Add historical imagery to an ArcGIS Pro project

Imagery is obtained from many sources, such as satellite, aerial cameras, and scanned historical aerial photos. Modern satellite images and aerial cameras tend to have relatively accurate location information, but scanned historical aerial photos usually do not contain spatial reference information. This means that you cannot use them in a GIS system along with other geospatial data—the software would not know where to locate them on the map.

There are various methods to remedy this issue. The more advanced workflows involve orthorectification techniques, but they also assume that you have a certain amount of information about the images. In this lesson, you'll focus on simpler georeferencing techniques, where you align the imagery to a map coordinate system.

Set up the project in ArcGIS Pro

First, you'll set up the project in ArcGIS Pro, and choose the desired projection coordinate system.

  1. Download the Aspen Georeferencing project package.
  2. Relocate the file AspenGeoreferencing.ppkx to a location you can easily find, such as your Documents folder.
  3. Double-click the AspenGeoreferencing.ppkx package to open it in ArcGIS Pro.
  4. If prompted, sign in using your licensed ArcGIS account.
    Note:

    If you don't have ArcGIS Pro or an ArcGIS account, you can sign up for an ArcGIS free trial.

    The AspenGeoref project opens in ArcGIS Pro.

    Next, you'll review the content of the project.

  5. In the map Contents pane, there are two layers, which together form the Imagery Hybrid basemap. Turn each layer off and back on to see what it represents on the map.

    Imagery Hybrid basemap

    The basemap layer display imagery and major roads, which will allow you to identify control points while georeferencing. The map is focused on the area of Aspen, Colorado.

    AspenGeoref map

Next, you'll add the historical aerial photo.

Add and locate the historical photo

You will add an image to your map that does not have statistics or georeferencing data. You will use tools in ArcGIS Pro to calculate the statistics so the image will load more smoothly as you work with it, and identify where the map was placed without its georeferencing data.

  1. On the ribbon, click the View tab, and in the Windows group, click Catalog Pane.

    Catalog Pane

  2. In the Catalog pane, click the arrows to expand Folders, AspenGeoreferencing, commondata, and raster_data2. Right-click AspenSource.tif and choose Add To Current Map.

    Add image to map.

    The Calculate statistics for AspenSource.tif window appears notifying you that the image you are adding does not have statistics and prompting you to calculate them. You will accept and calculate statistics for your image.

    Note:

    Statistics are useful to improve the display of imagery and perform certain tasks on it. Learn more about calculating statistics.

  3. In the Calculate statistics for AspenSource.tif window, click Yes.

    Calculate statistics for AspenSource.tif window.

    AspenSource.tif appears in the Contents pane, but for now, you don't see it on the map. A message in the upper right corner warns you that the image does not have coordinate system information.

    Georeference warning

    This is expected, because the image you added did not contain any spatial reference information. Before you remedy this, you'll locate the image on the map.

  4. In the Contents pane, right-click AspenSource.tif and choose Zoom To Layer.

    Zoom To Layer

    The map now shows the historical aerial photo taken in the region of Aspen. However, since it is not georeferenced, the application cannot locate it and displays it near the latitude and longitude (0,0), as you can see indicated in the lower part of the map view.

    Ungeoreferenced image displaying at 0,0

    Ungeoreferenced images are added at the intersection of the equator and the Greenwich meridian. To verify that current location, you'll zoom out until you can recognize geographic elements.

  5. Zoom out repeatedly with the mouse scroll wheel.

    At first the background is completely black, but after a while some land appears, and you can see that the image is located off the west coast of Africa.

    Map zoomed out to west coast of Africa

    Next, you'll return the map extent to the Aspen, Colorado, area and proceed with the georeferencing steps.

  6. On the ribbon, on the Map tab, in the Navigate group, click Bookmarks. Click the bookmark Aspen Colorado.

    Aspen Colorado bookmark

    The map viewer zooms back to the area of Aspen, Colorado.

Set the coordinate system

As a first step of the georeferencing process, you'll now set up the coordinate system in which you want to georeference the historical image.

Note:

All geospatial data and imagery must have a coordinate system or spatial reference defined. There are many coordinate systems to choose from, but one that is commonly used for data located in Colorado is NAD 1983 UTM Zone 13N. This is the one you'll use in this lesson.

  1. If necessary, in the Contents pane, click AspenSource.tif.
  2. On the ribbon, click the Imagery tab, and in the Alignment group, click Georeference.

    Locate georeferencing toolbar.

  3. On the ribbon, on the Georefence tab, in the Prepare group, click Set SRS.
    Note:

    SRS stands for Spatial Reference System.

    Set spatial referencing.

    The map properties for the AspenGeoref map appear on the Coordinate Systems tab.

    The current coordinate system for the map is WGS 1984 Web Mercator (auxiliary sphere).

    Coordinate Systems settings

    This projection is commonly used in web maps, but it is not recommended for data analysis. You'll switch to the coordinate system you selected for this project, NAD 1983 UTM Zone 13N.

  4. In the Coordinate Systems pane, type NAD 1983 UTM Zone 13N in the search box and press Enter.
  5. In the Coordinate Systems pane, expand the arrows next to Projected Coordinate System, UTM, and NAD 1983. Click NAD 1983 UTM Zone 13N and click OK.

    Set coordinate system to NAD 1983 UTM Zone 13N.

    The map now applies the NAD 1983 UTM Zone 13N coordinate system to layers in the map. Georeferencing the historical image will now take place in the same coordinate system and result in an image transformed to NAD 1983 UTM Zone 13N.

Next, you'll start positioning the ungeoreferenced image on the map.

Perform an approximate alignment

You'll now bring the image into the Aspen area, and position it manually to obtain a rough alignment with the basemap imagery.

  1. In the Content pane, verify AspenSource.tif is selected.
  2. On the ribbon, on the Georeference tab, in the Prepare group, click Fit to Display.

    Fit image to current map extent.

    The image is repositioned and is placed within the current map display.

    Review fitted image.

    The historical image is displayed ordered between the imagery basemap and the hybrid reference layer. The hybrid reference layer gives some useful information, such as the position of Highway 82. Next, you'll review the image to better understand its current position and scale.

  3. At the top of the map, collapse the Georeferencing: AspenSource.tif information pane to simplify the map display.

    Collapse information pane.

  4. On the ribbon, click the Appearance tab, and in the Compare group, click Swipe.

    Use Swipe tool to explore image.

  5. On the map, use the Swipe tool to review the historical image. Drag from top to bottom or side to side to reveal the content below the image. Try to distinguish features that can be used as reference points.

    Use Swipe tool to view the layer beneath.

    Notice that the image is oriented incorrectly. In particular, Highway 82 runs diagonally in the image. Next, you'll rotate the image.

  6. Click the Map tab, and in the Navigate group, click Explore to deactivate the Swipe tool.
  7. On the ribbon, click the Georeference tab, and in the Prepare group, click the arrow for Fixed Rotate and choose Rotate Right.

    Fixed Rotate tool

    The image rotates 90 degrees clockwise.

    Image rotated right

    You can now see that the highway runs in the same direction in the image (in white) and on the basemap.

  8. Explore the image further by zooming and panning with the mouse. You can also turn the historical image off and back on to better review the alignment with the basemap.

    You'll notice that the image covers a large area of basemap but actually represents a much smaller geographic area to the southeast of the city of Aspen. Notice the distinctive river bends that show where the bends in the historical image should align with the basemap.

    Image extent

  9. On the Georeference tab, use the Move, Scale, and Rotate tools to improve the placement of the image.

    Use Move and Scale to refine image fit.

    As you use these tools, transparency is applied to the image to assist in identifying the correct position.

    Tip:

    While using the Move and Scale tools, press the C key to momentarily revert to pan and zoom as needed.

    Adjust image position using the Move, Scale, and Rotate tools.

    Note:

    This process is not intended to achieve a perfect registration of the image; rather, you are refining the approximate placement to make it easier to visually find matching features between the input image and the basemap.

    The process of aligning an image with a reference layer is also called registration. As you move, scale, and rotate, you may notice the location registration is improving, but it is impossible to achieve the same level of registration across the whole image. As you fix registration in one area, it modifies registration in other areas. In the next step of the georeferencing process, you'll use control points and a transformation method to gain a better overall registration across the image.

    When you are satisfied with the approximate placement of the map, you'll reactivate the Explore tool.

  10. On the Map tab, in the Navigate group, click Explore to disable the georeferencing tools.
    Caution:

    For the success of the georeferencing process, do not save this image until the end of the next section, after you have added the control points.

Next, you will add control points to the image and apply a transformation.


Create control points and apply a transformation

You'll add control points to the image and apply a transformation to complete the georeferencing process.

Create control points

When you georeference an image, you define its location using map coordinates and assign the coordinate system of the map frame to the image. You do this by adding control points that identify a location in the image and its corresponding location in the reference basemap. In this section, you will identify and add several control points.

  1. On the Map tab, in the Navigate group, click Bookmarks, and choose Manage Bookmarks.

    Manage Bookmarks

    The Bookmarks pane appears for quick access to several bookmarks that were created for this lesson. They represent several suggested control point locations.

  2. On the Georeference tab, in the Adjust group, click Auto Apply once to turn it off.
    Note:

    When the Auto Apply tool is turned off, it does not have a blue background.

    Auto Apply turned off

    When Auto Apply is active, it automatically applies a transformation to the source layer and updates the display as control points are added, removed, or modified. For the purpose of this exercise, you want Auto Apply to be off to be able to observe your control points in their original location. You'll apply the transformation in the next section.

  3. In the Bookmarks pane, click 1st Control Point, and click Zoom To.

    Zoom To button for 1st Control Point bookmark

    The map extent updates to the location for the first control point. You'll now display the control point table and start adding control points.

  4. Click the Georeference tab, and in the Review group, click Control Point Table. Reposition the table below the Map view.
    Note:

    If needed, drag the top of the control point table window to adjust its size, ensuring that you can still see the map.

    Expand control point table window if necessary.

  5. On the Georeference tab, in the Adjust group, click Add Control Points.

    Add Control Points tool

    To add a control point, you'll first click a location on the source layer (the historical image), and then click the corresponding location on the target layer (the reference basemap). These locations are called the From and To points.

  6. In the map view, identify a location for the first control point at the curve along Highway 82. First, find the desired location on the historical image source layer (From point (source)).

    From point selected on the bend of Highway 82 on the historical image

    Note:

    The result of your prior rough alignment may vary, and thus the placement of Highway 82 may appear differently relative to the source image.

  7. When you're ready, click the location.

    A red square appears representing the placed From point.

  8. Locate and click the same location on Highway 82 on the basemap layer.

    Location of target or To point

    Tip:

    For a quick view of your underlying reference layer (in this case, the imagery basemap), press the L key once to temporarily turn off the source raster layer (historical image layer), and press L again to turn it back on.

  9. When you're ready, click the location to place the To point (target).

    There are now two cross-shaped points representing the From point (red) and the To point (green). This is your first control point.

    To point is placed at the bend of Highway 82 on the imagery basemap.

  10. Review the AspenGeoref:AspenSource.tif control point table.

    The first control point is now listed. The Source X and Source Y values represent the coordinates for the From point, and the X Map and Y Map values represent the coordinates for the To point.

    Review control point table..

    Note:

    The values in the table may vary based on the current position of your source image and the exact position of your control points.

  11. Using the 2nd Control Point bookmark, add a second set of control points following the same workflow as for the first control point, and add the From (source) and To (target) points at another curve of Highway 82.

    The 2nd Control Point bookmark and the From point (source) placed

  12. Review the AspenGeoref:AspenSource.tif control point table and verify that the second control point has been added.

    Review control point table.

  13. Using the 3rd Control Point bookmark, add a third control point at the intersection of Highway 82 with a small secondary road.

    Third control point location

  14. Using the 4th Control Point bookmark, add a fourth control point at the intersection of Highway 82 with another small secondary road.

    Fourth control point location

Now that you have all four control points, you'll apply a transformation to align the From and To points.

Apply a transformation

In this section, you'll apply the default transformation to the historical image based on the four control points you created.

  1. With the mouse scroll wheel, zoom out until you can see the entire historical image.
  2. On the Georeference tab, in the Adjust group, click Apply.

    Apply

    The source image is automatically shifted, scaled, and rotated to make every source and target control point pair coincide as much as geometrically possible.

    Map after alignment

  3. If necessary, refine the adjustment by adding additional control points and clicking Apply again.
  4. Review the control point table.

    Review your control points and residuals.

  5. In the control point table, verify the transformation method applied is 1st Order Polynomial (Affine).
    Transformation method set to 1st Order Polynomial (Affine)
    Note:

    The first-order polynomial transformation is commonly used to georeference an image. Use a first-order or affine transformation to shift, scale, and rotate a raster dataset. This transformation method ensures that straight lines on the raster dataset will remain mapped as straight lines in the output raster dataset. Squares and rectangles on the raster dataset may be changed to parallelograms of arbitrary scaling and angle orientation.

  6. In the control point table, click the 1st Order Polynomial (Affine) drop-down arrow.

    There are various transformation types available in the list. You only see five of them, because other transformations require a higher number of control points to be created.

    Review various transformation types.

    For now, you'll keep the 1st Order Polynomial (Affine) transformation.

  7. In the control point table, review the individual residual values associated with each control point.

    The residual value of a control point is the difference between where the source point and the target point display on the map. Residuals closer to zero are considered more accurate.

  8. At the top of the map, expand and review the on-screen georeferencing details.
    Note:

    Your Total RMS Errors values may differ from the example below.

    Review RMS values.

    For each control point, the residual is the difference between where the From point displays in the map and the actual location that was specified. The total error is computed by taking the root mean square (RMS) sum of all the residuals to compute the RMS error. The value describes how accurately the selected transformation method is able to fit all of the control points to their specified location. When the error is particularly large, you can remove and add control points to adjust the error. However, a large error may also mean that the selected transformation method is unable to accurately fit the points to their specified location. However, if the points are all properly selected and a large error is reported, it typically means that a different transformation is required.

    The forward residual shows you the error in the same units as the data frame spatial reference. The inverse residual shows you the error in the pixel units. The forward-inverse residual is a measure of how close your accuracy is, measured in pixels. Residuals closer to zero are considered more accurate.

  9. In the map view, zoom out to display the full image.

    Zoom to image extent.

  10. Review the transformed image.

    Notice that for relatively flat areas, the first-order transformation provides adequate results. However, in areas that have more hills, a different transformation type will provide better results.

    The result is reasonably satisfactory overall, so you'll save it.

  11. On the Georeference tab, in the Save group, click Save.

    Save the georeferencing changes.

    Saving the image writes the georeferencing information in an associated auxiliary file. This information includes where the image should be positioned and how much it should be shifted, scaled, and rotated.

    Note:

    Choosing Save as New (instead of Save) will create a new image that is permanently transformed. It will resample the raster cells as needed, instead of only recording the transformation parameters in an external auxiliary file. ArcGIS Pro doesn't require you to do this permanent transformation, so using Save is sufficient in many cases. However, the permanent transformation is useful if you plan to perform analyses on the image, or if you want to use it with another software package that doesn't recognize georeferencing information stored in the auxiliary file. You can learn more about this in Overview of georeferencing.

  12. On the Quick Access Toolbar, click Save to save the project.

In this section, you applied a first-order polynomial transformation to the image. In the next section, you will compare the results of a first-order polynomial transformation to the results of a spline transformation.


Compare transformation methods

Georeferencing is the process of transforming a scanned map or aerial photograph so it appears spatially correct in a map. Once you have defined your control points, you choose among different transformations to best align those points. In the previous section, you used the first-order polynomial transformation. In this section, you will compare the results of a first-order polynomial transformation to a spline transformation.

The first-order polynomial transformation you performed in previous steps may differ based on the number and location of the control points you selected. To make the comparison to spline more predictable, you'll now close that map and open instead a first-order polynomial transformation that was prepared for you with 10 control points.

Evaluate control points and transformation methods

  1. Close the AspenGeoref map.

    Close map tab.

  2. If necessary, on the ribbon, click the View tab, and in the Windows group, click Catalog Pane.
  3. In the Catalog pane, expand Maps, right-click FirstOrder, and click Open.

    Open the FirstOrder map.

    Next, you'll open a map with a spline transformation.

  4. In the Catalog pane, right-click the Spline map and click Open.

    This map displays the same historic source image, transformed using a spline transformation. In this map, the image appears to be warped and transformed more than in the FirstOrder map. The reason is that a spline transformation applies localized stretching around the different control points to get the best possible fit, whereas the first-order polynomial transformation only allows image-wide shifting, scaling, and rotating.

    Spline image

    Both maps use the NAD 1983 UTM Zone 13N projection like the map you built earlier in the lesson.

  5. In the map Contents pane, click AspenSpline.tif.
  6. On the ribbon, on the Imagery tab, in the Alignment group, click Georeference.

    Georeference

    The map updates to display the image and the 32 control points that were used for that spline transformation.

    Display spline control points.

    Note:

    A spline transformation requires at least 10 control points and often uses many more.

    On the map, notice that the red symbols appear at all of the control points in the image, but the green symbols for the target positions are not visible. This is because the mathematics of the spline transformation perfectly fit every control point to its specified location, so the From points are directly on top of each other.

    Note:

    For better georeferencing results, it is recommended that you distribute the control points around the entire raster dataset rather than concentrating them in one area.

    Also, keep in mind that your georeferenced data is only as accurate as the data to which it is aligned. To minimize errors, you should use target data that is at the highest accuracy and resolution available.

  7. On the Georeference tab, in the Review group, click Control Point Table.
  8. If necessary, change the size of the table pane to better view the rows.
  9. Review the 32 control point table.

    The Source X and Source Y columns represent the coordinate values of the source points, and the X Map and Y Map columns represent the coordinate values of the target points.

    Review spline control point table.

  10. On the map, review the on-screen image georeferencing details.

    Notice the RMS Errors values are 0 or close to 0.

    Review spline RMS errors.

    Note:

    Although the RMS error is an assessment of the transformation's ability to fit the desired control points, a low RMS error does not ensure an accurate registration. For example, the transformation may still contain significant errors if you have a poorly placed control point.

    The case of the spline transformation is a bit special: it almost always gives an RMS of nearly zero, because it makes it a priority to have the From and To points overlap perfectly, even if it means stretching the rest of the image. As a result, a value close to zero does not necessarily mean that the image is optimally georeferenced.

  11. Close the control point table.

    You'll now hide the control points from the Spline map and display the two maps side by side for comparison.

  12. On the ribbon, on the Georeference tab, in the Close group, click Close Georeference.

    Close Georeference

  13. Drag the Spline map tab and dock it next to the FirstOrder map for side-by-side comparison.

    Dock the maps side by side.

    Next, you will link the maps for easier inspection.

  14. In the Catalog pane, if necessary, expand Maps.
  15. Press Ctrl while clicking FirstOrder and Spline to select both maps.

    Both FirstOrder and Spline maps are selected in the Catalog pane.

  16. On the ribbon, click the View tab, and in the Link group, click the lower half of Link Views, and choose Center And Scale.

    Center and scale linked maps.

    The maps are now linked. As you zoom in on one map, the second map updates to the same extent.

    Review linked maps.

  17. Zoom in and pan different areas of the FirstOrder map. As the Spline map updates to match, compare the alignment of both images in their respective maps to the basemap, paying attention to mountainous slopes on the east and west of the images.
    Note:

    The FirstOrder map image has a first-order polynomial applied. This is one of the first transformations available to you as you begin georeferencing the image. It requires no set number of control points and allows for a consistent but diminishing movement of the image you are working on. However, the more control points you add, the less effective this transformation becomes. If you are adding, for example, five or six control points, your image will shift greatly and begin to line up with your reference map. But as you add more and more control points, the shifts become minimal. To obtain further adjustments, you'll need to switch to a transformation such as spline.

    A spline transformation requires a larger number of control points because it stretches the image, essentially moving a part of the image exactly to the control point location. This process of using stretches and warping to move portions of the image is referred to as rubber sheeting.

  18. Continue your exploration and select an area in the FirstOrder map that appears to have poorer image registration than in the Spline map and answer the following questions:
    • How does image transformation differ in valleys and slopes?
    • Does the spline image show a higher level of accuracy?
  19. When you are finished exploring the maps, save the project.
    Note:

    As a general recommendation, when you're georeferencing, start with a first-order transformation.

    If you need a more precise fit and can find a large number of control points, you can consider using a higher-level transformation, such as spline. For best accuracy, distribute the control points around the image and on terrain extremes.

    While the spline transformation can lead to a more precise fit, it is more time consuming. Also, because of the rubber sheeting process, straight lines may become curved, which could be a problem in some cases (for example, an urban landscape depicting buildings could appear misshapen).

    As a result, you should carefully choose the most appropriate transformation for your needs.

As a result of the georeferencing process you performed, the developer completing the environmental impact assessment in the Aspen area can now use the historical image for a comparative analysis of the past and present vegetation coverage and river flow.

You can find more lessons like this on the Introduction to Imagery & Remote Sensing page.