Using a Trimble GeoExplorer 6000 Series GeoXT Handheld, I recorded uncorrected GNSS positions, for example:

Horizontal estimated accuracy in the field 4 meters.
Vertical estimated accuracy in the field 10 meters.
Position Dilution of Precision PDOP = 2

  1. Am I correct in saying that these accuracies are specified as RMS and have 68% vertical confidence and a 63–68% horizontal confidence?
  2. Is this accuracy ±? For example, is the horizontal accuracy ±4m or is it 4m across = ±2m?

The precision of GNSS devices is indeed usually given in RMS. Trimble is not an exception. In your case, you would have 63% of confidence that your point is located within an horizontal circle of radius 4 m around the measured position. And 68% confidence that your elevation is within +/- 10 m from the measured one. PDOP of 2 is quite good, so you can expect submetric precision after corrections.

As a remark, make sure that the elevation is in an appropriate elevation datum, because this could be an important source of inaccurate measurement.

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