How to Collect and Correct GCP Data Without NTRIP or a GNSS Rover

Establishing accurate Ground Control Points (GCPs) is essential for producing geospatially correct maps, 3D models, and volumetric data. While many drone operators rely on real-time corrections via NTRIP or GNSS rover kits like the Emlid RS2 and Reach RX, I use a simple, cost-effective workflow that achieves centimeter-level accuracy using only the DJI D-RTK 2 base station.

This approach works entirely offline—no cell service, no rover required—and is ideal for operators working in remote areas or looking to avoid expensive GNSS upgrades.

✅ GCP Data Collection and Correction Workflow

1. Mark and Prepare Ground Control Points

I place aerial GCP targets at key, visible locations across the mapping area. These are durable, high-contrast markers placed evenly across the site and labeled with unique identifiers.

2. Log Static GNSS Data Using DJI D-RTK 2

The DJI D-RTK 2 base station can operate in static mode to record precise GNSS data. I treat each GCP as an individual observation point by doing the following:

- I place the base station directly over the center of the GCP target, using a plumb bob or tripod center pole for vertical alignment.

- The tripod is leveled both at the legs and by using the bubble on the base mount to avoid tilting the antenna.

- I power on the base station and allow it to run in **static GNSS logging mode** for **a minimum of 15 minutes**—though I often extend that to 20–30 minutes if I’m in a location with poor satellite visibility or partial tree cover.

- During this time, the base is collecting raw satellite observations, which are stored in `.dat` format inside the unit.

- Once logging is complete, I power off the base station, relocate it to the next GCP, and repeat the process.

This approach closely mirrors professional survey workflows, and although it’s more time-consuming than using a rover, it produces highly accurate results without needing extra gear.

3. Convert RTCM3 to RINEX Using RTKLIB

After data collection, I transfer the `.dat` log files from the base station to my computer. These logs are recorded in RTCM3 format and must be converted to RINEX for processing.

I use RTKLIB (specifically RTKCONV) to:

- Convert each log file into standard RINEX format

- Retain all satellite observation and navigation data

- Label each output file based on the GCP ID for easy reference

4. Submit to CSRS-PPP for Post-Processing

I upload each RINEX file to Natural Resources Canada’s free CSRS-PPP (Precise Point Positioning) service. The system analyzes the satellite data using precise orbit and clock corrections to calculate corrected coordinates.

- Results are typically accurate to **within 2–5 cm horizontally and vertically**

- I can export the results in various coordinate systems (e.g., WGS84, UTM)

- Final coordinates are imported into my photogrammetry or GIS software to anchor the data

💡 Why I Don’t Use a Rover (Yet)

Many drone mapping professionals use rover kits like the Emlid RS2 and RX to speed up GCP collection—but I’ve opted for a leaner, more cost-effective setup. Here’s why:

- My DJI D-RTK 2 already provides accurate static logging

- I avoid spending thousands on extra gear I don’t currently need

- The workflow is completely offline-capable—no NTRIP, no cellular service required

- It’s slower, but perfect for small teams, solo operators, or early-stage drone businesses

⚙️ Why This Workflow Works

- Static GNSS logging provides high-quality satellite observation data

- RTKLIB conversion ensures compatibility with precision correction systems

- CSRS-PPP delivers verified position data with survey-grade accuracy

- Entirely offline and ideal for rural, remote, or budget-conscious operations

Author:

Robert Lawrence, Advanced RPAS Pilot and Founder of High Attitude Drone Solutions, Inc.

Precision Mapping & Aerial Imaging for Construction, Land Development, and Industry.