Tutorial: Create drone imagery products using ArcGIS Pro Ortho mapping

Available with Standard or Advanced license.

Available for an ArcGIS organization with the ArcGIS Reality license.

In ArcGIS Pro, you can use photogrammetry to correct drone imagery to remove geometric distortions caused by the sensor, platform, and terrain displacement. After removing these distortions, you can generate Ortho mapping products.

First, you will set up an Ortho mapping workspace to manage the drone imagery collection. Next, you will perform a block adjustment, followed by a refined adjustment using ground control points. Finally, you'll generate a digital terrain model (DTM), and an orthorectified mosaic, or orthomosaic.

Ortho mapping requires information about the camera, including focal length and sensor size, as well as the location at which each image was captured. This information is commonly stored as metadata in the image files, typically in the EXIF header. It is also helpful to know the GPS accuracy. In the case of drone imagery, this should be provided by the drone manufacturer. The GPS accuracy for the sample dataset in this tutorial is better than 5 meters.

License:

ArcGIS Pro 2.6 or later is required to complete this tutorial.

Create an Ortho mapping workspace

An Ortho mapping workspace is an ArcGIS Pro subproject that is dedicated to Ortho mapping workflows. It is a container in an ArcGIS Pro project folder that stores the resources and derived files that belong to a single image collection in an Ortho mapping task.

A collection of 12 drone images is provided for this tutorial. The folder labeled GCP contains a ground control point (GCP) file, labeled YVWD_WGS84_EGM96.csv, and images of the GCP locations at the site.

To create an Oortho mapping workspace, complete the following steps:

1. Download the tutorial dataset. unzip it, and save contents to C:\SampleData\Drone_tutorial.
   1. Unzip the package in the C:\SampleData\Aerial Imagery directory.
2. In ArcGIS Pro, create a project using the Map template and sign in to your ArcGIS Online account if necessary.
3. Increase the processing speed of block adjustment.
   1. On the Analysis tab, click Environments.
   2. Locate the Parallel Processing Factor parameter in the Environments window and change the value to a value appropriate for your system.

      Setting the Parallel Processing Factor value higher than what your system can accommodate will lead to parallel processing failures. A common requirement is that for each logical processor, you need 2GB of RAM. For example, if you are working with a system that has 6 cores, 12 logical processors, and 16 GB RAM, setting the parallel processing factor to 100 percent would require a minimum 24 GB of RAM for the process to run successfully. A more appropriate Parallel Processing Factor value based on this example would be 50 percent. which would require approximately 12 GB of RAM.

4. On the Imagery tab, in the Ortho Mapping group, click the New Workspace drop-down menu and choose New Workspace.
5. In the Workspace Configuration window, type a name for the workspace.
6. Click the Type drop-down menu, and choose Drone.
7. Click the Basemap drop-down menu, and choose Topographic.
8. Accept all other default values and click Next.

   The Image Collection window appears.

9. In the Image Collection window, click the Sensor Type drop-down menu, and choose Generic.

   This option is used because the imagery was collected with an RGB camera.

10. Click Add, browse to the tutorial data location, and select the Images folder.

Note:

Most modern drones store GPS information in the EXIF header. This will be used to automatically populate the table shown below. However, some older systems or custom-built drones may store GPS data in an external file. In this case, you can use the Import button next to the Geolocation parameter to import external GPS files.


11. Ensure that the workspace Spatial Reference and Camera Model values are correct.

    The default projection for the workspace is based on the latitude, longitude, and altitude of the images. This projection determines the spatial reference for your ortho products, including the orthomosaic and DEM. For this dataset, you’ll use the default projection.

12. Click the Spatial Reference button .
13. Click Next.
14. Accept all the default settings in the Data Loader Options window and click Finish.

    

    • If you have access to the internet, the Elevation Source value is derived from the World Elevation Service. This provides an initial estimate of the flight height for each image.
    • If you do not have access to the internet or a DEM, choose Constant Elevation from the Elevation Source drop-down menu and enter an elevation value of 414 meters.
    • The parameter, Flight Height Above Terrain (m), refers to the image exclusion height, where images having a flight height above terrain that is less than this value will not be included in the workspace.

    Once the workspace has been created, the images, drone path, and image footprints are displayed. An Ortho Mapping container is added to the Contents pane, where the source imagery data and derived Ortho mapping products will be stored.

    

    The initial display of imagery in the workspace confirms that all images and necessary metadata were provided to initiate the workspace. The images have not been aligned or adjusted, so the mosaic will not look correct.

Block adjustment

After the Ortho mapping workspace has been created, the next step is to perform a block adjustment using the tools in the Adjust and Refine groups. The block adjustment first calculates tie points, which are common points in areas of image overlap. The tie points are then used to calculate the orientation of each image, known as exterior orientation in photogrammetry. The block adjustment process may take a few hours depending on your computer setup and resources.

To perform a block adjustment, complete the following steps:

1. On the Ortho Mapping tab, in the Adjust group, click Adjust .
2. In the Adjust window, make sure Perform Camera Calibration is checked.

   This indicates that the input focal length is approximate and that the lens distortion parameters should be calculated during adjustment. For drone imagery, this parameter is checked by default, as most drone cameras have not been calibrated. This option should not be checked for high-quality cameras with known calibration.

   The camera self-calibration requires that your image collection has in-strip overlap of more than 60 percent and cross-strip overlap of more than 30 percent.

3. Expand the Advanced Options section.
4. Ensure Quick adjust at coarse resolution only is unchecked.

   If this option is checked, an approximate adjustment will be performed at a coarse, specified resolution. If this option is not checked, tie points are first computed at a coarse resolution, followed by a refined adjustment at image source resolution. A one-step adjustment is appropriate for the sample data in this tutorial since the sample dataset is small, and the adjustment will be performed quickly.

5. Check Use Orientations from Metadata.
6. Make sure Perform Camera Calibration is checked.

   This indicates that the input focal length is approximate, and that the lens distortion parameters will be calculated during adjustment. For drone imagery, these options are checked by default, since most drone cameras have not been calibrated. This option should not be checked for high-quality cameras with known calibration.

   The camera self-calibration requires that an image collection has in-strip overlap of more than 60 percent and cross-strip overlap of more than 30 percent.

7. Ensure Fix image location for high accuracy GPS is unchecked.

   This option is used only for imagery acquired with differential GPS, such as Real Time Kinematic (RTK) or Post Processing Kinematic (PPK) GPS.

8. Uncheck the Compute Posterior Standard Deviation for Images check box.
9. Expand the Tie Point Matching section.
10. Uncheck Fix Image Location for High Accuracy GPS.

    This option is used only for imagery acquired with differential GPS, such as Real Time Kinematic (RTK) or Post Processing Kinematic (PPK).

11. Expand the Tie Point Matching section.
12. Under the Image Resolution Factor drop-down menu, choose 8 x Source Resolution.

    This parameter is used to define the resolution at which tie points will be calculated. Larger values will allow the adjustment to run more quickly. 8 x Source Resolution value is suitable for most imagery that includes a diverse set of features.

13. Under the Image Location Accuracy drop-down menu, choose High.

    GPS location accuracy indicates the accuracy level of your GPS data collected concurrently with your imagery and listed in your corresponding EXIF data file. This is used in the tie point calculation algorithm to determine the number of images in the neighborhood to use. The High option is used for GPS accuracy of 0 to 10 meters.

    

14. Accept all other defaults and click Run.

    After the adjustment has been performed, the Logs file displays statistical information such as the mean reprojection error (in pixels), which signifies the accuracy of adjustment, the number of images processed, and the number of tie points generated.

    The relative accuracy of the images is also improved, and derived products can be generated using the options in the Product category. To improve the absolute accuracy of generated products, GCPs must be added to the block.

Add GCPs

GCPs are points with known x,y,z ground coordinates, which are often obtained from ground survey, that are used to ensure that the photogrammetric process has reference points on the ground. A block adjustment can be applied without GCPs and still ensure relative accuracy, although adding GCPs increases the absolute accuracy of the adjusted imagery.

Import GCPs

To import GCPs, complete the following steps:

1. On the Ortho Mapping tab, in the Refine group, click Manage GCPs.

   The GCP Manager window appears.

2. In the GCP Manager window, click the Import GCPs button .
3. In the Import GCPs window, under GCP File, browse to and select the YVWD_WGS84_EGM96.csv file, and click OK.
4. Under Set GCP Spatial Reference, click the Spatial Reference button and do the following in the Spatial Reference window:
   1. For Current XY, expand Geographic World, and choose WGS84.
   2. For Current Z, expand Vertical Cooridnate System > Gravity-related > World, and choose EGM96 Geoid.
5. Under Geographic Transformations, click the Vertical tab and choose WGS 1984 to EGM 1996 Geoid 1 from the drop-down menu.
6. Click the folder icon below GCP Photo Location, browse to and select the folder containing the images of the GCP locations.
7. Click OK to accept the changes and close the Spatial Reference window.
8. Under Geographic Transformations, click the Horizontal tab and choose WGS 1984 (ITRF00) to NAD83 from the drop-down menu list.
9. Click the folder below GCP Photo Location, browse to and select the folder containing the images of the GCP locations, and click OK.

   

   Once the GCPs have been imported, the table in the GCP Manager will be populated.

Add tie points for selected GCPs

To add tie points, complete the following steps:

1. In the GCP Manager window, select GCP9. Click the View GCP Photo button to display the GCP image chip, and use the Dynamic Range Adjustment button to increase the image contrast.
2. Click the Add Tie Point button to add a tie point in the image viewer for each image.

   The tie points for other images will be automatically calculated by the image matching algorithm where possible, although each tie point should be checked for accuracy. If the tie point is not automatically identified, add the tie point manually by selecting the appropriate location in the image.

3. Highlight GCP11 and click the Delete GCP button to remove it from the list of GCPs.

   The location and image chip for GCP11 does not provide enough context to accurately place a tie point. This issue illustrates one of the common challenges that you might encounter with your metadata.

4. After each GCP has been added and measured with tie points, select GCP10 and right-click to change it to Check Point.

   This will provide a measure of the absolute accuracy of the adjustment since this point will not be used in the adjustment process.

5. After adding GCPs and checkpoints, the adjustment must be run again to incorporate these points. Click Adjust.

   

Review adjustment results

Adjustment quality results can be viewed in the GCP Manager window by analyzing the residuals for each GCP. Residuals represents the difference between the measured position and the computed position of a point. They are measured in the units of the project's spatial referencing system. After completing adjustment with GCPs, three new fields—dX, dY, and dZ—are added to the GCP Manager table and display the residuals for each GCP. The quality of the fit between the adjusted block and the map coordinate system can be evaluated using these values. The root mean square error (RMSE) of the residuals can be viewed by expanding the Residual Overview section of the GCP Manager window.

Additional adjustment statistics are provided in the adjustment report. To generate the report, on the Ortho Mapping tab, in the Review group, click Adjustment Report.

Generate a DSM

The stereo image pairs of an image collection are used to generate a point cloud (3D points) for which elevation data can be derived. The derived elevation data is classified as either a digital terrain model (DTM), which includes only the ground surface, or a digital surface model (DSM), which includes the elevations of trees, buildings, and other above ground features.

Note:

Elevation values can be derived when the image collection has a good amount of overlap to form the stereo pairs. Typical image overlap necessary to produce point clouds is 80 percent forward overlap along a flight line and 60 percent overlap between flight lines.

Follow the steps below to generate a DSM using the wizard.

1. On the Ortho Mapping tab, click the DSM button in the Product group.

   The Ortho Mapping Products Wizard window appears.

2. Click Next to advance the wizard to the Point Cloud Settings window.
3. In the Point Cloud Settings window, for Matching Method, choose Semiglobal Matching from the drop-down menu.

   This method is typically used for images of urban areas and captures more detailed terrain information.

4. Ensure the Filter Ground Objects check box is checked on.
5. Ensure Maximum Object Size to Filter is set to 10 meters.

   Objects smaller than this threshold will be filtered as ground, otherwise objects will be treated as above-ground features, such as buildings, bridges, or trees.

6. Ensure Point Ground Spacing is blank.

   This defines the spacing, in meters, at which the 3D points are generated. The default is five times the resolution of the source imagery. For this image collection, points will be generated every 15 cm.

7. Accept all remaining default settings and click Next.

   For information on Advanced Settings, see Create elevation data using the ortho mapping DEMs wizard.

   
8. In the DEM Settings window, for Cell Size, use the default value of 5 x GSD.

   This will determine the resolution of the DSM, which is five times the imagery resolution, in this case.

9. Accept the remaining default settings and click Finish.

   The DSM will be generated.

   

Generate a orthomosaic

An orthomosaic is an orthorectified image product mosaicked from an image collection. Geometric distortion has been corrected and the imagery has been color balanced to produce a mosaic.

1. On the Ortho Mapping tab, in the Product group, click Orthomosaic to start the Orthomosaic Wizard.
2. Click Next.

   The Orthorectofication Settings pane appears.

3. In the Orthorectification Settings window, under Elevation Source, select Use DEM Product and choose Digital Surface Model.
4. Click Next.

   The Color Balance Settings pane appears.

5. In the Color Balance Settings pane, uncheck Select Mosaic Candidates and accept all other default options.
6. Click Next.

   

   The wizard guided workflow advances to the next pane, Seamline Settings.

7. In the Seamline Settings window, under the Computation Method drop-down menu, choose Veronoi. Click Next.
8. Click Next.

   The wizard guided workflow advances to the next pane, Orthomosaic Settings.

9. Accept all the default settings in the Orthomosaic Settings pane, and click Finish.

   The orthomosaic will be generated, listed in the Contents pane, and loaded into the map display.

   

Summary

In this tutorial, you created an Ortho mapping workspace for drone imagery and used tools on the Ortho Mapping tab to apply a photogrammetric adjustment with ground control points. You then used tools in the Ortho Mapping Products Wizard group to generate a DSM and orthomosaic. For more information about ortho mapping, see the following topics:

• Ortho mapping in ArcGIS Pro
• Create an ortho mapping workspace for drone imagery
• Adjustment options for ortho mapping drone imagery
• Add GCPs to your ortho mapping workspace
• Generate elevations data using the DEMs Wizard
• Generate an orthomosaic using the Orthomosaic Wizard

The imagery used in this tutorial was acquired at the Yucaipa Valley Water District and provided by Yuneec USA, Inc..