Topic: GPS drifting

[promapper]

I left my control unit on for a couple of hours with the boat sitting in my driveway and the track points shows the drift in the GPS location.  Based on the map scale on the left side of the screen it looks as though the location drifted by as much as 40-50 feet.

I thought it would have been tighter than that.  Is there a setting I may have missed?

[Humminbird_Greg]

I would have set the Trackpoint Interval and Track Min Distance menus to their lowest settings.  Other than that there are no menu setting that will affect the accuracy of the GPS Receiver.  That is controlled by the placement of the GPS Receiver (any obstructions?) and which GPS and WAAS satellites the GPS Receiver can receive signals from.
What were the EPE and HDOP numbers during this time?
What Fix Type did you have during this?

[ITGEEK]

The GPS accuracy can also be affected by atmospheric
conditions, and solar flares.
I would try it on a different(stable weather) day,
and see if it changes.
Also, make sure your battery is good, and is
fully charged.  I would think, a borderline battery may
cause fluctuations in the unit, and thus in the GPS
accuracy.

[promapper]

I will wait for clear weather and repeat the test noting the fix type etc.

[Humminbird_Greg]

This is really not the best way to judge the accuracy of the GPS Receiver.  Consider that the only time it will drop a trackpoint will be when the position exceeds the settings for both the Trackpoint Interval and Track Min Distance menus.  The trackpoint Interval menu setting should not be a problem as it is time based but the Track Min Distance will be.  In a perfect world, the unit would have 0.0 meter accuracy 100% of the time.  While over a 24 hour period it could potentially drop 86,400 trackpoints (1 per second) it wouldn’t drop any as the 1 foot minimum Track Min Distance menu setting would not have been exceeded.  In the real world here you may not see where the unit resolved its position to a very high degree of accuracy as it did not move at least the required distance of the Track Min Distance menu setting.  A better way to look at the accuracy would be to take the NMEA output and plot it on a graph.  Of course you really would need to do this from a point on the earth that has been georeferenced to a much higher precision than our GPS Receivers are capable of.  We have two of those points on here at Humminbird.

Remember too that GPS Receiver accuracy is generally expressed in a 50% CEP (Circular Error Probable) rating.  So an accuracy of 1.5 meters (CEP) would mean that 50% of the time the GPS Receiver is within 1.5 meters (4.92 feet) of its actual position on the earth.

[promapper]

So if I understand this correctly the only time it will drop a new point is when both conditions are met, both time and distance.  Makes perfect sense to minimize the amount a data collected.  No sense in collecting position points if the antenna has not moved.

However does not a gps receiver from the same position continually refine the solution and gain more accuracy the more data it collects from the satellites.  I am pretty certain that the high order monuments you have at HB were collected in a static mode over a period of time, which would depend on the equipment in use.  But then the surveyor also had the advantage of post processing the data with all of the corrections and anomalies dealt with by interpreting the results.  With the number of satellites now available he could even throw out several without effecting the results negatively.  The HB software is probably not that intuitive given all of the variables and I would not expect it to be.

All in All,  I believe I understand the reasoning in what I observed.  Given that the unit was running for 2 hours it could have collected 7200 points of which only 40 were plotted on the track.

looking at it that way it puts it in a whole new perspective.

My bad, Thanks for the lesson.

[Humminbird_Greg]

So if I understand this correctly the only time it will drop a new point is when both conditions are met, both time and distance.

Correct.