c/techniques•Posted byu/AnnaSurveys•2 months ago

Troubleshooting RTK accuracy - common causes of 10-20cm offsets

Had a call from a newer surveyor this week who was getting consistent 10-20cm offsets between his RTK positions and known control points. Helped him troubleshoot it and figured the checklist we went through might help others.

If your RTK coordinates are consistently off by 10-20cm (not random scatter, but a systematic shift), here are the most common culprits in order of likelihood:

1. Antenna height error. This is the number one cause. Make sure you're measuring to the correct reference point on your antenna (ARP vs L1 phase center) and that your software is applying the right offset. Different receivers measure from different points.

2. Datum/realization mismatch. Your base and rover need to be on the same datum realization. NAD83(2011) and NAD83(CORS96) can differ by several centimeters. Check your NTRIP mount point metadata.

3. Geoid model. If your horizontal is fine but vertical is off, you probably have the wrong geoid model selected or none at all. GEOID18 is current for CONUS. Make sure its actually applied, not just selected in the menu.

4. Base coordinate error. If you set up your own base, did you use a long enough observation? A 15-minute OPUS solution can easily be off by 5-10cm. Do at least 2 hours for reliable results, 4+ hours is better.

5. Stale corrections. Check the age of your corrections. If its consistently above 5 seconds, your corrections are lagging and accuracy degrades. This usually means your internet connection is too slow or intermittent.

Most of the time its #1 or #2. Always check the simple stuff first.

Comments

T. Weston PLS•2 months ago

Saving this post. Number 2 bites people constantly. I'd add - if you're mixing equipment brands (like an Emlid base with a Trimble rover), double check that both are outputting the same message format. RTCM3 is standard now but older setups might have quirks.

Also epoch matters. NAD83(2011) epoch 2010.0 coordinates can differ from epoch-corrected coordinates by a centimeter or more depending on your location.

Gary Bowman•2 months ago

I'd add one more - pole not plumb. A 2-degree tilt on a 2m pole is about 7cm of horizontal error. Combine that with one of the other issues and you're easily at 15-20cm. Use a bipod or get a receiver with tilt compensation.

Good list though. This should be required reading for anyone starting out with RTK.

nate_107•2 months ago

this explains so much. i've been getting exactly this kind of offset and just assumed my equipment was bad. gonna check the antenna height setting first thing tomorrow