Available with Advanced license.
In ArcGIS Pro, you can photogrammetrically correct drone imagery to remove geometric distortions induced 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 your 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 is better than 5 meters.
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 within 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 small collection of twelve 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.
- Download the tutorial dataset and save it to C:\SampleData\Drone_tutorial.
- In ArcGIS Pro, create a project using the Map template and sign in to your ArcGIS Online account if necessary.
The default Parallel Processing Factor is set to 50%. In order to accelerate the adjustment, you can set the Parallel Processing Factor to 100%. Under the Analysis tab, click Environments. Locate the Parallel Processing parameter in the Environments window and enter 100% in the dialog box.
- On the Imagery tab, in the Ortho Mapping group, click the New Workspace drop-down menu and select New Workspace.
- In the Workspace Configuration window, type a name for your workspace.
- In the Type drop-down menu, choose Drone.
- In the Basemap drop-down menu, choose Topographic.
- Click Next.
- In the Image Collection window, under the Sensor Type drop-down menu, choose Generic, as the imagery was collected with an RGB camera.
- Click the Add button to browse to and select the imagery folder.
- Make sure the Spatial Reference and Camera Model are correct. The default projection for the workspace is defined based on the latitude, longitude, and altitude of your images. This projection determines the spatial reference for your ortho products, including the orthomosaic and DEM. For this dataset, we’ll use the default projection. Click Next.
- Accept all default setting in the Data Loader window and click Finish.
Note:Most modern drones store GPS information in the EXIF header. This will be used to automatically populate the table below. However, for some older systems or custom-built drones, GPS data might be stored as an external file. In this case, you can use the import button next to the Geolocation parameter to import external GPS files.
Once the Workspace has been created, the images, drone path, and image footprints will be displayed. An Ortho Mapping category has also been added to the Contents pane. The source imagery data and derived ortho mapping products will be stored here.
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.
After the Ortho Mapping workspace has been created, the next step is to perform block adjustment using the tools in the Adjust and Refine groups. The block adjustment will first calculate tie points, which are common points in areas of image overlap. The tie points will then be used to calculate the orientation of each image, known as "exterior orientation" in photogrammetry.
- In the Ortho Mapping tab, in the Adjust group, click Adjust .
- In the Adjust window, make sure Quick adjust at coarse resolution only is unchecked. If this option is checked, an approximate adjustment will be performed at a coarse, user-defined 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 OK for the sample data in this tutorial, since the sample dataset is small, and the adjustment will be performed quickly.
- 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.
- Make sure 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.
- Under the Blunder Point Threshold drop-down menu, choose 5. Tie points with a residual error greater than this value will not be used in computing the adjustment. The unit of measure is pixels.
- 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. This default value is suitable for most imagery that includes a diverse set of features.
- 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.
- Click Run.
Add Ground Control Points
GCPs are points with known x, y, z ground coordinates, often obtained from ground survey, and used to ensure that the photogrammetric process has reference points on the ground. Block adjustment can be applied without GCPs and still ensure relative accuracy, although adding GCPs does increase the absolute accuracy of the adjusted imagery.
- In the Ortho Mapping tab, in the Refine group, click Manage GCPs to launch the GCP Manager tool.
- In the GCP Manager window, click the Import GCPs button .
- On the Import GCPs window, navigate to and select the GCP file (YVWD_WGS84_EGM96.csv). Click OK.
- Under Set GCP Spatial Reference, click the Browse button. For Current XY, expand Geographic, World, and choose WGS 1984. For Current Z, expand Vertical Cooridnate System, Gravity-related, World, and choose EGM96 Geoid.
- Under Geographic Transformations, click the Vertical tab and choose WGS 1984 to EGM 1996 Geoid 1 from the drop-down menu.
- Click the folder icon below GCP Photo Location and browse to and select the folder containing the images of the GCP locations. Click OK.
Add tie points for selected GCPs
- Once the GCPs have been imported, the table in the GCP Manager will be populated.
- 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.
- 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.
- Highlight GCP11 and click the Delete GCP button to remove it from the list of GCPs.
- After each GCP has been added and measured with tie points, select GCP10 and right-click to change it a Check Point. This will provide a measure of the absolute accuracy of the adjustment, as this point will not be used in the adjustment process.
- After adding GCPs and checkpoints, the adjustment must be run again to incorporate these points. Click Adjust.
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.
Review adjustment results
Adjustment quality results can be viewed in the GCP Manager by analyzing the residuals for each GCP. Residuals represents the difference between the measured and computed position of a point. They are measured in the units of the project spatial referencing system. After completing adjustment with GCPs, three new fields, dX, dY, 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.
Additional adjustment statistics are provided in the adjustment report. To generate the report, under the Ortho Mapping tab, in the Review group, click Adjustment Report.
Generate a digital surface model (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 can be derived when the image collection has a good amount of overlap to form the stereo pairs. Typical image overlap to produce point clouds is 80 percent forward overlap along a flight line and 60 percent overlap between flight lines.
- In the Ortho Mapping tab, In the Product group, click DSM to start the DSMs Wizard. Click Next.
- 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.
- Make sure Filter Ground Objects is checked.
- For Maximum Object Size to Filter, use the default value of 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.
- Leave Point Ground Spacing 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.
- Accept all remaining default settings and click Next.
For information on the Advanced Settings, see Generate elevation data using the DEMs wizard.
- In the DEM Settings window, for Cell Size, use the default value of 5 x GSD. This will determine the resolution of the DSM (five times the imagery resolution).
- Accept the remaining default settings and click Finish.
Generate an 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.
- In the Ortho Mapping tab, In the Product group, click Orthomosaic to start the Orthomosaic Wizard.
- Click Next.
- In the Orthorectification Settings window, under the Elevation Source, select Use DEM Product and choose Digital Surface Model.
- Click Next.
- In the Color Balance Settings, uncheck Select Mosaic Candidates and accept all other default options. Click Next.
- In the Seamline Settings window, under the Computation Method drop-down menu, choose Veronoi. Click Next.
- Accept all remaining default setting and click Finish to generate the final orthomosaic.
In this tutorial, you created an ortho mapping workspace for drone imagery and used tools in the Ortho Mapping tab to apply a photogrammetric adjustment with ground control points. You then used the Ortho Mapping Products Wizard to generate a DEM and an orthomosaic. For more information on these topics, see the following:
- 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.