DirtMate RTK: large position shift from a VRS mountpoint mismatch

What this looks like

You may have a reference frame mismatch if:

• Your DirtMate RTK reports a high-accuracy RTK fix (for example, 0.05 ft horizontal, 0.04 ft vertical), but positions are several feet off from known control points
• The offset is consistent across the site — all points are shifted by roughly the same amount in the same direction
• Your site coordinate system (CRS) and DirtMate configuration appear to be set up correctly
• The shift happened suddenly, with no warning on the device
• Points laid out previously no longer align with field-verified locations

What causes it

Many Virtual Reference Station (VRS) corrections services offer multiple mountpoint options. One common option is called AUTO.

The AUTO mountpoint provides corrections in a global reference frame (such as ITRF2014). DirtMate RTK is typically configured to a regional reference frame (such as NAD83 for US sites). When these two reference frames don't match and no transformation is applied, positions can be off by several feet — even though the device shows a strong, high-accuracy RTK fix.

The size of the offset depends on your location. As a reference, the difference between ITRF2014 and NAD83 in the southeastern United States is approximately 3–4 ft horizontal and 4–5 ft vertical. Use the NGS HTDP tool to look up the expected offset for your region.

How to confirm the cause

Use these steps to confirm that your VRS corrections service is the source of the shift before making any changes.

Option 1: Test with a different corrections source

1. Check your DirtMate and site CRS settings. Confirm the DirtMate CRS matches the site CRS in Propeller. A CRS mismatch on the device would cause a different kind of error.
2. Connect to a different NTRIP corrections service (for example, RTK Data, Smartnet, or your state DOT CORS network).
3. Re-check the same control point with the alternate source.
   • If the shift disappears: the corrections service and its AUTO mountpoint are the cause. Proceed to the fix below.
   • If the shift stays: the problem is likely in the DirtMate CRS configuration itself. Contact Propeller support for further help.

Option 2: Use a second DirtMate as a base

If a second DirtMate RTK is available on site:

1. Set the second DirtMate up on the known control point as a base station.
2. Use the first DirtMate as a rover.
3. If the position shift disappears with this local base setup, the VRS corrections service is the cause.

Option 3: Use a 3rd party GNSS receiver as a base

f a third-party GNSS receiver is available (for example, a Trimble, Leica, or similar unit with its own base station capability):

1. Set up the receiver over the known control point as a base station, broadcasting its own local corrections.
2. Connect the DirtMate RTK as a rover to those local corrections instead of the VRS service.
3. If the position shift disappears, the VRS corrections service and its mountpoint configuration are the cause.

This workflow is similar to the second DirtMate option and produces equivalent results — use whichever equipment is available on site.

How to fix it

Switch the VRS mountpoint from AUTO to LOCAL (or whichever mountpoint explicitly names NAD83 or your regional reference frame) on all DirtMate RTK devices.

The LOCAL mountpoint locks corrections to NAD83 at a fixed epoch, which is consistent with the coordinate systems used on most US sites.

Steps to switch and verify

1. While still on the AUTO mountpoint, occupy your site's known control point and record the current coordinates.
2. Switch the VRS mountpoint to LOCAL.
3. Power cycle the DirtMate RTK.
4. Re-occupy the control point and compare the new reading to the expected coordinates.
   • If they match: the fix worked. All future layout with the LOCAL mountpoint will be consistent with your site CRS.
   • If they do not match: some previously captured points may have been recorded in the wrong reference frame. Contact Propeller support — a coordinate translation may be needed to bring those points into your site's coordinate system.

Things to know

• High accuracy does not mean the right reference frame. DirtMate RTK reports how precise (repeatable) the fix is — not whether the corrections are in the right coordinate system. A device can show 0.05 ft accuracy and still be several feet off if the corrections source is mismatched.
• The AUTO mountpoint can change without warning. Some VRS providers update the reference station or correction model behind the AUTO mountpoint between sessions without notifying users. This can cause unexpected shifts between sessions on the same job.
• Adjusting the pole height does not fix this. Changing the pole height adjusts a reported elevation offset but does not correct a reference frame mismatch. If the pole height was previously adjusted to compensate for a position shift, review that setting after switching to LOCAL.
• This applies to multiple corrections services. PointOneNav, RTK Data, Smartnet, and others may all offer an AUTO mountpoint option. Always confirm which reference frame a mountpoint uses before starting layout work on a new site.

I still can't do it!

We wrote these articles to equip you with everything you need to get the job done on your own, but we understand that sometimes this isn't sufficient.

If you're stuck, the Propeller hardware support team may be able to help. You can contact them by emailing hardwaresupport@propelleraero.com.au.