GPS Photogrammetry-based sensor digitization

EGI Geodesic Photogrammetry System

EGI’s GPS technology is used to derive the 3D position of each EEG sensor on the scalp surface. The sensor coordinate map is important for increasing the accuracy of electrical source imaging, relative to the default positions, as it describes the true position of the EEG sensors relative to the whole head. This capability improves the accuracy of source localizations performed with EGI’s GeoSource 3 source estimation software.

The GPS 3 product uses EGI’s photogrammetry technology to capture images of all the sensors on the head simultaneously with the click of a button, a great convenience for researchers, clinicians, and their research participants and patients. The photographs provide a permanent record of the sensor positions for a given exam, allowing for verification and analysis at any time after the exam is complete.

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New with GPS 3.1 software

  • The GPS 3.1 solver application now includes the functionality of the prior GeoScan File Converter
  • Ability to control the camera exposure to reduce glare during GPS acquisition
  • Improvement to instructions for calibration file
  • Bug fixes and feature requests

The GPS is an innovative photographic technology for sensor localization. The GPS uses 11 cameras mounted in a geodesic array to photograph sensors on the patient’s head, allowing the simultaneous recording of all sensor positions. Patients are then free to go immediately. 

At any time after photography, the location of the sensors are determined using semi-automated software with machine vision technology. The final location of the sensors in the solved point cloud can be compared to the original pictures to verify accuracy.

The Geodesic Photogrammetry System has been shown to result in similar accuracy to that obtained with electromagnetic digitizers (Russell, Eriksen, Poolman, et al. (2005)). However, unlike electromagnetic digitizers, photogrammetry is not susceptible to electromagnetic interference, and therefore provides reliable data even in electrically noisy environments.

EGI Geodesic Photogrammetry System Process

In addition, measurements with electromagnetic digitizers require a patient to remain still for the entire measurement, which can be difficult for some participants, particularly pediatric patients or patients with behavioral challenges. In contrast, the GPS 3 method can be used with any patient, as the measurement is complete within seconds.

Russell, Eriksen, Poolman, et al. (2005). Geodesic photogrammetry for localizing sensor positions in High Density EEG. Clinical Neurophysiology 116, 1130-1140.

The Geodesic Photogrammetry System includes:

  • a precision geodesic dome of 11 cameras
  • gantry with effortless counterweight movement
  • Net Station Photogrammetry software module
  • data acquisition computer and monitor
  • calibration object
  • manuals and reference materials
  • on-site installation and training
  • full year support contract GPS supplement 


EGI High Density EEG NeuroimagingHigh Density EEG neuroimaging
Precise sensor localization is critical for accuracy in High Density EEG neuroimaging.


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Geodesic Photogrammetry System 3

EGI's Geodesic EEG System product line


Part of the Geodesic EEG Product Line

This instrument is not intended for use in diagnosis or treatment of any disease or condition. It is a scientific research instrument designed for performing measurements and acquiring data for neurophysiological research. MagstimEGI makes no representation of the suitability of the instrument for any particular research study.