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Assess the accuracy of orthomosaics and elevation models

With the diverse array and increasing number of sensors capturing imagery, as well as the availability of suitable processing software, it is now easier than ever for GIS users to generate their own 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 orthoimagery to private mapping companies or license speculatively created products. Ortho imagery in the form of orthomosaics and true orthos are often leveraged as base maps used to visualize context and extract feature locations with automated and manual measurements. The positional accuracy of these data products directly impacts the accuracy of these measurements, and therefore impacts the validity of any analyses and decisions based upon them.

The American Society for Photogrammetry and Remote Sensing (ASPRS) has set forth Standards, Guidelines, and Best Practices to define, control, and report the positional accuracy for digital geospatial data. Horizontal accuracy of orthoimages is calculated by measuring clearly identified and randomly distributed locations ("check points") with known coordinates. Similarly, for elevation products (either bare earth DEMs, DTMs, or DSMs), the vertical accuracy can be verified with vertical check points.

Within ArcGIS, you can leverage the Positional Accuracy Assessment web app (referenced below) to determine the accuracy of ortho images. This app allows you to input your ortho imagery as a web service, upload and measure your check points, and then generate accuracy assessment reports.

Prior to using the Positional Accuracy Assessment app, it is necessary to have the imagery accessible as an image service. This can be achieved by hosting and sharing the imagery in ArcGIS Online or by serving the imagery using ArcGIS Image Server.

There are four main workflow steps when conducting an accuracy assessment using the Positional Accuracy Assessment app:

  1. Create or define 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 need to be converted into a feature class defined as a set of points with the appropriate spatial reference. This can be done using the standard tools in ArcGIS Pro such as Open Table and Display XY Data to create a feature class. Review the content, then use Share as Web Layer to publish to ArcGIS Online or ArcGIS Enterprise. When creating such control point feature classes, tag the item with the tag 'Control.' This will make it quickly searchable in the app. Esri will also be providing a tool to create the control point layer directly in the web app.
  2. Create an accuracy assessment check layer—When users measure the location of the point, they need to be driven to the approximate location of the points and then identify and measure the correct feature. It is important to only provide an approximate location and 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.
  3. Measure the locations—Measure the location of the "check points" at visible locations in the imagery layer. This step 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 application will then bring up a map showing the imagery and drive you to 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 need to zoom in and measure the required location accurately.
  4. Generate an accuracy report—This step 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).

Explore the following resources to learn more about assessing ortho imagery accuracy in ArcGIS. (Not sure where to start? Look for the star by Esri's most helpful resources.)

Imagery Workflows resources

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ArcGIS help

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ArcGIS blogs, articles, story maps, and technical papers

Review the following supplemental guidance about concepts, software functionality, and workflows:

Esri Community

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