20-Jul-2008 -- Revisited my first confluence, this time with a precision Trimble GNSS R8 GPS instrument, capable of defining the point within 5 cm (or even less in unobstructed areas).

The point was found to be about 1.2 m east of the latest attempt with an autonomous hand-held GPS. The old stick was still there and can be seen in the view-west picture and the overview picture.

Calculations of the position error gave the result that I was 6.6 cm from the exact confluence position, measured as an average of 10 samples of 3-epoch determinations (5.0 cm south and 4.2 cm east of the desired point). The read-out position from the instrument display varies a little for each sample. The instrument's own estimation of the measurement accuracy is given in the display as 0.013 m in position and 0.029 m in elevation, figures that seem to be taken as a standard deviation for the displayed position.

In the GPS photo the displayed latitude 65 degrees 00 ' 00.00340" N can be calculated to be 1852 * 0.00340 / 60 m = 0.105 m from 65 degrees N exactly. Similarly the longitude is 1852 * cos 65 *0.00490 / 60 m = 0.064 m from 20 degrees E exactly. The shown sample is taken a few minutes after the 10-sample average.

The displayed elevation is the height above the WGS84 ellipsoid. The RH70 elevation, which is corrected for the height of the geoid, is 232.59 m.

Eight days later I was back and measured the gravity at the confluence. The purpose of this visit was to establish a reference gravity station in order to incorporate an old survey into the standard ECS62 gravity bouguer anomaly data. The gravity instrument I used was an 35 years old Worden, which had been restored to full function because the more modern Scintrex Autograv CG-3 (18 years old) I used until last year was out of order permanently. The RH70 elevation is used in the bouguer anomaly calculation and must be known with an accuracy of a few centimeters to give the desired accuracy in bouguer anomaly. The bouguer anomaly is used to determine if dense bodies (preferably orebodies) may exist in the survey area.