Obtaining the Tutorial Data

Before beginning the tutorial, you will copy all of the required tutorial data onto your Desktop. Option 1 is best if you are completing this tutorial in one of our short courses or from the GIS/Data Center and Option 2 is best if you are completing the tutorial from your own computer.

OPTION 1: Accessing tutorial data from Fondren Library using the gistrain profile

If you are completing this tutorial from a public computer in Fondren Library and are logged on using the gistrain profile, follow the instructions below:

1. On the Desktop, double-click the Computer icon > gisdata (\\file-rnas.rice.edu) (R:) > Short_Courses > Mapping_Imagery.
2. To create a personal copy of the tutorial data, drag the Imagery folder onto the Desktop.
3. Close all windows.

OPTION 2: Accessing tutorial data online using a personal computer

If you are completing this tutorial from a personal computer, you will need to download the tutorial data online by following the instructions below:

1. Click Imagery.zip above to download the tutorial data.
2. Open the Downloads folder.
3. Right-click Imagery.zip and select Extract All....
4. In the 'Extract Compressed (Zipped) Folders' window, accept the default location into the Downloads folder and click Extract.
5. Drag the unzipped Imagery folder onto your Desktop.
6. Close all windows.

Setting Up a Reference Map

1. On the Desktop, double-click the Imagery folder.
2. Double-click the Imagery.aprx project file to open the project in ArcGIS Pro.

Before you can georeference an image, you must have existing GIS data with known coordinates containing similar features to those in the image. While you can georeference to the built in basemaps, additional vector data can be helpful, because vector data can lay on top of the imagery you are trying to georeference, so they can be seen simultaneously, whereas relying on the basemap would require repeatedly toggling the image on and off or adding a transparency to the image, so that both the image and the basemap are visible, but this type of display can be more difficult to interpret. Since you will be georeferencing an aerial image of the Rice campus, have vector layers related to campus features would be helpful.

1. In the Catalog pane on the right, expand the Databases folder.
2. Expand the Imagery.gdb geodatabase.
3. Click the first buildings feature class to select it.
4. Hold downShift and click the last streets feature class to select all four feature classes within the geodatabase.
5. Right-click the streets feature class and selectAdd To New Map.

Ensure that all four feature classes (streets, sidewalks, parking, buildings) appear in the Contents pane on the left. If you are missing any layers, drag them from the Catalog pane on the right directly into the Map view. The layers of the Rice campus should look familiar. If desired, update the symbology of each of the layers to a color of your choice by right-clicking on the color patch beneath each layer name and selecting a color. For example, you might choose to make the parking and streets layers two shades of gray, the sidewalks layer tan, and the buildings layer red.

Now you will add in a JPEG file depicting an aerial view of campus. This image is just like any other image file you are familiar with and does not contain any sort of spatial reference.

1. In the Catalog pane, expand the Folders folder.
2. Expand the Imagery folder.
3. Right-click the aerial.jpg raster image and select Add to Current Map.

If the Add to Current Map command does not work, you can click the Add Data button within the Map tab on the ribbon and then browse to and select the same image.

1. In the Table of Contents, right-click the aerial.jpg layer and select Zoom to Layer.
2. In the Table of Contents, right-click the streets layer and select Zoom to Layer.

Both layers appear in the data frame, but, as expected, the image cannot be lined up with the other data layers, because it has not been georeferenced and no projection has been defined.

Occasionally, you will come across an undefined data layer for which you cannot locate complete metadata telling you the coordinate system and projection for the data. In other cases, you may wish to make use of raster data that never had a coordinate system in the first place, such as this aerial photograph or a scanned historical planning map. Whenever the spatial reference for a layer is unknown or non-existent, you must georeference the layer by lining it up visually with other layers that have a known projection.

Georeferencing Imagery

Since the aerial photograph never had a coordinate system associated with it, you will need to georeference it based on the other campus layers using control points.  Control points are recognizable points in both layers that occur in the same geographic location.

1. On the ribbon, click the Imagery tab.
2. Click the Georeference button to open the Georeference tab.

Before you can begin to align the aerial image with the other layers, you will need to be able to see them all in the same view.

1. In the Contents pane on the left, right-click the streets layer and select Zoom to Layer.
2. In the Georeferencing toolbar, click the Fit to Display button.

The full extent of the image now appears behind the current layers.  It is always good to zoom in to the approximate extent of the image before using the Fit to Display command.  Notice that while both layers are now visible, they are not yet aligned.

You now need to identify a common point in both the aerial image and the vector layers.

1. In the Table of Contents, uncheck the parking and sidewalks layers to make the aerial image easier to view.
2. Visually, identify the football stadium in both the aerial layer and the buildings layer.
3. On the Tools toolbar, click the Zoom In button.

1. On the western portion of campus, click and drag a box around the football stadium in both layers to zoom into the area in which you will add your first control point.

1. On the Georeferencing toolbar, click the Add Control Points button.  Your cursor will change to crosshairs.

1. Click on the southwest corner of the upper deck of the stadium in the aerial layer.

1. With a line linking to the first point you clicked on, click on the southwest corner of the upper deck of the stadium on the buildings layer.

The two points will snap together, lining up the image at that particular control point.

1. In the Table of Contents, right-click the streets layer and select Zoom to Layer to return to the full extent of the image.

At this point, the image is aligned at the one control point, but the scale of the image does not match the scale of the vector layers, so nothing else is yet aligned. You will need to select another control point to begin to establish the proper scale. Remember, it is best to select a point that is not horizontally or vertically in line with the existing control point.

1. Visually, identify Fondren Library in both the aerial layer and the buildings layer.
2. On the Tools toolbar, click the Zoom In button.

1. In the center of campus, click and drag a box around Fondren Library in both layers to zoom into the area in which you will add your second control point.

1. In the Georeferencing toolbar, click the Add Control Points button.
2. Hold down the Spacebar to temporarily override snapping, so that you can accurately click on the southwest corner of Fondren Library in the aerial layer, without being forced to snap to the vector layers.

1. Release the Spacebar to reactivate snapping and, with a line linking to the first point you clicked on, click on the southwest corner Fondren Library on the buildings layer.

The two points will snap together, lining up the image at that particular control point.

1. In the Table of Contents, right-click the streets layer and select Zoom to Layer to return to the full extent of the image.

At this point the majority of the image appears to be aligned. If you decide to add another control point, you should do so wherever there is currently the most discrepancy between the image and the vector layers.

Notice that, in the northern-most section of campus, the buildings are not quite aligned.  You will add a third control point in that area.

1. Visually, identify Martel College in both the aerial layer and the buildings layer.
2. On the Tools toolbar, click the Zoom In button.
3. In the northern-most portion of campus, click and drag a box around Martel College in both layers to zoom into the area in which you will add your third control point.

1. In the Georeferencing toolbar, click the Add Control Points button.

1. Hold down the Spacebar and click on the northwest corner of Martel College in the aerial layer.

1. Release the Spacebar and, with a line linking to the first point you clicked on, click on the northwest corner Martel College on the buildings layer.
2. In the Table of Contents, right-click the streets layer and select Zoom to Layer to return to the full extent of the image.

At this point, the image looks well aligned with the other vector layers.

1. On the Georeferencing Toolbar, click the View Link Table button.

This table lists all the control points you have added. The residual, in the final column of the table, is a measure of how distorted the image has become. A lower residual indicates that the georeferenced image has less distortion from the original image. It not necessary to obtain a residual of zero, as this is difficult unless both layers are from the same source or were already in the same coordinate system.

If you added a poor or incorrect control point, you could select it in the table and click the Delete Link button to remove it. You could also simply uncheck it, so that it is no longer factored in to georeferencing calculations. You would then want to continue adjusting your control points until you achieved the desired result.

1. Close the Link Table.

Notice that none of the final three control points are in line with each other vertically or horizontally, the image appears to be well aligned, and the residual is at or close to 0.  These are all indications that the georeferencing was successful.

1. When you are satisfied with your control points, on the Georeferencing toolbar, click the Georeferencing drop-down menu and select Update Georeferencing.

Notice that the control points have disappeared.

1. On the Standard toolbar, click the Save button.
2. Close ArcMap.
3. On the desktop, double-click the ImageryTutorialData folder.

Before beginning this tutorial, there was only one aerial file: aerial.jpg. During the tutorial, you generated the three additional files. The JGWX file contains the spatial referencing information that you just created. Because this new file has been created, the image will now line up spatially whenever you add it into a new data frame. In other words, you only need to georeference a layer once.

The OVR file contains an index of the image that was created when you selected to build pyramids upon first loading the image. The pyramids provide a quicker rendering of the image when you zoom in and out in the Map Display window. The XML file contains the metadata used by ArcCatalog to describe the newly georeferenced aerial image.

If you were to delete these three accessory files, the image would no longer be georeferenced and would be returned to its original state.