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How To: Set ground control points for use in Site Scan for ArcGIS

Summary

Instructions provided describe how to set ground control points (GCPs). GCPs are fixed features visible in Site Scan for ArcGIS source images, with accurate and precise, known positions. They are used to enhance the absolute accuracy of Site Scan outputs.

Without GCPs, Site Scan outputs are solely georeferenced using the drone’s GNSS receiver and inertial sensors. While the relative accuracy within the models is high, the absolute accuracy, or where the model is located on Earth, may shift between 2-8 feet.

Procedure

Setting ground control points

  • Visibility
    Easily identifiable features can be used, for example, the corner of the site trailer, the center of a manhole, or targets can either be laid or painted on the ground
  • Ground control points must be immobile
  • Use contrasting patterns where the surveyed location can be easily identified, such as chevrons, checkered patterns, or crosses, as shown in the images below.
Image of a GCP target Image of a GCP target
Image of a GCP target Image of a GCP target

Number of GCPs and layout

A minimum of three points is required. For optimal results, a minimum of five control points is recommended for a site of any size.

To maintain high accuracy throughout the processed outputs, Esri recommends using additional points for large or oddly shaped sites, or sites with many elevation changes.

For sites wider than 20 acres (eight hectares), Esri recommends adding one GCP for each subsequent 15 acres (six hectares).

To set the first five points, visualize the five faces of a dice. One point near each corner, but not on the very edge of the mapped area, and one near the center. Distribute other points evenly between the first five.

Sample GCP layouts

Image of sample GCP layout
Image of sample GCP layout
Image of sample GCP layout

Active construction sites may have difficulty laying a control point near the center of the site, and it is acceptable to omit this point if unpractical. Esri recommends adding at least two additional points along the perimeter.

Image showing active site

Measuring GCP position

  • The location of the points must be obtained by a surveyor or field engineer using conventional survey methods such as total stations and survey-grade GPS, or from other reliable geographical data sources.
  • Site Scan for ArcGIS GCP processed data can only be as accurate as the original GCP position. Ideally, under 1-2 cm.
  • The coordinate reference system (CRS) in which the points are collected must be known, along with the units used, or the EPSG code.
  • Surveyors typically deliver a *.txt or *.csv file containing the CRS and units or EPSG code, and XYZ data for each point, along with a description.
  • Assign the same CRS to your Site Scan for ArcGIS Manager project.
Image showing CRS assign

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