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 themenough 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,
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.
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
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.
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.
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
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)
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.