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How to prepare GCPs for UAV mapping without RTK corrections
Learn how to establish accurate ground control points (GCPs) for UAV mapping using GNSS postprocessing without relying on RTK corrections or a local base station.
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Products used in this tutorial
Want to create accurate maps with your drone but don’t have access to RTK corrections? You still have an option.
You can use a GNSS receiver to measure your Ground Control Points (GCPs) and post-process the data later, without needing an internet connection or real-time correction service in the field.
This method takes more time than using RTK, since you need to record static GNSS observations and process them afterwards. However, it can be a practical and free alternative when RTK corrections are not available.
In this tutorial, we’ll guide you through the process, from collecting static measurements at each GCP to post-processing them in the office, so you can use accurately positioned control points in your UAV mapping project.
Required hardware
- RTK Portable 2
- Budget Survey Multiband GNSS Antenna
- Magnetic Stand for Survey GNSS Antenna
- Ground Control Points (GCP) for drones
- PC or laptop with Microsoft Windows (for configuration)
- Android or iOS smartphone/tablet (for GNSS logging in the field)
Required software
How to Survey GCPs for UAV Mapping Without Real-Time Corrections
Configure your receiver for postprocessing
Before recording the log files the u-blox receiver needs to be configured using the u-center software. The configuration should produce raw data for post-processing, and at a rate of one GNSS calculation every 30-seconds. This is done by loading a specific configuration file, and setting up a custom update rate.
- Connect the provided USB-C to USB-A cable from the RTK Portable 2 receiver to the computer.
- Open u-center and connect using the appropiate COM port. Before configuring the receiver, we have to check the firmware version of the receiver. Go to View > Message View > UBX > MON > VER, and if the firmware version is below 1.51, please update the firmware.
- Download the Post-processing RAW data (PPK) over UART1 & USB at 1Hz configuration file. To load the configuration file go to Tools > Receiver Configuration and select Generation u-blox Generation 9. Select the configuration file from your hard disk and click Transfer file > GNSS.
- Open the main menu View > Messages View. To set the receiver to update every 30 seconds, go to UBX > CFG > RATE and set the Measurement Period to 30000 ms, and press Send.
- To permanently store the new configuration in the boot memory, in the menu bar, go to Receiver > Action > Save Config to save your configuration.
- Now your receiver is perfectly configured for GNSS postprocessing.
Log GNSS position of GCPs
Now it is time to go to the survey site.
- Place ground control points (GCPs) across your mapping area. To ensure your final 3D model will be accurate accurate, follow these rules:
- Place GCPs near each corner of the surveying and at least one in the center.
- Keep GCPs spaced 50–100 meters apart; add more along long edges if needed. The more GCPs you use, the better accuracy of the 3D map you will have.
- Ensure GCPs are clearly visible from the air and not obstructed by trees or other objects.
- If the terrain is uneven, place GCPs at both high and low elevation points.
- Since static measurements require leaving the antenna in a fixed position for at least one hour, make sure to place your GCPs on flat, level surfaces.
- Screw the GNSS antenna to the magnetic stand and place it in the center of GCP.
- Screw the SMA connector of the antenna cable to the RTK Portable 2 receiver connector.
- To power the RTK Portable 2 receiver, connect the provided USB-C to USB-C cable to both the receiver and the mobile device. Alternatively, you could power the receiver using a power bank.
- Open SW Maps and create a new project, if needed. Go to Main menu > External GNSS Connection. Set Connection Mode to Bluetooth LE (Low Energy) and press the refresh button.
- Select the BT+BLE_Bridge_XXXX device, check the Log to File option, and press Connect.
- After a successful connection, SW Maps will start recording a GNSS raw data log file to the mobile device’s memory. Make sure that the mobile device has enough battery life to record the amount of time you want to be logging this specific fixed position, and if needed add an additional power source for it.
Be aware that the data recorded is in raw format, not NMEA, and therefore SW Maps might not be able to translate it into valid coordinates in its interface. Do not worry if you do not have a valid position in SW Maps while recording; we will obtain it afterwards in post-processing.
- Record GNSS data for at least 1 hour. A minimum 1-hour log file is required for reliable results, but for best results, we recommend recording data for 24 hours.
- To stop recording, simply Disconnect the device in SW Maps.
- Repeat the steps 11-15 in case you need to measure several points.
- Open SW Maps and create a new project, if needed. Go to Main menu > External GNSS Connection. Set Connection Mode to USB Serial and press the refresh button.
- Select the u-blox GNSS receiver device, check the Log To File option, and press Connect.
Postprocess your GNSS measurements to get centimeter-level accurate position
- Find the log files on your mobile device:
- On Android, the SW Maps log files are saved under Android/media/np.com.softwel.swmaps/RawData.
- On iOS, the SW Maps log files are saved under SW Maps/rawfiles.
- If you want to send the log files to a post-processing service from your PC, first transfer them from your mobile device to your computer. Select **Share** from the file’s context menu and choose your preferred sharing platform (e.g. email, Google Drive, etc.). Alternatively, you can use your phone to access an online post-processing service directly from your mobile device.
- Open the PostProcessing Master online service https://postprocessingmaster.com/ and press START.
- Login to your account and upload the SW Maps log file to start postprocessing. Drag and drop the file into the web browser or browse and select it. Note that the PostProcessing Master online PPP service does not require you to convert the file to any other format before upload. It can directly handle data collected from the most common GNSS receivers in their native format, such as u-blox UBX, Septentrio SBF and Unicore OBSVM.
- The log file processing will start.
- Once the processing is finished, you will receive an email with the resulting coordinates. Click the Copy result button to keep the resulting coordinates in a document on your computer, or access them later by logging back into your account. The results are kept on the server for 7 days.
- Under Processing details, you can find the formal internal precision of the resulting coordinates.
- That’s it! With your files processed, you now have your precise coordinates ready to use.
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