With the diverse array and increasing number of sensors capturing imagery, as well as the availability of processing software, you can generate ortho imagery and digital elevation models (DEMs) to inspect, monitor, and explore the natural and built environment. Many organizations also contract out the generation of ortho imagery to private mapping companies or license speculatively created products. Ortho imagery in the form of orthomosaics and True Orthos are often used as base maps to visualize context and extract feature locations with automated and manual measurements. The positional accuracy of these data products directly impacts the accuracy of the measurements and the validity of any analyses and decisions based on them.
The American Society for Photogrammetry and Remote Sensing (ASPRS) has set standards, guidelines, and best practices to define, control, and report the positional accuracy for digital geospatial data. Horizontal accuracy of ortho images is calculated by measuring clearly identified and randomly distributed locations (checkpoints) with known coordinates. Similarly, for elevation products (bare earth DEMs, digital terrain models [DTMs], or digital surface models [DSMs]), the vertical accuracy can be verified with vertical checkpoints.
In ArcGIS, you can use the Positional Accuracy Assessment web app to determine the accuracy of ortho images. This app allows you to input ortho imagery as a web service, upload and measure the checkpoints, and generate accuracy assessment reports.
Before using the Positional Accuracy Assessment app, you must have the imagery accessible as an image service. You can do this by hosting and sharing the imagery in ArcGIS Online or by serving the imagery using ArcGIS Image Server.
The following are the main stages of an accuracy assessment using the Positional Accuracy Assessment app:
- Creating or defining a control point layer—The control point layer is a point feature service that defines the location of well-defined points that have been accurately surveyed. Typically, the coordinates, metadata, and descriptions of these points are available as tables (such as .csv or .xlsx). These must be converted into a feature class defined as a set of points with the appropriate spatial reference. You can do this using the standard tools in ArcGIS Pro such as Open Tabular Data and XY Table To Point to create a feature class. Review the content, and use Share as Web Layer to publish to ArcGIS Online or ArcGIS Enterprise. When creating control point feature classes, tag the item with the Control tag. This makes it quickly searchable in the app.
- Creating an accuracy assessment check layer—When users measure the location of the point, they must see the approximate location of the points and identify and measure the correct feature. It is important to only provide an approximate location, not the exact location. Similarly, in some organizations, the accurate location of the control points is restricted and should not be made available to users who perform the measurement. This enables the control point feature service to be converted to an accuracy assessment check layer where the location of the point is randomly moved. The app provides a preparation step that prompts for the control point layer, which is the name of the accuracy assessment check layer to create a tolerance value. The tolerance value is the magnitude of the random variations that will be applied to each point.
- Measuring the locations—Measure the location of the checkpoints at visible locations in the imagery layer. This stage requires the definition of the image layer, as well as the accuracy assessment check layer, and defines the name of the measurement layer to be created. The app then displays a map showing the imagery and each point showing a circle that defines the approximate location as well as the details from the control point file and image if available. You must zoom in and measure the required location accurately.
- Generating an accuracy report—This stage performs an analysis using the original control point layer and the measured points to create an accuracy assessment report that can be viewed or exported as a PDF. The app prompts for the control points and measure layer (and, optionally, for a web map to be used as a base).
ArcGIS help topics
Review the following reference materials for ArcGIS products:
- Learn more about web services and how to create them .
- Learn about the general workflow for orthomosaic and DEM generation with ortho mapping.
- Complete the Drone2Map 2D products workflow to learn how to create high-resolution orthomosaics, True Orthos, and DEMs.
- Learn about the CSV text file format in ArcGIS Online and ArcGIS Enterprise for checkpoints.
- Learn about reality mapping in ArcGIS Pro to generate True Orthos and 2.5D DSMs.
- Learn how to upload photos to Site Scan Manager and set ground control points for use in Site Scan for ArcGIS.
ArcGIS blog articles
Review the following for information about concepts, software functionality, and workflows:
- Learn about the different kinds of ortho images created in ArcGIS.
- Read an explanation of the value of True Orthos and how they differ from orthomosaics.
Esri Community
Visit the Esri Imagery and Remote Sensing community to connect, collaborate, and share experiences regarding accuracy assessment.