Is RTK GNSS vertical accuracy (±2-5 cm) sufficient for construction work, such as checking road gradients or foundations?

Yes, quite sufficient. Modern RTK systems deliver 1.5-3 cm vertical accuracy under working conditions, whilst construction standards ACI 117 require ±19 mm for foundation elevation marks and ±12 mm for road bases over 3 metres. RTK fits comfortably within these limits. Even accounting for multipath and vibrations from machinery, if everything's set up properly, errors rarely exceed 2-3 cm.

What RTK actually delivers on site

The technology works simply: a base station receives satellite signals, calculates errors and transmits corrections to the rover via radio or internet. Phase measurements of the carrier wave provide centimetre-level accuracy instead of metres, as with standard GPS.

In practice, at distances up to 15-20 km from the base station, we get 1-2 cm horizontally and 2-3 cm vertically. Testing of Oscar equipment showed a median vertical error of 1.44 cm in open areas and 3.08 cm near tower blocks. These are real figures from construction sites, not laboratory conditions.

How RTK compares with construction tolerances

Type of work	Permitted tolerances	RTK delivers	Suitable?
Foundations horizontally	±6 mm	8-16 mm	Borderline
Foundations vertically	±19 mm	15-24 mm	Fine
Road surfacing	±6 mm over 3 m	15-20 mm	Needs checking
Floor slabs	±19 mm	15-24 mm	Suitable
Site levelling	±50 mm	20-30 mm	Plenty of margin

What about multipath and vibration

Multipath occurs when signals from satellites bounce off buildings, metal, machinery and reach the antenna twice. The receiver gets confused and produces inaccurate coordinates. On construction sites this is the main problem, as there are loads of reflecting surfaces everywhere.

Manufacturers tackle this in different ways. Surveying antennae have a special shield underneath (ground plane) which dramatically reduces reception of reflected signals. Multi-frequency receivers (L1, L2, L5, E5) detect anomalies in the data and filter out rubbish. Processing algorithms also clean distorted measurements.

Vibrations from bulldozers and rollers genuinely interfere, but not critically. Research by Wisconsin Department of Transportation showed that if you mount equipment on a vibration-isolating tripod, the problem nearly disappears. Plus averaging measurements over 5-10 seconds removes short-term jumps.

How it works for different jobs

Roads and gradients

Roads require 0.5-1% gradient accuracy, that's 5-10 mm per metre of length. RTK can manage this, but only during base preparation and earthworks. When it comes to laying asphalt with a tolerance of ±6 mm over three metres, surveyors switch to total stations or laser levels with 1-3 mm accuracy.

On large road projects, RTK is fitted to graders and bulldozers - the machine controls its blade automatically using satellite data. It's quick and convenient for rough work, then control measurements follow with other instruments.

Foundations and concrete

Standard ACI 117 states: the top of foundations may deviate from design by a maximum of ±19 mm in height. RTK with 15-24 mm vertical error sits right in this range. If using Network RTK (several base stations working together), accuracy improves to 15 mm + 0.5 ppm, which provides a good safety margin.

Builders often work like this: RTK for setting out axes and quick checks, then for final sign-off they bring in geometric levelling with 0.5-1 mm accuracy per kilometre. The combined approach provides both speed and quality assurance.

Making it work reliably

Choose the right time and place. Measurements are best taken when there are 8+ satellites visible and PDOP is below 2 (lower is better). Keep away from tall buildings and metal structures. If machinery is working nearby, fit vibration isolation under the tripod.

Network RTK beats a single base. Services like System.NET or RTKHUB provide more stable accuracy, especially when your own base is far away. Statistics show network solutions deliver 8 mm + 0.5 ppm horizontally and 15 mm + 0.5 ppm vertically even over large distances.

Always check the solution status on the rover screen. It should show "Fixed", not "Float". In Float mode, errors can reach 50 cm - that's rubbish. Measure critical points twice with a 10-20 minute gap. Every 2-3 hours, check against benchmarks with known heights - that way you'll spot if something's gone wrong.

The bottom line

RTK with 1.5-3 cm vertical accuracy covers about 80-85% of construction tasks. That's earthworks, setting out, foundation checks, preliminary site levelling. For particularly precise work - final levelling, installing equipment with tight tolerances - RTK is worth using as a backup tool alongside a total station or level.

Multipath and vibrations genuinely affect results, but it's not fatal. Modern receivers with decent antennae and a sensible approach to measurements keep errors within 2-3 cm even in difficult conditions. The key is understanding where RTK is absolutely spot-on, and where you'd better double-check with another method.