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TIPS: Using GPS elevation with GPR

Using GPS elevation with GPR 1

When we think about GPS, we usually only think about our lateral position on the Earth, or, in other words, our XY position. But GPS also provides Z, your elevation. If you are collecting GPR data with GPS, the elevation is pretty much free information. When GPR data with GPS files are opened in the EKKO_Project software, the software automatically adds GPS position values for every GPR trace collected, including the elevation.

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Figure 1:
Switching the vertical axis from Depth to Elevation

GPR lines are typically plotted with a depth axis but when elevation information is available, the data can be plotted with an elevation axis simply by switching the vertical axis from depth to elevation in the LineView Module > View Settings > Axes menu option (Figure 1):

Plotting data with elevation can assist with data interpretation. For example, the left side of Figure 2 shows a GPR line with a boundary that appears to be dipping at an angle from the surface. After switching to an elevation axis, we see that the dipping boundary is actually flat and it is the surface that is sloping.

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Figure 2:
GPR line plotted with the vertical axis in depth (left) and elevation (right) shows that the sloping reflector in the depth plot (red dashed box) is flat-lying in the elevation plot.

Like most things in life, you need to be careful of the quality of free things. In general, the GPS Z value is less accurate than the XY positions. Historically, the error in Z is often 4 to 5 times larger than the error in XY values. To get an accurate elevation value, you generally need a more advanced GPS unit. Figure 3 shows the results of elevation corrections of GPR data collected with GPSs of different accuracies. As the plots in Figure 3 show, it is important to ensure the accuracy of the GPS positioning is sufficient for your survey requirements. Elevation information, when accurate, helps with data interpretation; reducing topographic data distortions and by providing better lateral spatial positioning of the targets. Best of all, this capability in the EKKO_Project software is available with no extra effort.

GPR TopCon

Figure 3:GPR lines plotted with elevation values from a higher performance GP compared with a more rudimentary GPS device.