Improve Photogrammetry Accuracy! Correct Use of Ground Control Points and Practical Workflow

Table of Contents

• What is a Ground Control Point (GCP)?

• Why GCPs are necessary in photogrammetry

• Correct usage and installation procedures for GCPs

• Key points for GCP installation (tips for improving accuracy)

• Challenges of GCP installation

• Latest technologies that solve GCP challenges

• Simple surveying with LRTK

• FAQ

What is a Ground Control Point (GCP)?

To obtain high-accuracy results in photogrammetry using drones, the use of ground control points is indispensable. In this article we explain in detail what GCPs are, their role, how to use them correctly, and the procedures for installing them. We also introduce practical workflows and the latest technologies to help you improve photogrammetry accuracy.

In photogrammetry, a ground control point (GCP) is, simply put, a “ground point whose exact coordinates (horizontal position and elevation) are known in advance.” These GCPs are included in aerial photos taken by drones and used as reference points when creating 3D models or survey data from the images. In English they are called Ground Control Point (GCP), and in Japanese they are also abbreviated as “GCP.”

Photogrammetry reconstructs terrain and structures in 3D by stitching many photos together, but without references the position and scale of the resulting data are uncertain. GCPs act as a real-world “measuring stick,” providing accurate position and scale to the reconstructed model and are therefore essential.

Why GCPs are necessary in photogrammetry

In drone-based photogrammetry, the position information obtained during capture (such as GPS) can have positional errors on the order of several meters. In particular, the standalone GPS in commercial drones can have meter-level offsets due to satellite orbit errors and atmospheric effects, which cannot meet the accuracy required for public surveys (error within 5 cm (2.0 in)). To correct meter-level errors down to centimeter-level, it is necessary to install GCPs—points on the ground with known coordinates—and use them for surveying.

GCP coordinates are obtained using high-precision surveying instruments (e.g., RTK-GNSS receivers or total stations), as described later. By using precisely measured GCPs as a reference against error-prone GPS data, photogrammetry results can be improved to centimeter-level accuracy.

In addition, points called checkpoints (verification points), which serve a role similar to GCPs, are also installed. Checkpoints are used to independently verify the coordinate accuracy assigned to the modeled 3D data; they are not used in the processing and are only used to check errors. By comparing coordinates of checkpoints measured beforehand with those extracted from the model, if the discrepancy is within 5 cm (2.0 in), it proves that the photogrammetry data has ensured sufficient accuracy.

Correct usage and installation procedures for GCPs

Let’s go over the basic workflow for using GCPs in photogrammetry. The general steps from planning and installing GCPs to aerial capture and data processing are roughly as follows.

• GCP and checkpoint placement planning: Consider the shape and terrain of the survey area and plan where to place GCPs and checkpoints. Choose an appropriate number of points inside and around the area so the entire survey range is covered (placement tips are described later).

• Install aerial markers: Place aerial markers (ground-mounted markers) at the planned GCP and checkpoint locations as visible markers. Ensure the markers are large and high-contrast enough to appear in aerial photos, and secure them with nails or weights so they do not move in the wind.

• Measure GCP coordinates: Use surveying instruments to measure the precise coordinates of each installed GCP and checkpoint. Typically, a GNSS surveying instrument (RTK-capable GPS receiver) that can connect to a reference station, or a total station, is used to obtain plane coordinates and elevations with millimeter accuracy. Confirm measurements on site as needed and record the data.

• Conduct aerial capture with a drone: Fly the drone and take aerial photos covering the survey area. It is important that the markers placed at the GCPs appear in multiple photos, so capture images with high overlap. Confirm that all GCPs and checkpoints are sufficiently visible in the photos.

• Process with photogrammetry software: Import the captured image data into specialized software and perform photogrammetry (SfM/MVS processing). During processing, enter the coordinates of the GCPs measured on site into the software and mark the corresponding points on the photos for each GCP. The software will georeference the overall model to those reference points and generate point clouds and orthophotos.

• Accuracy verification: On the completed 3D model, compare the coordinates of the checkpoints measured beforehand with the coordinates of the same locations derived from the model to check errors. If the error falls within the target accuracy (e.g., within 5 cm (2.0 in)), the accuracy is considered sufficient. If a large error appears, consider measures such as adding additional GCPs or reprocessing the photos.

The above is the basic flow when using GCPs. Particularly important are the GCP placement plan and accurate coordinate measurement. The quality of these can be said to determine the accuracy of photogrammetry results. The next section explains specific tips for GCP installation.

Key points for GCP installation (tips for improving accuracy)

Here are placement tips to effectively utilize GCPs. By keeping the following tips in mind, you can further improve photogrammetry data accuracy.

• Distribute them evenly across the survey area: Place GCPs so they are well balanced throughout the survey area. Avoid concentrating them only on the outer perimeter; distribute them evenly inside as well. The spacing between GCPs is generally desirable to be within 100 m (328.1 ft), and if possible, install them at equal intervals of 30–80 m (98.4–262.5 ft) for stable accuracy.

• Consider elevation when there is significant relief: For terrain with large relief, install GCPs in both high and low locations. Including points with variation in elevation improves vertical accuracy in the generated model. In sites with extreme elevation differences, it is important to place points at multiple elevations.

• Place them in open locations clearly visible from the air: Install GCPs where they can be seen by the drone camera without obstruction. Trees or buildings nearby can block markers from the photos. Choose positions that are not hidden by obstacles from oblique as well as nadir views.

• Raise markers for LiDAR surveying: GCPs are also necessary when surveying with drone-mounted laser scanners (LiDAR), but point cloud data lacks color information, making marker identification more difficult. As a countermeasure, fix the marker on a stand or stake so it is elevated off the ground, making it easier to distinguish in the laser data.

• Measure GCP coordinates in advance: It is not necessary to perform the drone flight and GCP surveying on the same day. If time allows, measure GCP coordinates before the flight day, which makes the day of flight smoother. On the flight day you can focus on marker placement and aerial capture, shortening the total field time.

Challenges of GCP installation

While GCPs are effective for improving photogrammetry accuracy, their operation has several challenges.

• Time and labor intensive: Installing, surveying, and collecting GCPs require considerable effort and time. One survey found that GCP-related tasks (coordinate measurement and marker installation/removal) account for 30–40% of total fieldwork time. Standard guidelines recommend placing adjacent GCPs within 100 m (328.1 ft), so even a few hectares may require a dozen or so points, and tens of hectares may require over a hundred GCPs. Markers must be removed after flight, so the larger the site, the greater the workload.

• Cost per project: For each site surveyed, you must plan, measure coordinates, install, and remove GCPs, incurring personnel and equipment costs each time. Unless you continuously survey the same location, GCP work becomes a repeated labor for each project.

• Difficulty installing in some locations: In places with poor footing, hazardous access, or legally restricted areas, installing GCPs can be difficult. Forcing installation can pose safety risks. Depending on terrain and environment, it may be impossible to place GCPs as desired.

Latest technologies that solve GCP challenges

Against these challenges, technologies have emerged in recent years to reduce or omit GCP-related work. Representative examples are RTK/PPK technologies for drones and GNSS-equipped aerial markers. Let’s look at each.

Reducing GCPs with RTK/PPK

An approach to reduce GCPs is improving drone-side positioning accuracy using RTK (Real-Time Kinematic) and PPK (Post-Processed Kinematic). An RTK-equipped drone has a high-precision GNSS on board and receives correction information in real time during flight, improving per-photo position data to centimeter level. PPK corrects satellite positioning data from the drone after flight to achieve similar high accuracy.

Using these technologies can drastically reduce the number of GCPs required in photogrammetry. For example, RTK-enabled drone trials have reported cases where conventional setups using about five GCPs met accuracy requirements with only one GCP. The PPK method has also been recognized in the Ministry of Land, Infrastructure, Transport and Tourism standards as a “method that can directly position camera locations,” and cases have emerged where zero GCPs meet public survey accuracy requirements. However, even when omitting GCPs entirely, accuracy confirmation using checkpoints is still necessary. Also, implementing RTK/PPK systems requires compatible drones and dedicated software, so consider the balance with initial investment costs when adopting them.

Efficiency with GNSS-integrated aerial markers

Another approach is to automate or simplify the surveying work of GCPs using tools such as aerial markers with built-in GNSS receivers. Traditionally, the position of markers placed at GCPs was measured separately with a GNSS device or total station, but with GNSS-module embedded markers they record their coordinates simply by being installed. Multiple markers can be observed simultaneously while the drone is conducting aerial capture, and by matching and processing the observation logs with photo data in cloud software later, reference point surveying and positional correction of images can be completed automatically.

Using such smart markers eliminates the need to set up surveying instruments at each GCP. In addition, software automates cumbersome post-processing tasks like marker recognition on images and point cloud integration, so both fieldwork time and data processing time can be greatly reduced. There are case studies reporting that workload was reduced to less than half compared to traditional methods, making this technology a notable contributor to improving the efficiency of GCP operations.

Simple surveying with LRTK

In addition to the above technologies, new solutions have recently appeared to make on-site surveying itself easier. One is the pocket-sized positioning device “LRTK” that uses a smartphone. According to the [LRTK official site](https://www.lrtk.lefixea.com/), LRTK is a pocket-size RTK-GNSS receiver that can be attached to a smartphone or tablet, allowing anyone to easily perform centimeter-level positioning as an all-purpose surveying tool.

Attach LRTK to a smartphone, press a button at the point you want to measure, and high-precision position information such as latitude, longitude, and elevation is obtained and recorded in the cloud. It weighs only about 125g and is lightweight, and with a dedicated pole (monopod) a single person can efficiently measure ground points. Traditionally, specialized equipment was required for measuring GCP coordinates, but with LRTK this can be easily done with just a smartphone.

For example, if you measure GCP coordinates with LRTK before drone photogrammetry, you can apply those values to the aerial images and obtain high-accuracy results. Even with a drone that does not have RTK, pre-measuring reference points with LRTK enables creation of highly accurate 3D models. LRTK also offers functions beyond point measurement, such as distance and area calculations based on measured points and AR-based staking out, making it an innovative tool aimed at enabling “every person on site to be a surveyor” in civil engineering and construction.

Thus, GCPs remain the key to improving photogrammetry accuracy, but the installation and surveying work itself is being simplified through technological innovation. By skillfully combining GCPs and the latest tools, it is becoming possible for small teams to efficiently perform high-accuracy surveying. Drone photogrammetry technology will continue to evolve; actively utilizing the latest solutions can help you balance accuracy improvement and efficiency.

FAQ

Q: What is the difference between GCPs and checkpoints? A: GCPs are reference points with known accurate coordinates used to align (correct) the model in photogrammetry. Checkpoints (verification points) are placed to evaluate model accuracy; they are not used in processing and are installed to assess error. Simply put, GCPs are for “correction,” and checkpoints are for “accuracy verification.”

Q: How many GCPs should be placed for drone surveys? A: The required number varies with the size and shape of the survey area, but commonly at least about five points (such as the corners and the center) are used. For public-survey-level accuracy, placing points within 100 m (328.1 ft) is recommended. Larger areas require more points; for a few hectares plan 10–20 points, and for very large sites plan several dozen or more GCPs.

Q: Where should GCPs be placed for maximum effectiveness? A: Basically place them to surround the survey area and also spread evenly internally. Start by placing near the four corners of the area, add points in the center and along long sides to cover the whole site. For uneven terrain, place points in both high and low locations. It is also important to choose positions with good visibility so a single GCP appears in as many photos as possible.

Q: If I use an RTK-equipped drone, are GCPs unnecessary? A: RTK-enabled drones can greatly reduce the number of GCPs needed. In some cases zero GCPs suffice, but generally it is recommended to place at least one GCP as a reference or to set up a separate checkpoint to check the result. RTK positioning can still have errors or communication interruptions, so having at least one ground reference provides reassurance.

Q: How are GCP coordinates measured? A: GCP coordinates are measured using surveying instruments such as GNSS satellite positioning (RTK) or total stations. These yield accurate position coordinates to centimeter or millimeter levels. Recently, small RTK devices that attach to smartphones (e.g., products like LRTK) have appeared, allowing high-precision coordinates to be obtained easily without dedicated equipment.

Q: What kind of aerial markers are suitable for GCPs? A: High-contrast markers such as black-and-white crosses or checkerboard patterns are commonly used. Standards for public surveys require markers in aerial photos to be at least 15 pixels in size, and in practice panels of about 50 cm (19.7 in) to 1 m (3.3 ft) are often used. Commercial aerial markers are available, but you can also make them by painting boards. The important thing is to prepare markers with sufficiently large size and color patterns that are easily distinguishable in images (yellow may be used in some situations). Plastic specialized markers are now sold commercially and can be used as needed.