TIPS: Positioning GPR Surveys – GPS Offset
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TIPS: Positioning GPR Surveys – GPS Offset

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any GPR surveys are done in Locate & Mark mode, meaning the operator monitors the data display and interprets the data in real time. When features of interest are found, their location is determined using the back-up arrow and the position is marked with a flag, paint or immediate excavation occurs.

Other surveys are done in Survey & Map mode. In this type of data collection, typically a large area needs to be surveyed, and productivity is important. Data is collected with a single channel or, for even higher productivity, a multi-channel system like a SPIDAR system is used to cover a wider swath in each pass. The large data volume from such surveys means that real time interpretation is difficult or impossible and, in most cases data are often collected without the operator even looking at the data display. Data are interpreted during post-survey analysis of the data using the EKKO_Project software for cross sections and depth slices or 3D for grids. Post-acquisition processing and interpretation become essential when addressing complex sites with a variety of ancillary data. Other survey data, remote sensing images, and construction records must often be referenced to place the GPR data in context.

When a feature of interest is found in the GPR data, its physical location must be determined. In the early days and even now in some situations, positioning is often established using site-specific local grids and surveyed-in marker positions. Nowadays it has become quite commonplace to collect GPR with GPS to provide geo-referenced positioning for the GPR survey lines and grids.

SmartCart GPS mounts are designed so the GPS receiver sits directly above the GPR system so the data from each system is coincident. In other deployment configurations like low frequency bi-static, SmartChariot or SmartTow, it may not be possible or convenient to have the GPS directly over the GPR system. Providing the GPS is positioned at a defined XYZ offset from the GPR data reference point (normally at the mid-point between the GPR antennas), the EKKO_Project‘ software provides the tools to correct the GPS position of the GPR data.
For example, this feature corrects the GPR positions of data collected with the SmartChariot; the GPS mount is +0.5 meters ahead of the GPR in the Y position and +0.5 meters above in the Z position.
An extension of this capability is to use a common GPS for the positioning of each GPR sensor in multi-channel SPIDAR system, provided the local XYZ offset of each GPR sensor is recorded.

In order to apply this correction, EKKO_Project requires two pieces of information: the XYZ offset from the GPR antenna to the GPS antenna, and the orientation of the system for each point along the line. The XYZ position offset is provided by the user at time of data import The system orientation is automatically computed using a set of advanced filters to recreate the path taken by the GPS antenna and determine which nay the system was pointed.

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