The GPS surveyed height of a point on the ground is derived from the GPS receiver position (1), the height of the survey pole (3), and the location of the antenna within the GPS receiver (2).
When PointMan measures a ground point (G) the measured vertical elevation is the value shown as (2) in the image below. We need to adjust that down to the ground so that we are measuring the actual physical point on the ground (G).
In most cases, we use a value called the antenna height in PointMan to accomplish this. A standard survey pole height is 2.0 m, or 6.561 feet long – this is the height referenced as (3) in Figure 1. The issue is that that is not the correct height location of the antenna within the GPS receiver. In any GPS receiver, the actual location of the measured height is at the point where the GPS signals are electrically focused. This is different from where the survey pole is attached to the antenna. In Figure 1 we show that electrically focused point at the top of the receiver, this is just for illustration purposes, it usually is somewhere within the physical housing of the GPS receiver but not necessarily at the top, in fact, on some high precision antennas, it can be a virtual point located outside the physical housing.
The height difference between where the survey pole is attached to the GPS receiver and the location of the antenna signal’s electrically focused point (commonly known as the phase center) is called the phase center offset. This value is specific to the antenna model and will be supplied by the GPS manufacturer. For PointMan, we would need to add that to the antenna height input value to properly indicate the true height of the antenna above the ground.
Using an example will help illustrate the point.
In this example, we will use the Trimble SPS986 antenna. Trimble Access knows this antenna model and will properly integrate the phase center offset in its height calculations. So, the elevation of the point on the ground that they measure is:
709.887 feet – This number is from Trimble Access.
From PointMan, we can get our example numbers (ones of interest in red – my emphasis)
Antenna: 6.562 ft
Site Name: Test
Site Northing: 711427.870840464
Site Easting: 1823937.51877073
Altitude: 716.9685 ft
Elps Altitude: 605.965 ft
Elevation: 710.4065 ft
Altitude (716.9685) is the height where the survey pole attaches to the antenna, Antenna (6.562) is the length of the survey pole – distance 3 in figure 1. So, to get the height on the ground, you would subtract the height of the survey pole from the collected height of the antenna to get the height of the point on the ground.
716.9685 – 6.562 = 710.4065
And 710.4065 is the value shown as Elevation of the point on the ground in PointMan – okay all good so far.
Problem: Where the survey pole attaches to the GPS receiver is not where the GPS signals are focused or where the GPS antenna is physically located, we need to add in the height indicate as (4 – phase center offset) to get the true height of the antenna above the ground.
This information we can get from Trimble – see screenshot below.
The Trimble site where you can select the antenna type and get the proper phase center offset is:
These values are from Trimble for their SPS986 Antenna, it shows the vertical offset numbers you need to properly calculate the antenna height – which one you use depends on how you attached the survey pole to the GPS receiver. The two I am interested in are the bottom of the antenna mount and the bottom of quick release methods, ordinarily, we would use the bottom of the antenna mount value of 0.14440 m (this would be the value of 4 in Figure 1) In this case the setup was also using what is called a quick mount, this is an additional adapter on top of the survey pole and is the same as method #2 Bottom of quick release method. This gives a value of 0.18440 m– this is the value we are looking for as the additional height above ground (4) where the antenna is located.
The correct antenna height calculation is: (using the phase center offset of 0.18440 meters (0.6050 feet)
716.9685 – (6.562 + 0.6050) = 709.8015 feet
Compare this with the Trimble Access height value of: 709.887 feet gives a difference of 0.086 feet – this is within expected error bounds.
In PointMan, instead of the 2.0 m or 6.562 feet antenna height standard, we would need to add on the phase center offset as well, in this case, 6.562 feet + 0.6050 feet = 7.167 feet would be the correct antenna height.