Conflicting WGS84 coordinates between Trimble and Garmin GPS receivers

Question

I am a contractor at a government facility in Nevada. We have our own in-house survey department who, for a number of reasons, still uses NAD27 State Plane Coordinates (Nevada Central Zone 2702) in survey feet (referred herein as SPC27). They also provide lat-long coordinates in NAD83 degrees, converted using Corpscon, I believe. For my project, they have established several local control points. When I occupy these same control points with my 15-year old but WAAS-enabled hand-held Garmin 76, I get WGS84 coordinates (and SPC27 coordinates converted using Corpscon) very close to those of the surveyors (within several feet).

When my co-worker occupies the same control points with his Trimble GeoHX (differential GPS with post processing), he gets WGS84 coordinates that are consistently 0.0009 degrees farther east, or when converted to SPC27 with his older CoordTrans software, are 250 feet farther east (and 18 feet farther north).

But when he converts his seemingly wrong WGS84 coordinates to SPC27 using his newer Trimble Pathfinder Office software, his resulting SPC27 coordinates plot closely with mine and those of our surveyors.

So, occupying the same control point, my Garmin and his Trimble give WGS84 readings that are off by 0.0009 degrees longitude (or 250 feet when converted to SPC27 using Corpscon). Yet this difference seems to be corrected when his WGS84 coordinates are converted to SPC27 using his Pathfinder Office software.

The error (250 feet) is about what I've read to expect in my geographic region between SPC NAD27 and SPC NAD83. But why would our raw WGS84 GPS readings be this far apart?

Answer 1

0.0009° is 3.24 arc sec, which is 3.24 * cos(40°) * 101 ft/asec = 251 feet, where I’m assuming your latitude is ~40° (northern Nevada). This suggests that the transformation by the CoordTrans software is not including any datum shift of any kind.

I don’t the know the details of how you are interfacing the Trimble to the CoordTrans software, but perhaps the Trimble is set to do its own conversion to NAD27 coordinates for your benefit, producing the 0.0009° shift, and that is also being fed to CoordTrans which recognizes the shift has already been done and simply translates lat-long to feet.

Since the newer Pathfinder software is providing a correct transformation, this suggests that it’s looking at the underlying WGS84 coordinates of the Trimble and doing the transformation to NAD27 on that basis.

In summary, perhaps in actuality the Trimble is displaying and CoordTrans is seeing NAD27 data, while Pathfinder sees WGS84 data.