---
title: Surveying Without a VRS
order: 5
---

# Surveying Without a VRS

Without a VRS, a RoboDot base averages its own position. Your points are precise relative to that base, but the whole job can sit a few meters away from NAD83(2011). You can fix this afterwards: send the base's observation log to NOAA's OPUS, then shift your points by the difference.

## What you need

* A base that stays on one spot for the whole session and records observations.
* From the base: the `.rub` log and the `.sht` file it saved when it set its point.
* From each rover: its `.sht` files from the same session.

## 1. In the field

1. Set up the base and let it average its position: **Auto Survey**, or **Set Position > Average Position** on the Status page. Don't move, release or restart the base until you finish. Each new average gives the base a new position.
2. Make sure the base is recording observations: **Record On Boot** in Settings, or **Record** on the Status page.
3. Survey with your rovers as usual.
4. Keep the base logging for at least 2 hours for OPUS Static. With less data, use OPUS Rapid-Static, which takes 15 minutes to 2 hours.

## 2. Get the base's corrected position from OPUS

1. When you've finished, press **Stop Recording** on the base. The **Files** page lists files only while the base isn't recording. Download two files from this setup: the `.rub` log, and the `.sht` file the base saved when it set its point.
2. Convert the `.rub` to RINEX 2.11 at [robognss.com/tools/convert-to-rinex](https://robognss.com/tools/convert-to-rinex). If the log is over the converter's 50 MB limit, [contact us](https://www.robota.us/contact).
3. Submit the RINEX file to [OPUS](https://www.ngs.noaa.gov/OPUS/) with antenna type **NONE** and antenna height **0.000**. OPUS then reports the position of the antenna itself, the same point the base averaged and sent to your rovers.
4. OPUS emails you the result.

:::info
[Application Workflows](operating-modes) uses an OPUS height of pole height + 0.094 m. That height gives you the ground mark. For this shift, use 0.000, so that both positions describe the same point.
:::

## 3. Shift your points

1. Open [robognss.com/tools/shift-survey-points](https://robognss.com/tools/shift-survey-points) and calculate the offset from two base positions.
2. **Original position:** open the base's `.sht` file and find the row named **Base Antenna PC Position**. Enter its Latitude, Longitude and Height(M). This is the position the base sent to your rovers.
3. **Corrected position:** paste the OPUS report. The tool reads its NAD83(2011) coordinates.
4. Load the rover `.sht` files from this base setup, then download the shifted files.

:::warning
**Read position from a RINEX file** gives the right original position only with a RINEX file from RoboDot Utility, which copies the base's position from its log into the file. A RINEX file from the RoboGNSS converter or another converter usually has zeros or a rough estimate there instead. The `.sht` row is right either way.
:::

## 4. Convert to State Plane, UTM and NAVD88

Open [robognss.com/tools/datum-convert](https://robognss.com/tools/datum-convert) and set both the source and the target to **NAD83(2011)**. Load the shifted files. The tool adds UTM and Texas State Plane columns and NAVD88 elevations from GEOID18 (contiguous US only).

## Checks

* **One OPUS solution per base setup.** A new setup starts whenever the base moves, is released, or averages again, for example after a restart with Auto Survey on. Each setup needs its own OPUS result and its own shift. If one rover `.sht` covers two setups, split it by its GPS Time column first.
* **The same point on both sides.** In OPUS, use antenna type NONE and height 0.000. For the original position, use the `.sht` Latitude, Longitude and Height(M) columns, not Ground Elevation.
* **Shift once.** The shifted points are already in NAD83(2011), because the shift includes the change of frame. Don't run them through a WGS84 or ITRF to NAD83 conversion afterwards.
* **Shift the `.sht` files, not a CSV export.** When a geoid model is on, the CSV export adds an orthometric column. The shift tool refuses files with that column, because those elevations would no longer match the shifted heights. Get NAVD88 elevations from the converter after the shift.
* **Enough data.** OPUS Static takes 2 to 48 hours of data; OPUS Rapid-Static takes 15 minutes to 2 hours. Check the quality figures in the OPUS report before you use the result.
* **Already on a VRS?** If the base's point came from a VRS that works in NAD83(2011), your points are already in NAD83(2011). Skip OPUS and the shift.