a control point is added.

6. Continue this step until enough control points are added for the desired transformation.

Note: A minimum of four control points should be placed. Spline is the preferred transformation for accuracy and requires 10 or more control points, but there are other transformations when this number of control points is not possible. The more control points that are added, the more precise the transformation will be. Also, it is best to have the control points evenly distributed throughout the map to reduce the chances of transformation error.

Transforming the Image

Transforming the image allows the raster image to be manipulated so the control points are aligned.

1. To transform the image, ensure that all necessary control points have been placed.

Note: You can add more control points even after the image has been transformed.

2. Click the down arrow under the Transformation icon in the adjust section of the georeference tab. Then select the desired transformation.

Note: Spline is preffered because it renders the most accurate transformations.

3. The image is now transformed and can be saved or edited if desired.

Deleting Control Points

Do not fret if a control point is misplaced or results in a skewed image. They are easy to delete!

1. To delete a control point, click Control Point Table in the Review section of the Georeference tab.

2. A table will open at the bottom of the screen. This table shows all of the control point data. If the incorrect control point is unknown, click the checkbox next to a point to uncheck it. If the image becomes unskewed, this is the faulty control point.

3. To delete a control point, click Delete in the Review section of the Georeference tab.

Saving the Project

When finished with the project, it is important to ensure that it is saved properly. Clicking the Save icon in the top left corner of the screen will not save the georeferenced data.

1. Click Save in the Save section of the Georeference tab. This will update the existing raster file that was imported into the project.

1. Click Export Control Points in the Save section of the Georeference tab. This will create a text file that contains the control point data.

2. Name the file the same as the JPG and the TIF files and save it in the same folder where the TIF is located.

After all of this is done, it is important that the project is closed correctly.

1. Click Close Georeference under the Georeference tab of the toolbar.

The project can be closed and ArcGIS Pro can be exited.

Ensuring Proper File Locations

Once a map has been georeferenced, some of the files must be moved to the folder containing the completed map information.

1. Open Digitization (\ulib.okstate.edu)(T:), Maps, Maps_00, McCasland.
2. Locate the files for the map that was georeferenced.
3. Select the JGWX and JPG files as well as the XML document by holding Ctrl on the keypad and clicking on all three files. They should be highlighted after they are clicked. Leave the TIF file.

4. Right click in the highlighted area and click Cut.
5. Open the McCasland_Georeferencing folder. Right click within the folder and select Paste. The three selected files should appear in this folder.

Conclusion

Georeferencing is a very usefull skill and is made simple by ArcGIS Pro. It only takes a little practice and time to transform a raster image by adding geographic information to its pixels.

Further Reading/Resources

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