Available with Advanced license.
This workflow describes how to import, compute, and measure ground control points (GCPs) in your ortho mapping workspace.
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. In a block adjustment, overlapping images are tied together to minimize error between images using tie points and ground control points, if available. Block adjustment can be applied without GCPs and still ensure relative accuracy, but adding GCPs increases the absolute accuracy of the adjusted imagery. Check points are GCPs that are excluded from the adjustment calculation and are used to help determine the accuracy of the adjustment.
The distribution of GCPs in a photogrammetric block is an important part of the accuracy of the adjusted block. Control distributed around the perimeter of the block have a greater influence on adjustment accuracy than control near the center of the block. The figure below shows recommended GCP locations prior to block adjustment.
GCPs can be added manually by selecting points in a reference image, imported from survey data using the Import GCPs tool, or computed using a reference image with the Compute GCPs tool. You can access these tools from the Manage GCPs menu in the Refine group under the Ortho Mapping tab. When working with satellite or aerial digital data, you can add GCPs to your workspace before the initial adjustment is computed. When working with drone or scanned aerial imagery, you must perform a block adjustment before you can add GCPs.
Add GCPs manually
If you do not have GCPs from ground survey, you can collect them from a georeferenced or orthorectified raster layer, raster dataset, mosaic dataset, or image service. Be sure the radiometric, temporal, and spatial resolution of the reference image is similar to that of the image collection. GCPs should be collected using identifiable features on the ground level that are present in both the image collection and the reference image. Recommended features include intersecting street paint striping, street corners, or other natural or developed features on the ground.
To add GCPs manually, complete the following steps:
- Add the reference image to the 2D map view.
- On the Ortho Mapping tab, in the Refine group, click the Manage GCPs button .
- In the GCP Manager window, click the Define Z Correction button to define any vertical transformations that are required to transform heights extracted from the basemap or reference image into the spatial referencing of your project.
If the vertical coordinate system of the project DEM is not defined, select None under Geographic Transformation.
- Click the Browse and select spatial reference button .
- In the spatial reference window, under Current Z, select the vertical reference system of the project DEM, which is attached to the image collection.
- Click OK.
- In the Contents pane, turn off the image collection layer. This ensures that GCPs are selected from the reference image.
- In the GCP Manager window, click the Add GCP or Tie Point button .
- Ensure that the reference imagery is selected in the Contents pane. Navigate to an identifiable feature in the reference imagery and click it.
A red cross appears over the feature in the map and a new GCP entry is entered in the GCP Manager.
- In the GCP Manager window, locate the corresponding GCP feature in the image viewer and click the feature to place a tie point. Tie points for any other overlapping images are automatically calculated.
Once a tie point has been successfully added to an image, the GCP symbol changes from red to green. This indicates that the GCP has been measured. The location of a measured point can be changed by remeasuring or clicking the desired point.
- Repeat this process until a sufficient number of GCPs have been collected.
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, ensure that your reference image has good georeferencing quality in terms of geopositional accuracy and clarity and that the resolution is similar to your source imagery. For example, the default ArcGIS Online World Imagery Service may be a good reference for computing GCPs for your satellite data, but it may not be a good enough reference for high-resolution and highly accurate aerial imagery.
- On the Ortho Mapping tab, in the Refine group, click the Manage GCPs drop-down menu and select Compute GCPs.
- In the Compute Ground Control Points window, in the Reference Image drop-down menu, select or browse to a reference image.
- In the Elevation Source drop-down menu, select or browse to an elevation source. When working with satellite imagery, check Geoid Correction.
- Specify the image location accuracy of the image collection, as well as the point similarity, point density, and point distribution of your output GCPs.
- Check Adjust and click Run.
Import existing GCPs
If you have existing ground survey data, you can import your GCPs into the ortho mapping workspace.
- Click the Manage GCPs drop-down menu and select Import GCPs. Browse to the location of the GCP file, which can be a .csv or .txt file, feature class, or shapefile.
The fields in the file should include the GCP Label, X coordinate, Y coordinate, Z coordinate, XY accuracy, and Z accuracy.
- Under Set GCP Spatial Reference, browse to and specify the horizontal and vertical coordinate systems of the GCPs. If the coordinate system of the GCPs is different than that of the ortho mapping workspace, specify the horizontal and vertical geographic transformations to apply.
When working with a satellite ortho mapping workspace, the vertical coordinate system will automatically be converted to WGS84 Ellipsoidal.
- Under Field Mapping, specify which fields in your GCP file correspond to the field mapping parameters listed.
- If your GCP data includes photos of the locations where GCPs were measured, browse to and select the folder containing these photos under GCP Photo Location.
The name of the photo should match the GCP Label field. If there are multiple photos for each GCP, the name of the photo should include a number appended to the Label field, which will be used as a unique identifier. For example, GCP20_1.jpg and GCP20_2.jpg are the photos for GCP20.
- Click OK to import GCPs.
If your GCPs and the elevation source of your mosaic dataset have different z-values, your imagery can appear shifted in the map view after adjustment. This may occur if you specified a constant z-value when defining the orthomosaic. To visualize adjustment results on GCPs, click the Show GCP button to display GCPs in the GCP Manager window. Any visible image shift is corrected when you readjust the imagery with a refined elevation source and generate the ortho mosaic product.
Add tie points
After GCPs are imported, click the Manage GCPs drop-down menu and select GCP Manager, which allows you to add the tie points for your GCPs. In photogrammetry, this process is known as creating a measured tie point, or a tie point that has been provided to an image that corresponds to a GCP.
- Select a GCP from the list.
The corresponding images that overlap with the GCP appear in the image list at the bottom of the GCP Manager window.
- Click the Add GCP or Tie Point button to add an image tie point in the image viewer for each overlapping image.
The tie points for other images will be automatically calculated when possible, but each tie point should be checked for location accuracy. To ensure placement accuracy, you can use the Ctrl+M keyboard shortcut to turn the measured tie point on or off, and make positional adjustments accordingly. When a tie point has been successfully added to an image, the gray tie point symbol changes to blue in the image viewer list and the associated GCP symbol changes from red to green in the 2D map view.
- Repeat this process until all GCPs have been measured.
Add check points
The steps below describe how to convert GCPs to check points for accuracy assessment. Check points are conceptually the same as GCPs; they are points with known ground coordinates and link to features visible in multiple overlapping images. However, they are not used as inputs to control the adjustment process. Instead, check points measure the accuracy of the adjustment and the resulting orthoimage. For each check point, the distance between its known ground location and the location of the corresponding pixel after the adjustment process is used to calculate the overall absolute accuracy of the block of images. Accuracy measurements are summarized as the root mean square error (RMSE) using the check points and can be found in the Adjustment Report.
GCPs can be converted to check points for postprocessing accuracy assessment.
- After your GCPs have been added and measured with tie points in your ortho mapping workspace, select the GCP you want to convert to a check point in the GCP Manager pane.
- Right-click the GCP and select Change to Check Point.
The check points are displayed as pink triangles on the ortho mapping workspace map, and the Label icon in the GCP Manager changes from a blue circle to a blue triangle.
Once you add GCPs or tie points, you must click Adjust to rerun the block adjustment and utilize these points.
Notes and tips
- The Clear Links button is used to delete all tie points associated with selected GCPs.
- A single tie point can be deleted by right-clicking the image name in the GCP Manager image viewer and selecting Delete Tie Point.
- The Delete GCP button is used to delete a selected GCP and all the associated tie points for that GCP.
- The Delete All button can be used to remove all GCPs from the GCP Manager window.
- The Undo button can be used to restore GCPs that were deleted during the measuring session. Once adjustment has been run, deleted GCPs cannot be restored.
- In the image viewer, you can zoom to the full extent of the selected image using the Zoom button , stretch the selected image based on the pixel values within the display using the Dynamic Range Adjustment button , or enable or disable the selected GCP in the image viewer using the Show GCP button .
- The x- and y-coordinates of a GCP are measured from the map, and the z-coordinate is obtained from the elevation source. If you add a reference image that has a different geographic coordinate system from the map, or the elevation source has a different vertical coordinate from the map, ensure that the coordinate transformations are correctly defined.
- If GCPs are collected from a reference image, they should be collected from the ground surface. Consider image resolution when you choose features, so they will be clearly depicted in your imagery.
- If you don't have a GCP file and you need to measure GCPs on a reference map, use the Manage GCPs tool to enter a GCP by clicking a location on the map and adding the corresponding tie points in the viewers. The accuracy of these secondary GCPs depends on the accuracy of the reference map.
- Double-click a row in the GCP Manager window to pan to the GCP location in the image viewer at the current display scale.
- When the pointer is in point collecting mode, you can press and hold the C key to switch to the explore pointer.
- The shortcut to turn dynamic range adjustment (DRA) on and off is Ctrl+D.
- The shortcut to zoom to the full extent of the image is Ctrl+F.
- Make sure to include enough well distributed check points to facilitate a representative accuracy assessment.
- Try running an adjustment with check points, assessing the accuracy in the Adjustment Report, then changing different GCPs to check points. Rerun the adjustment and see how the resulting RMSE changes.
To learn about creating ortho mapping products, see Generate ortho mapping products.