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Question What is the MagArrow GPS accuracy in position/elevation/altitude?

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The 3 Meter accuracy of the GPS in the MagArrow I is 50% Circular Error Probable (50% CEP).  That means that if accumulating location data at a fixed point for a long period of time 50 percent of the readings will be within  a circle with a 3 meter radius of the actual location, and 50% will be outside that circle.  It also requires the GPS antenna have a clear view of the sky with no multipath interference.

 A good measure of GPS accuracy is to look at the HDOP number.  It should be less than 1.

Altitude values as a rule of thumb will be half as accurate due to the geometry of satellites.  It will be much less accurate if the satellites towards the horizon are blocked, which is often the case.  In other words the best accuracy for altitude requires a wide view of the sky.

MagArrow II has much improved GPS specs: 50% CEP 1.5m; less than 1m with SBAS.

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Here are a few additional details relating to the measurement of elevation:

GNSS Accuracy
Accuracy depends on multiple aspects of the GNSS system: among them are clock accuracy, atmospheric effects, and satellite geometry.

Satellite geometry
"Satellite geometry" refers to how the currently visible satellites are distributed in the sky - close to each other or scattered around. The best satellite geometry includes satellites that are near the axes on which you hope to locate your receiver; for example, to locate your receiver on the East/West axis, it's helpful to have good reception from satellites low in the sky in the East and in the West.

If you also have satellites that are low in the sky near the South and North horizons, you will have good accuracy on the horizontal (latitude/longitude) plane. It's best to have satellites scattered around the sky, overhead as well as near the horizon all around.

MagArrow records HDOP, a standard measure of satellite geometry's effect on horizontal (or latitude/longitude) accuracy. A smaller number (less than 1.0 is very good) indicates that the visible satellites are in good positions to contribute to accuracy.

Vertical Accuracy
The reason that GNSS systems aren't as accurate on the vertical axis as on the horizontal axes is that no satellites are visible in a full half of that axis: the half that is below the horizon. Consequently, vertical accuracy is on average about half that of horizontal accuracy; calculated offset from true elevation is on average about twice that as on the horizontal axes.

While on average HDOP can therefore be used to estimate VDOP (the similar measurement of the effect of satellite geometry on the vertical axis), that estimate is only a rule of thumb; it is possible to have an excellent HDOP, reflecting very good horizontal satellite geometry, while having poor vertical satellite geometry. In those cases, good HDOP does not indicate good VDOP. Keeping in mind that possibility, a combination of good HDOP and many satellites in view usually indicates good VDOP.

Practical effects

Some data processing techniques (upward continuation, for example) can include elevation as an input. Customers who are considering using GNSS elevation in those techniques should conduct a careful analysis of their data and develop test routines to verify that all their data meet the requirements of the technique and its application to a particular survey. Some customers who require very accurate elevation data incorporate LIDAR data and drone elevation data into their analyses.