Georectifying images

Before tackling this tutorial, you will need to download and install a dataset following these instructions:

Create a folder called georectify somewhere under your personal directory (e.g. C:\Users\jdoe\Documents\Tutorials\georectify\).
Download the data for this exercise and unzip its contents into the georectify folder.

Open the ArcGIS Pro project

Open the georectify.aprx file.

The project consists of a barebones map document with an ESRI provided imagery basemap.

The extent should be set to the Colby College campus. If it's not, you can set the extent to the campus via the Bookmarks >> Colby Campus link.

Loading a plain image file

In this exercise, we will use aerial imagery of the campus as a backdrop to help georectify a png file. The png file is located in your project folder and is called colby1996.png. It's an aerial view of the campus taken back in 1996.

In the Catalog pane, load the colby1996.png into the map document via a drag-and-drop operation

The image file is not a GIS file in that it has no coordinate system information that the software can use to properly locate the image on the map.

In the Contents pane, right-click on the colby1996.png file and select Zoom To Layer.

The software is placing the image somewhere in the Amazon basin.

ArcGIS does not place an unrectified raster layer haphazardly in a map. If the layer is not provided with coordinate system information, the software will assume that the upper left hand pixel is assigned the (0,0) coordinate value and that each pixel is 1 mapping unit in size (mapping unit is in meters in this map project). The image is 1782 pixels wide by 1555 pixels tall, therefore, ArcGIS assumes that it covers an extent that is 1782 meters wide by 1555 meters tall (as can be seen in the adjacent figure that has a grid superimposed on the map).

Next, we will assign control points to the png layer telling ArcGIS where the layer should be placed on the earth's surface.

Adding control points

First, let's zoom back to the Colby campus extent. Feel free to use the bookmark shortcut.

Select the colby1996.png layer in the Contents pane.
Next, click on the Georeference tool in the Imagery toolbar.

If the Georeferencing button is ghosted out, you probably did not select the colby1996.png layer in the Contents pane.

Clicking the Georeference button will add a Georeference ribbon to your toolbar.

In the steps that follow, you will link features on the png file to locations on the earth's surface. But first, we will move the image to the extent encompassing Colby campus. This will make it easier to toggle back-and-forth between the png file and the Colby campus backdrop.

While making sure that the map extent is set to the Colby campus extent (you may need to click on the Bookmark), click on the Fit to Display button in the Georeference tab.

This should place the png file within the extent of your current map window. Next, we will add control points.

When creating control points, you want to use features that offer clear and distinct properties that can be easily matched between the unrectified and georeferenced layers. Examples of such features are road intersections and building corners (or any feature that defines a sharp angle).

As a first control point, we will use Eustis' roof line.

In the Georeference ribbon, click on Add Control Points.

You create control points by first clicking on a feature from the unrectified image (colby1996.png layer in this example), then clicking on the matching feature of the georeferenced image (World imagery layer).

Click on the northern end of the Eustis building's rooftop peak.

You'll note the helper pop-up text reminding you that the first control point is to be placed on the unrectified image (source point).

Next, click on the matching location on the Word Imagery layer. You might want to turn off the colby1996.png layer to see the backdrop layer.

This creates your first control point. If you toggle the colby1996.png layer on and off, you should see the roof peak ends overlap between both layers.

But, note that the other parts of the layers may not overlap properly. It takes several control points to properly georectify an image. The number of control points will usually be driven by the amount of distortion in the image being georectified.

Next, you will add additional control points until you have a properly georectified image. You want to have a minimum of six control points ideally spread out across the layer's extent.

As you add control points, ArcGIS will transform the colby1996.png layer on the fly.

You can view the list of control points by clicking on the Control Point Table button in the Georeference ribbon.

You can see how each control point influences the layer's transformation by checking each control point on and off. Ideally, each control point should have equal weight in the image's transformation. If unchecking one control point results in a significant change in the image's transformed appearance (i.e. more so than when unchecking other control points), this may indicate that the control point may not have been properly placed.

You can use this table to check and remove (if needed) badly placed control points.

The georectified image will only persist in this ArcGIS Pro session.In other words, loading the colby1996.png file into another map document will still result in a misplaced layer. To make this georectification permanent, you will need to export the the layer to a dedicated raster file.

In the Georeference ribbon, click on Save as New.

Save this layer as a tif file in the project folder.
All other default parameters should be fine for this exercise.
Click on Export.

You should now see the newly created TIF file in your Contents pane.

You do not need to save the control points to make use of the newly created TIF file in other projects. But, if you plan to revisit the control points for the unrectified file, you can export the control points to a dedicated table via the Export Control Points button in the Georeference ribbon. We will not bother with this step, but note that once you close the Georeferencing session, you lose all control points.

To end the georeferencing exercise and to close the Georectify ribbon, click on Close Georeference button.

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