Georeferencing Guide for OSU Maps and Spatial Data Employees

About

Last Updated 09/06/2019
Created by OSU Maps and Spatial Data

Table of Contents

• Introduction
• Selecting a map
• • 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.

Selecting a map

1. Go to the Georeferencing Tracking spreadsheet.
2. Pick a project to work on, then click on its respective sheet.
3. Select a map from the sheet that isn’t marked as done. Take note of the file name.

Locating the File

1. To locate the map file, follow the instructions for your chosen project.

2. Select the folder that contains the file name of the map you chose and locate the corresponding TIF File. See the example below for the McCasland Collection.

   

3. Open the file in Adobe Photoshop. Click File, Save As.

   

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

   

5. 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

Opening a Project

1. You should have a project located in C:\Georeferencing\<your name>. If you don’t have one, follow the instructions on this document to create one.

Adding Files

With your project created/opened, 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 (C:\Georeferencing\<your name>) 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 the Imagery tab, then under it, click Georeference. A new Georeference tab will 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.

   

Helpful Layers

Depending on what you are georeferencing, there may be one or more useful layers to add to your map to aid the process.

Latitude and Longitude (graticule)

If your map has latitude/longitude lines on it, you can use them to take a georeferencing shortcut.

1. Click on the Map tab, then click Add Data. Navigate to C:\Georeferencing\Georeferencing_Helpers\graticule.
2. Select the file called graticule.lyrx, then click OK.
3. You should see a layer appear that looks like this.
4. It may not look very helpful at the moment, but if you zoom in, you will see that each line is clearly labeled, particularly those divisible by five.
5. When identifying and adding control points (see below), use the intersections of latitude longitude lines.

Township and Range

Some maps, especially but not limited to the WPA collection, contain detailed information from the Public Land Survey. This information can be used for georeferencing as well.

1. Click on the Map tab, then click Add Data. Navigate to C:\Georeferencing\Georeferencing_Helpers\.
2. Select the file called ok_township_range.lyrx, then click OK.
3. You should see a layer appear that looks like this.
4. If you need sections, you can add the layer sections.lyrx from the same location. It will increase load time, however, so only add sections if you can make use of them.

Aerial imagery

When georeferencing aerial photos, you can make use of two layers. One is relatively current imagery that’s better positioned than the Satellite basemap on ArcGIS. The other is a layer of historical 1:24k topographic maps from the USGS. These will be very helpful when dealing with older, more remote aerials.

1. Click on the Map tab, then click Add Data. Navigate to C:\Georeferencing\Georeferencing_Helpers\.
2. Select the file called AerialReference.lyrx, then click OK.
3. You will see the layer appear in the table of contents (left hand side of the application). Click and drag it so that it is below the map/photo you are going to georeference. The order layers appear in the TOC determines how they are drawn on the map, and if the aerial/topo layer is on top, they will cover up your georeferencing object.
4. Repeat 1-3, with USA_Historical_Topo_Maps.lyrx.

Oklahoma County boundaries

Sometimes, you just need some county boundaries!

1. Click on the Map tab, then click Add Data. Navigate to C:\Georeferencing\Georeferencing_Helpers\.
2. Select the file called okla_counties.lyrx, then click OK.

Adding Control Points

Now that you’ve got your helper layers in place, it’s time to add 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.

   

For maps with latitude longitude or township/range/sections

1. Locate and select the point of intersecting lines on the raster image (for latitude/longitude, township/range). A red box should appear, indicating the selection on the map.

   

2. Turn off the raster layer by clicking the check box next to its name in the Contents pane.

3. Find and select the corresponding intersection on the original world map. Be as precise as possible to ensure minimal error. Once this is done, a control point is added.

   

For aerial photos or maps without grids

1. Use the historical topo layer, aerial layer, county boundaries, etc to identify the location of the map/aerial being georeferenced. Some good spots to use as control points include:

   • The corner of a building where it is closest to the ground.
   • Section road intersection (combined with the sections layer mentioned above)
   • Distinctive pavement (like cul-de-sacs)

   Some spots that you should NOT rely on, as they change too rapidly:

   • Vegetation
   • Any kind of water, especially rivers/creeks!
2. Locate and click the point on the raster image being georeferenced. 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 intersection on the original world map. Be as precise as possible to ensure minimal error. Once this is done, a control point is added.

Important

An absolute 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 Transformation icon in the Adjust section of the Georeference tab. Then select the desired transformation.

   

   Note: Spline is preferred 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!

![Bad Control Point](/guide-georeferencing-msd/images/BadControlPoint.PNG)

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.

   

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

   

3. 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.

4. 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. Locate the files for the map that was georeferenced (should be in C:\Georeferencing\<your georeferencing folder>).

2. 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.

   

3. Right click in the highlighted area and click Cut.
4. Open the Georeferencing folder for your project on the Digitization (T:) drive. 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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