The Receiver Is Not the Point
RTK solves the position of the antenna, not the point at the bottom of the rod. To report the elevation of the occupied point, the workflow must use the true distance from the pole tip or mark to the antenna reference point and must also account for the antenna phase center. A precise fixed solution cannot repair an incorrect physical antenna-height relationship.
What RTK Actually Measures
When the rover pole is placed on a benchmark, the antenna is suspended above the mark. The GNSS solution therefore belongs to the antenna, while the elevation the user wants belongs to the pole tip or occupied point. The field software bridges those two locations by using the entered rod height and the antenna phase-center correction. If either part of that relationship is wrong, the displayed ground elevation will be wrong even when the RTK solution itself is excellent.
It helps to picture the measurement as a short vertical chain: occupied point to antenna reference point, then antenna reference point to antenna phase center. The first distance is physical and can be checked with a reliable measurement. The second is the antenna phase-center offset, often referred to in field practice as the APC correction, and it must be accounted for by the RTK receiver or software.
The pole tip, ARP, and antenna phase center are not the same point. The operator measures the rod from the tip or mark to the ARP. Whether the APC offset gets added by the crew or by the software depends on the specific setup, and assuming the wrong one produces a real height error.
Do Not Trust the Rod Graduations Blindly
A telescoping rod may be labeled at 2.000 meters or another standard height, but the graduation is not proof of the true tip-to-ARP distance. Manufacturing tolerances, a worn tip, a replaced point, a quick-release adapter, a spacer, or the way the receiver seats on the pole can all change the actual height. Even a small difference transfers almost directly into the reported elevation.
For reliable elevation work, extend and lock the rod exactly as it will be used, install every adapter and receiver component, and manually measure from the actual pole tip or occupied point to the receiver-defined ARP. Record that verified value rather than assuming the printed graduation is exact. The check should be repeated whenever the pole, tip, adapter, mount, or receiver arrangement changes.
In practice, the more common mistake is not a bad rod graduation. It is the measurement-method setting in the field software. Most data collectors ask how the antenna height was measured: vertical to the ARP, slant to the bottom of a notch, slant to a manufacturer-marked point, or true vertical. Each option tells the software to apply a different internal offset before the height ever reaches the antenna phase center. Selecting the wrong method silently mismeasures the antenna height even when the entered number is completely accurate.
Entering the correct number for antenna height is not enough. If the measurement-method setting does not match how that number was actually measured, the software applies the wrong internal offset regardless of how carefully the height was measured.
A rod marked 2.000 meters is not automatically a 2.000-meter antenna height. The graduation is only a setting reference until the complete field assembly has been measured from the actual tip to the ARP.
Account for the Antenna Phase Center
The ARP is a physical reference that can be measured, but the GNSS signal is associated with the antenna phase center. That phase center is offset from the ARP. Some field software expects the user to add that APC offset to the measured rod height and enter a single combined total. Other software asks only for the height to the ARP and adds the correct offset automatically once the antenna model is selected. Neither pattern is a universal default, and assuming the wrong one produces a real, silent error.
The practical requirement is to confirm, from the software's own documentation, which pattern it actually uses before relying on a new combination of receiver and software. If the software expects a combined total, enter the verified tip-to-ARP distance plus the published APC offset. If it expects the ARP height alone, enter that and let the software add the offset. Do not assume a new setup works the same way a previous one did.
If every rover elevation is high or low by nearly the same small amount, compare that amount with the rod-height error, adapter thickness, or unaccounted APC relationship. A constant vertical shift often points to the physical antenna setup rather than the satellites, geoid, or datum.
The Base Height Matters Too
The same physical measurement problem exists at a local RTK base. The assigned base coordinate describes the antenna position, so the base antenna height must correctly relate the monument or setup point to the base antenna. An incorrect base height can shift the vertical reference transferred to every rover working from that base. The rover may repeatedly obtain FIX and show excellent precision while all elevations carry the same inherited bias.
The rover height and base height therefore play different roles but demand the same discipline. A rover rod-height error affects the point being measured. A base antenna-height error affects the starting reference for the entire session. Both measurements should be made to the correct ARP, and APC must be accounted for consistently in the RTK workflow.
A Field Check Before Blaming the Datum
When an RTK elevation disagrees with a NAVD88 benchmark, first confirm that the comparison is truly apples to apples. Then inspect the physical setup. Verify the pole tip is on the intended mark, the rod is plumb, the true tip-to-ARP height has been measured, all adapters are included, and the APC relationship is being handled once by the receiver or software. Repeat the same review for a local base if one is being used.
These checks do not replace the geoid and datum review from the previous chapters. They complete it. A meaningful elevation comparison requires both a correct reference system and a correct physical reduction from the antenna to the point. Either side can make the final number appear wrong.
RTK measures the antenna, not the ground point. Manually verify the complete rod height from the actual tip or mark to the ARP, do not assume the rod graduations are exact, and make sure the APC relationship is accounted for exactly once. Check the base antenna height with the same care because a base error can shift every rover elevation in the session.