Evaluate the accuracy of ground control points

Rancho Del Mar is a small neighborhood on the west coast of Baja California, Mexico, that is looking to expand. As an employee of the property development company in charge of the project, you need to plan additional space for future growth that will be safe from hurricanes. To do so, you need accurate representations of the area. You have already collected drone imagery of the neighborhood. Before you use it to create an elevation models and orthoimage mosaic, you'll evaluate the accuracy of the ground control points.

Ground control points are used to georeference the images to the correct position when creating imagery products. These products will be used for analysis, development, and disaster planning of the new neighborhood, so they must have a high level of accuracy. In this tutorial, you'll import ground control points, add image links, and adjust the imagery. After adjustment, you'll assess and improve the accuracy of the ground control points.

This tutorial was last tested on September 22, 2022, using ArcGIS Drone2Map 2022.1. If you're using a different version of Drone2Map, you may encounter different functionality and results.

Requirements

• ArcGIS Drone2Map version 2022.1 or later
• Publisher or Administrator role in an ArcGIS organization: ArcGIS Online (see options for software access) or ArcGIS Enterprise (learn more)

Import ground control points and add image links

To begin, you'll download and open an existing Drone2Map project and gain familiarity with its components. Then you'll import ground control points (GCPs) and check points (CPs) using two methods. You will add image links to one of the points.

Set up the project

First you will download the imagery of Rancho Del Mar and open it in ArcGIS Drone2Map. You'll review the images that are available.

Note:

The data used in this lesson is 1.95 GB and will take several minutes to download.

1. Download RanchoDelMar.zip.
2. Locate the downloaded file and unzip it to drive C, so the folder path is C:\RanchoDelMar.

   Caution:

   Some data connections will be broken if you unzip the file to a location other than drive C.

   

3. Open the extracted RanchoDelMar folder. Double-click RanchoDelMar.d2mx to open the project in ArcGIS Drone2Map. If prompted, sign in to your ArcGIS account.

   Note:

   If you don't have an organizational account, see options for software access.

   The project appears in ArcGIS Drone2Map. The Contents pane lists three layers: Image Centers, Flight Lines, and Control. The Control layer is currently empty. Later, you will import GCPs into this layer.

   

4. On the map, click one of the blue Image Centers points.

   The Image Viewer window appears, showing a high resolution image for that area.

   

   There are 174 images in the project.

5. Close the Image Viewer window.
6. Right-click anywhere on the map and click Clear.

   

   Next, you'll examine where the data for the project is stored.

7. Below the Manage pane, click the Catalog tab.

   

8. In the Catalog pane, on the Project tab, expand Folders and expand RanchoDelMar.

   ArcGIS Drone2Map organizes project components into various subfolders.

9. Expand RanchoDelMar.gdb.

   

   The project's geodatabase contains the three feature layers listed in the Contents pane. Any new data that you import into the project will also be stored here.

10. Expand the Images folder.

    The images referenced by the Image Centers layer are stored here.

ArcGIS Drone2Map uses geolocated images and creates tie points between neighboring images so that they can be merged together to form imagery products. The images in this project have not yet been merged together.

While GPS data from drones is accurate enough for most general analysis and product generation, for this project, you must be confident that you have the highest accuracy possible. To ensure this accuracy, you'll incorporate ground control points when you stitch the images into an image collection.

Note:

Some drones that use real-time kinematic (RTK) correction to apply high accuracy correction to GPS information on the fly. Even with these drones, it is still useful to collect GCPs to be used as CPs that help validate your drone's accuracy.

Import ground control points from a .csv file

GCPs are high accuracy points captured on the ground either before or after a flight that are used to adjust output products created in ArcGIS Drone2Map. These points are typically marked on the ground so they are visible in the imagery and can be used to create geolocation links between images. It is recommended that you use between five and ten GCPs well dispersed across the image area.

You will import a ground control point from a .csv file.

1. On the ribbon, click the Home tab. In the Control group, click the bottom half of the Control button and click Import Control.

   

2. In the Import Control window, choose Import from CSV or text file.
3. Click OK.
4. For Import Control From, click the Browse button.

   

   The Browse to control file window appears. It is already open to the RanchoDelMar folder associated with the project.

5. In the RanchoDelMar folder, click Rancho_Del_Mar_GCP_EPSG4484.csv and click OK.

   

   In the Import Control window, in the Control Coordinate System section, the Current XY coordinate system was automatically defined as WGS 1984 UTM Zone 11N. The data in the .csv file was defined in a different projected coordinate system. You'll change the coordinate system to the correct one.

6. Next to Current XY, click the Set Horizontal and Vertical Spatial Reference button.

   

   The Spatial Reference window appears. You'll search for the EPSG number listed in the .csv file name.

7. In the Search bar, type 4484 and press Enter.

   

   The XY Coordinate Systems Available list filters to show only one item, Mexican Datum 1993 UTM Zone 11N. The Current XY button updates to list this new horizontal coordinate system.

   Tip:

   If the Current XY button does not update, click Mexican Datum 1993 UTM Zone 11N in the list.

   The Current Z button lists EGM96 Geoid as the vertical coordinate system. This is the correct choice for the data, so you will not change it.

8. Click OK.

   In the Import Control window, the Current XY and Current Z fields list the correct coordinate system.

   

9. Leave the Control Photos section empty.

   There are no control photos for this GCP.

10. In the Control Field Information section, ensure that the location fields were matched with the correct fields from the .csv file:
    • Lat [Y]—Set to Latitude.
    • Lon [X]—Set to Longitude.
    • Elevation [Z]—Set to Elevation.
    • Label—Set to Label.
11. For Accuracy Horizontal. Choose H_Accuracy. For Accuracy Vertical, choose V_Accuracy.

    

12. Click OK to import the GCP.

    In the Contents pane, the Control layer disappears temporarily and reappears when the import is complete. A green cross symbol appears on the map.

    

    In the Control Manager pane, the new GCP appears as the first row in a table. The table row has an orange warning symbol to indicate that the GCP is not yet linked to any images.

    

    Next, you'll create links between each GCP and the images that overlap it.

Add image links

In ArcGIS Drone2Map, you can link ground control manually or automatically. However, if you choose the automatic method, you still must create a few image links manually before the software can determine where other links may be. If you have a small set of GCPs, it is recommended that you manually link all of the images to ensure the greatest accuracy. You will manually link the GCP from the .csv file.

1. In the Contents pane, turn off the Image Centers and Flight Lines layers.

   

2. In the Control Manager pane, click the GCP 5 row to select it.
3. Click the Show image links editor button.

   

   The Image Links window appears. On the left, all of the project's images are listed. Those closest to the GCP are listed first.

4. In the list, click the first image to ensure it is the one displayed in the viewer.

   

5. In the viewer, zoom to the red X marked on the sidewalk, near the center of the image.

   

6. Click the center of the X to place a yellow image link marker.

   

   In the list, a green check mark appears next to the first image, to indicate that it has been linked.

7. In the list, click the next image.

   The next image appears in the viewer. It has a different orientation than the last image, but the same red X is visible on the sidewalk.

8. Zoom to the red X and click its center to add another image link.
9. Add image links to the next three images, each time aligning them with the center of the red X.

   Note:

   Optionally, continue through the list of images to find more images containing the red X and add an image link to each. The images that are most likely to contain the red X are all listed near the top.

10. Click OK.

    In the Control Manager pane, the warning symbol next to GCP 5 is replaced with a green check mark to indicate that the GCP now has image links.

    

11. Right-click anywhere on the map and click Clear to deselect GCP 5.

Import ground control points and check points

Next, you'll import nine more GCPs. The image links have already been created for these new GCPs in another Drone2Map project, so instead of importing a .csv file, you'll import a .zip file exported from the previous project.

1. At the top of the Control Manager pane, click the Import Control button.

   

2. In the Import Control window, choose Import from Drone2Map control export.

   Note:

   This method will import points with image links that were created in another Drone2Map project. You will use this method in this lesson to save time. Generally, you would only want to use this method if your flights over the same location use GCP markers that are static features. If you are using temporarily placed targets, the locations of the targets will be very hard to match exactly when placing them for additional flights.

3. Click OK.
4. In the Import Control window, click the Browse button.
5. In the Browse to Drone2Map control export window, in the RanchoDelMar folder, click GCP.zip.

   

6. Click OK.

   In the Import Control window, the Ground Control Point table populates with eight rows.

   

7. In the table, click the second row.

   The Image Links list next to the table updates to list five images that are linked to GCP 2.

8. Click each row of the table to see how many images links will be imported with each point.
9. Click OK to import the ground control points.

   In the Control Manager pane, the new points appear in the table. All of them have green check marks.

   

   The Type column shows that five of the points are GCPs and four are check points CPs. GCPs and CPs are collected in the same manner, but they are used for different purposes. GCPs are used to process imagery products and improve accuracy, while CPs are not included in processing. Instead, they are used to independently assess the accuracy of the model after processing.

The dX, dY, and dZ columns are empty. They will be populated with accuracy values after the images are adjusted. Your goal is to align the imagery as closely as possible with the GCPs. The dX, dY, and dZ values will help you to assess the accuracy of the alignment.

Adjust the imagery and evaluate its accuracy

Next, you will process the 174 images in your project to generate an image collection. Processing will stitch the images together using aerial triangulation and bundle block adjustment. After processing, you'll review the results of the adjustment and assess the accuracy of the GCPs. You'll reposition some of the image links and rerun the adjustment to improve overall accuracy.

Run the block adjustment

Next, you'll process the imagery to create an image collection.

1. Below the Control Manager pane, click the Manage tab to switch to the Manage pane.
2. Under 2D Products, ensure that Image Collection is the only item checked.

   

   Later, you plan to create an orthomosaic and digital terrain model, but for now, you will only create an image collection to save time in assessing ground control accuracy. Once the accuracy values are satisfactory, you can generate the output products that you need.

3. On the ribbon, click the Home tab. In the Processing group, click Start.

   

4. Also in the Processing group, click Log.

   The Processing Log view appears, showing the progress of the processing steps.

   

   Processing will take a long time to run, so you should move to another task and return to this lesson later.

Evaluate accuracy

After the adjustment process has completed, you'll review the accuracy of the GCPs and their individual image links. You'll assess the accuracy values for each GCP (dX and dY) and the reprojection errors for the image links. The closer these values are to zero, the more accurate the alignment.

1. Check the Processing Log view to confirm that adjustment processing is complete.

   When the process is complete, the last message in the log will be Processing completed successfully. The progress bar at the bottom of the Manage pane will remain at 100 percent.

   

   First, you'll review the Catalog pane to view the results of the adjustment process.

2. On the ribbon, on the Home tab, in the Layers group, click Catalog.

   

3. In the Catalog pane, if necessary, expand the RanchoDelMar folder and RanchoDelMar.gdb.

   There are four new items present in the geodatabase: the new image collection and its associated data.

   

   Next, you'll view the accuracy values for each GCP.

4. On the ribbon, on the Home tab, in the Control group, click Control Manager to reopen the Control Manager pane.
5. Drag the side of the pane to make it wider until you can read all of the columns in the table.

   

   The dX, dY, and dZ columns are populated. These are measurements of each control point's accuracy after adjustment. A value that is close to zero means that the point had to shift very little and will retain most of its initial accuracy.

   Note:

   The values in your table will be slightly different than the ones shown in the image above, due to how you placed the image links.

6. Review the d values in the table.

   GCP 4 has larger accuracy values than the other control points.

   These values are related to the placement of image links. If one or more image links are placed incorrectly, you will see a significant deviation in these values. It is likely that an image link for GCP 4 was incorrectly placed. You'll review the image links to fix this issue.

7. In the Control Manager pane, click the Show image links editor button.
8. In the Image Links window, for Control Point, choose 4.

   

   Before, the image links were sorted by distance. Now they are sorted by Reprojection.

   Reprojection errors are computed for each image link. This value represents the distance between the initial point and the adjusted point, measured in pixels. All the image links will shift slightly, but a reprojection error greater than 1 is considered high. Images with higher reprojection errors are listed first.

9. In the image list, click the first image to ensure it is the one shown in the viewer.

   Above the viewer, the Reprojection Error value for the selected image link is listed.

   

   In the image above, the value is 2.196. Values will differ slightly in your project.

10. In the viewer, zoom to the image link.

    The yellow image link marker is offset from the center of the red X.

    

11. In the viewer, click the center of the red X to reposition the image link.

    Above the viewer, the Reprojection Error disappears.

12. Click the next image in the list.

    It also has a Reprojection Error value greater than 1.

13. Reposition the image link for the second image.

    In the image list, five of the images have green check marks to indicate that they have image links. The reprojection errors for all of these image links are greater than 1 so you will reposition them all.

14. Click each of the linked images in turn and reposition its image link.
15. Click OK to close the Image Links window.

    Reviewing image links with high reprojection errors helps provide you with quality control to achieve the best products possible. Anytime you incorporate ground control into your project, it is recommended that you review the image links after the adjustment has been carried out.

Next, you will rerun the block adjustment process using the modified image links. This will not take as long as the initial process and will provide you with updated accuracy values.

Reprocess and re-evaluate

You'll process the imagery again to see if the changes you made to the image links for GCP 4 have helped with GCP accuracy. It is normal to run through the adjustment process multiple times before you obtain accuracy values that are suitable for your imagery products.

1. On the ribbon, on the Home tab, in the Processing group, click Start.
2. If the Processing Log view is not already open, click Log.
3. When processing is complete, close the Processing Log view and open the Control Manager pane.
4. Review the new dX, dY, and dZ values.

   

   The values have changed. All values should be less than 0.1 If they are not, the image links for that control point may be too far from the center of the X.

   Note:

   The absolute accuracy of each GCP has not changed. These values are recorded in the horizontal (H_Accuracy) and vertical (V_Accuracy) accuracy fields included with the GCP imports. Rather, the accuracy of each adjusted GCP has changed. The process followed in this lesson is intended to place each adjusted GCP location as close as possible to the initial imported location. These steps will not make the original data more accurate, but they will improve the overall accuracy of any imagery products created from it.

5. On the ribbon, on the Home tab, in the Processing group, click Report.

   

   The Processing Report window appears.

6. Scroll to the Geolocation Details section.

   The Projection Error value for each GCP and CP is less than 1. The accuracy values (dX, dY, and dZ) are all less than 0.1. For your purposes, these are acceptable accuracy values, so you are now ready to generate accurate imagery products.

   Note:

   If you still see high accuracy values for any GCP, review and reposition its image links again. You can also remove a GCP.

7. Close the Processing Report window.
8. Above the toolbar, click the Save button.

   

Having well aligned and accurate imagery when doing disaster planning is key to allocating the proper number of resources or predicting potential areas of concern. Since you have accurately adjusted the imagery to your GCPs as best as possible, it can now be processed into elevation models to evaluate risk of landslide or an orthomosaic to review the area for potential hazards or unsafe structures.

In this lesson, you imported GCPs, added image links, processed imagery, assessed the accuracy of the GCPs, and modified image links. This process allows you to create imagery products with greater accuracy. Optionally, continue to experiment with the methods that you learned in this lesson on the dataset that is provided and test the generation 2D or 3D products.

You can find more tutorials in the tutorial gallery.

Acknowledgements

• The imagery in this tutorial was captured by Esri.