Available with Advanced license.
In ArcGIS Pro, you can photogrammetrically correct satellite imagery to remove geometric distortions induced by the 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 satellite imagery collection. Next, you will perform a block adjustment, followed by a refined adjustment using ground control points. Finally, you'll generate an orthorectified mosaic, or orthomosaic.
ArcGIS Pro can process satellite images from many sensor platforms, as long as the image orientation is described by a rational polynomial coefficients (RPC) model or a rigorous sensor model. This model is typically imbedded in the image file or included as a separate metadata file.
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.
The imagery packaged for this tutorial was collected and provided by Maxar Technologies. It includes a pair of multispectral and panchromatic images, a table of ground control points, and a DEM.
- Download the tutorial dataset and save it to C:\orthomapping_satellite_tutorial.
- In ArcGIS Pro, create a project using the Map template and sign in to your ArcGIS Online account if necessary.
- 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 Satellite.
- For Basemap, select Imagery.
- Click Next.
- In the Image Collection window, under the Sensor Type drop-down menu, choose GeoEye-1.
- Under Folder Containing Images, click the Browse button and navigate to the tutorial data folder on your machine and select the imagery folder (GeoEye_NAD83_UTM10N).
- Under Spatial Reference, click the Browse button .
- In the Spatial Reference window, under Current XY, set the spatial reference to NAD 83 UTM Zone 10N.
The Current Z will be automatically set to EGM96 Geoid. To choose a different vertical coordinate system, Click Current Z and browse to a different coordinate system.
- Under Current Z, expand Vertical Coordinate System and under Gravity-related, expand World and select EGM96 Geoid.
This projection determines the spatial reference for your ortho products, including the orthomosaic and DEM.
- Click OK to close the Spatial Reference window and click Next.
- In the Data Loader window, under Elevation Source choose DEM. Under DEM, browse to the DEM provided with the tutorial dataset.
This DEM will be used to orthorectify the image collection during block adjustment.
- Most elevation data use orthometric heights, so you will need to apply a geoid correction. Under Geoid correction, make sure EGM96 is selected.
- Under Processing Template, choose Pansharpen. This template will automatically pansharpen the image collection using the panchromatic image and multispectral images in the sample data.
- Accept all other defaults and click Finish.
Once the workspace has been created, the images 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 may not appear geometrically correct.
After you have created your ortho mapping workspace, 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, under Transformation Type, select RPC. The Rational Polynomial Coefficients (RPC) transformation will be applied in the adjustment, which is used for satellite imagery that contains RPC information within the metadata.
- In the Adjust window, under Blunder Point Threshold, use the default value. 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 Tie Point Matching, accept all default values.
- Click Run to perform block adjustment.
- After the adjustment is complete, turn on the Tie Points layer in the contents pane to view the distribution of generated tie points on the map.
- Tie point residuals or accuracy reporting can be viewed in the logs file. In the Ortho Mapping tab, in the Review group, click Logs to access this file. Tie point residuals are displayed in the row labeled RMSE_Tie_Image(xy). The units for tie point RSME is pixels.
Collect Ground Control Points (GCPs)
GCPs are points with known x,y,z ground coordinates. They are often obtained from ground survey or existing data and used to ensure that the images will be accurately georeferenced in the ground coordinate system. Block adjustment can be applied without GCPs and still ensure relative accuracy, but adding GCPs increases the absolute accuracy of the adjusted imagery. If you do not have GCPs from ground survey, but you have a georeferenced raster layer (raster dataset, mosaic dataset, or image service), you can add it as a reference to compute GCPs. When choosing a reference image for GCP computation, make sure your reference image has good georeferencing quality in terms of geopositional accuracy and clarity, and the resolution is similar to your source imagery. For this tutorial, we'll collect GCPs from the Imagery basemap.
While it is recommended to use a reference image with known geopositional accuracy, the Imagery basemap is sufficient in cases where suitable reference images are not available.
- In the Contents pane, uncheck the Tie Points layer to turn off the visibility.
- Right-click Databases in the Catalog pane, select Add database, and browse to and select "GCPs_Vancouver.gdb in the tutorial dataset.
- Click OK.
- In the Catalog, under Databases, expand the GCPs_Vancouver.gdb and add the GCPs feature class to the map. You will use the points in this layer to locate and select GCPs on the imagery basemap.
- In the Contents pane, click on the symbol for the GCPs point layer and change it to "Cross 3."
- In the Ortho Mapping tab, in the Refine group, click Manage GCPs to open the GCP Manager.
- Click the Define Z Correction button . This tool is used to define any vertical transformations that are required to accurately transform heights extracted from the basemap.
- In the Define Z Correction window, click the Browse button .
- In the Spatial Reference window, click the Current Z box. Then, under Z Coordinate System Available, select None.
A vertical coordinate system (VCS) was not selected because the DEM attached to the Ortho Mapping workspace does not have a defined VCS. If a VCS is defined for the workspace DEM, a VCS and transformation must be selected. To determine the VCS of your DEM, right-click on the layer in the Catalog pane, select Properties, and expand Spatial Reference.
- Click OK twice to accept the changes and close the Spatial Reference and Define Z Correction windows.
- In the Contents pane, uncheck the Image Collection layer. This ensures GCPs are measured from the basemap layer.
- In the GCP Manager window, click the Add GCP or Tie Point button .
- Make sure the Imagery basemap is selected in the Contents pane. Navigate to one of the points in the GCPs layer and click on the corresponding feature in the Imagery basemap to place a GCP.
GCPs should be collected using identifiable features that are present in your image collection and the reference image, such as road intersections, street corners, or other geometric features.
- The approximate location of the GCP in the satellite imagery will appear in the GCP Manager image viewer. Locate the same feature in the image viewer and click on the feature to place a tie point. The tie points for the other image will be automatically calculated when possible, but each tie point should be checked for location accuracy. When a tie point has been successfully added to an image, the gray tie point symbol will change to blue in the image viewer list. Use the pointer to change the location of the tie point.
The x and y GCP coordinates will be derived from the Imagery basemap, while the z value will be collected from the workspace DEM.
- Repeat this process for each of the four GCP locations.
- After each GCP has been added and measured with tie points, click the Set GCP Accuracy button and enter 1 for the XY Accuracy and 3 for the Z Accuracy.
- Confirm that Update is set to All and click OK.
- After adding GCPs, 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 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.
Generate a digital elevation model (DEM)
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.
If an area is heavily wooded, or has other dense vegetation cover, it will not be possible to derive a DTM ground surface because the ground is not visible. This issue might also arise in a dense urban areas, where buildings obscure the ground. In this case, the most appropriate elevation surface is a DSM, which specifically creates a surface depicting the top of the urban environment.
- In the Ortho Mapping tab, in the Product group, click DTM to start the DEMs 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 2.5 meters.
- Accept all remaining default settings and click Next.
For information on the Advanced Settings, see Generate elevation data using the DEMs wizard.
- In the DTM Settings window, under the Format drop-down menu, choose Tiff Format.
- For Interpolation Method, select TIN Natural Neighbor Interpolation from the drop-down menu.
- Accept the remaining default settings and click Finish. The DEM generation can take a few minutes to complete.
Generate an orthomosaic
Next, you will 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 Elevation Source, check Use DEM Product and verify that Digital Terrain Model is selected in the drop-down menu.
- 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 Veroni. Click Next.
- In Orthomosaic Settings window, for Pixel Size, use the default value. This will determine the final resolution of the orthomosaic.
- Click Finish to generate the final orthomosaic. This process may take some time to complete.
In this tutorial, you created an ortho mapping workspace for satellite 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 from Satellite Imagery
- Adjustment options for ortho mapping satellite imagery
- Add GCPs to your ortho mapping workspace
- Generate elevations data using the DEMs Wizard
- Generate an orthomosaic using the Orthomosaic Wizard
The satellite imagery used in this tutorial was acquired and provided by Maxar Technologies.