Georectifying images in ArcGIS 10
Davis Library GIS Services Spring 2013
How to georectify an image in ArcMap 10
The University Library has a large collection of historical aerial photos for some North Carolina
Counties ( http://www.lib.unc.edu/reference/gis/USDA/index.html ). For this lab, you will georectify an
historical aerial photo of Orange County, NC from either 1975 or 1955. These images will be saved by
the library and made available for future users, so do your best!
About the images
The Library’s digital aerial photos are from the USDA Aerial Photography Field office. The images came
with indexes, which are used to find which image(s) cover any given study area. The USDA has all of the
images that were acquired on the flights. The images overlap quite a bit. This is called stereo coverage.
Since there is so much overlap between the images, the library only purchased about half of them-
enough to cover the counties completely with minimal image overlap. This is called physical coverage.
The indexes show all of the images, including those the library does not own.
The ID numbers of the images the
library owns are labeled on the indexes with
beige boxes.
Since the indexes show all images (stereo coverage), the
individual images cover a larger area than is readily
apparent from looking at the index.
How to access and prepare the scanned image for georeferencing.
1. Each student has been assigned one historical aerial image to georectify. The assignments are
listed on this Google Spreadsheet: http://tinyurl.com/bygq3lt
2. The indexes of these historical images are available in kml format and can be viewed in Google
Earth to help you determine the coordinates of the area (generally) covered by your aerial
photo. Each student also has a link to the index for their image in the Google doc (link above).
The index must be viewed in Google Earth.
3. Download the image you have been assigned, unzip the file and save the image to your course
workspace folder. You will need to access the image later, so be sure to remember where you
save it.
4. Locate your image on the index, either by using the map on the library website (listed below) or
using the index in Google Earth. Notice that the images are in the order in which they were
taken on the flight path - generally starting from west to east and then moving north to south,
Georectifying images in ArcGIS 10
Davis Library GIS Services Spring 2013
and returning south to north. You will need to find exactly where to georeference and then
rectify your image. There are two ways to do that:
A. If you do not have Google Earth available on your computer, use the website with the index
displayed on a Google Map, either
1975: http://www.lib.unc.edu/reference/gis/USDA/orange_1975/index.html
or
1955: http://www.lib.unc.edu/reference/gis/USDA/orange_1955/index.html
Use the Zoom and Pan tools on the Google map that shows the index of your image.
Click on the Apply Transparency button at the top of the map to find street names,
intersections or other land marks that you can use to find your approximate location on
another map (this will be most difficult option).
OR
B. (Preferred )View the index in Google Earth. There is a link to open the appropriate index in
Google Earth inside the Google spreadsheet (http://tinyurl.com/bygq3lt), there is also one
on the appropriate web page, either
1975: http://www.lib.unc.edu/reference/gis/USDA/orange_1975/index.html
or
1955: http://www.lib.unc.edu/reference/gis/USDA/orange_1955/index.html
Below the map, click the link: View this index in Google Earth. Once you have located your
image, zoom into it (use the scroll button on the mouse). The transparency slider is located
in the middle of the left hand side. Use this to find a street intersection or other land marks
that may not have changed over the years. You may notice the index image is off a little
from the map.
Georectifying images in ArcGIS 10
Davis Library GIS Services Spring 2013
5. Important: select Tools > Options and change it to Show Lat/Long in Decimal Degrees.
Click OK.
6. Click on the yellow push pin symbol located along the top. This can be used to get xy
coordinates for your image. Drag the push pin to the intersection or landmark you identified. A
dialog box will open that has the long and lat coordinates which can be copied and passed into
ArcGIS. (If you click OK in the dialog box, it will close. Reopen it by right clicking on the push pin
and clicking properties.)
Minimize Google Earth while you prepare your workspace in ArcMap.
Setting up the workspace
7. Open ArcMap 10 and open a Blank Map.
8. Click File in the menu ribbon. Select Add
Data then Add Basemap. Select Imagery.
9. In Arc GIS, click on the Go
To XY button on the
Toolbar ( ).
Use the drop-down menu
to select Decimal Degrees.
10. Paste the coordinates of the yellow pushpin you created in Google Earth into the Go To XY box in
ArcMap. Y is Latitude and X is
Longitude. Note that you will need to
place a negative sign in front of the X
value or the point will be placed in
China. Use the other tools to help you
Georectifying images in ArcGIS 10
Davis Library GIS Services Spring 2013
zoom in to the location of your image. You can: add labeled point, zoom, etc. Zoom to a workable
level. Later when you no longer need the labeled point you can click on it and press delete.
11. Create a bookmark so you can find the location again easily. Click Bookmarks in the menu
ribbon and then Create. In the pop-up window, name the bookmark. Return to this
extent/zoom level easily by clicking on Bookmarks and selecting the bookmark.
12. Turn on the Georeferencing Toolbar. Click Customize in the menu ribbon. Select Toolbars and
then select Georeferencing. (If there is already a check mark next to it, then just leave it
selected.) The Georeferencing toolbar now will appear below the ribbon menu bar.
Add the image
13. Click the Add Data button ( . Click the Connect to Folder icon and find the folder that
contains the image to be georeferenced that you saved in step 3.
• Select the image in the main portion of the Add Data window and click Add.
• Click YES to create pyramids (pyramids will improve performance by speeding up display
of the data as you zoom in and out).
• Click OK when the “Unknown Spatial Reference” warning appears. The image name will
appear in the Table of Contents menu but the image will not appear in the map canvas.
(Note: click only once on the image before clicking Add. Clicking twice will open up the image to
allow a user to select individual band of the image. If you click twice, select Up One Level to
move up in the file hierarchy)
14. In the Georeferencing toolbar, make sure that the Layer field contains the name of your file.
15. Select the Georeferencing drop-down menu in the Georeferencing toolbar and then select Fit to
Display. The image to be georeferenced will appear in the main window on top of the basemap.
Georectifying images in ArcGIS 10
Davis Library GIS Services Spring 2013
Clicking the box next to the name of the image in the Table of Contents
window will turn the image on and off.
16. In the Table of Contents window, right-click the name of the image and select ‘zoom to layer’. If
the image does not appear, left click the small check box to select the image in the table of
contents.
Georeferencing
17. Find a feature present in both maps (e.g., a street intersection, building, monument, or feature
present both in the image and the basemap). Click on the Add Control Points button in the
Georeferencing toolbar. The arrow now will turn into a crosshairs marker.
Georectifying images in ArcGIS 10
Davis Library GIS Services Spring 2013
18. Click on a point on the image to be georeferenced. Then click on the corresponding location on
the basemap. It is important to always click on the image to be georeferenced first, then the
corresponding location on the basemap. Since the image may be overlaying the map, once the
first point is created, turn the image off in the table of contents, and then place the
corresponding point. Click once in each location. A green crosshairs marker will appear on the
image being georeferenced and a red crosshairs will appear at the location on the base map.
The points will be numbered as you place them. If the first click in the wrong place, right click
and cancel point.
• Another way to find your way between the two images is to use the transparency
setting. Right-click the name again and select Properties. In the Layer Properties
window, select the Display tab and change the Transparency value to 25.
The image will auto-adjust once a pair of control points has been added. (Note: You can turn off
this auto adjust feature by selecting the Georeferencing drop-down menu in the Georeferencing
toolbar and unselecting Auto Adjust. When Auto Adjust is unselected, a blue line will appear
between a placed control point and point on the image. To adjust the orientation of the image
based on the point placement, select Update Display from the Georeferencing drop-down
menu.)
19. After placing points, select Update Display (if the Auto Adjust feature is turned off).
20. Open the Link Table in the Georeferencing toolbar.
The Residual column in the Link Table
reports the amount of correction the GIS
must perform for each point. A large
Residual for a particular point may
indicate that the point was erroneously
placed.
21. Continue to place points. Distribute points throughout the image to avoid image warping. Pay
attention to the residuals as you place points. Note what happens as you place more points.
To delete a pair of control points, select View Link Table in the Georeferencing toolbar, select
the point, and click the delete button.
Hold down the mouse wheel to pan in the viewing window. Click Bookmarks and the bookmark
to return to the initial extent and zoom level. A mixture of panning, zooming, and turning the
Georectifying images in ArcGIS 10
Davis Library GIS Services Spring 2013
image to be georeferenced off may be required to identify locations/items present on both
maps. Points should be located near the center and near all four corners.
22. On the Georeferencing toolbar, look at the options listed under Georeferencing >
Transformation. Try and make note of the different transformations and see how this affects
the way the map looks and the residual values.
23. Select Update Georeferencing from the Georeferencing toolbar drop-down when finished.
24. When you are finished adding control points and feel the image is properly adjusted, click the
Georferencing drop-down and select Rectify.
Accept the default Cell Size. Leave NoData blank. Resample Type: Nearest Neighbor (for
discrete data)
Output location:
Create a folder named for your onyen in the submission folder in the course space:
\afs\isis\html\courses\2010fall\geog\370\006\@submission\lab4\yourlabday\youronyen
(note: \afs\isis\ is often mapped to the J: drive)
***IMPORTANT!!***
When you specify the output location, navigate to folder you created. The file name goes in
the Name: box.
(see image on the next page)
Georectifying images in ArcGIS 10
Davis Library GIS Services Spring 2013
Save the image in the TIFF format in the folder. Keep the original file name. ArcMap will create
three files – a TIFF, a .rrd, and a .aux. Make sure all files are there.
Part II. Answer these questions in a Word document and save it to the same folder:
1. In your own words, explain the process of georectifying a digital image.
2. How might you or another researcher use this technique in the future?
3. Explain the different rectification methods available. Which did you choose?
4. What happened to the RME as you added more points? Why might this be?
More resources:
Fundamentals for georeferencing a raster dataset
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009t000000mn000000
Georeferencing rasters in ArcGIS (video)
http://resources.arcgis.com/gallery/video/image-management/details?entryID=BEE233D1-1422-2418-
3442-F45730292D87
Georeferencing a raster dataset
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Georeferencing_a_raster_dataset/009t
000000mq000000/
Bibliography
Allen, D. W. & Coffey, J. M. (2011).GIS Tutorial 3: Advanced Workbook. Redlands, Calif.: ESRI Press.
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