Complete Guide to Using and Placing GCPs (Ground Control Points) Even Beginners Can Understand!

Table of Contents

• What is a GCP (Ground Control Point)?

• Why GCPs are necessary

• Basic procedure for surveying using GCPs

• Tips for placing GCPs

• Cautions when using GCPs (common mistakes)

• Latest technologies and the trend toward GCP-less workflows

• Simple surveying with LRTK

• FAQ

What is a GCP (Ground Control Point)?

GCP (Ground Control Point) is a reference point on the ground whose exact coordinates (horizontal position and elevation) are known. When conducting photogrammetry such as drone aerial photography, a visible mark (an aerial target) is placed at this control point so it appears in the aerial images. By using those control points as reference when creating maps or 3D models from photos, you can align the model’s position and elevation to an actual geodetic coordinate system, dramatically improving the survey results’ accuracy. Simply put, control points are the “key to alignment” in aerial photogrammetry and act as anchors that fix the model to real-world Earth coordinates.

In addition to control points, you also set up verification points (checkpoints). Like control points, verification points have known accurate coordinates, but they are used later to check accuracy (they are not used in the computation in the software; they are used only for error verification).

Why GCPs are necessary

It is possible to perform some surveying with only a drone or a camera with GPS, but without GCPs you can experience position errors on the order of several meters. For example, ordinary GPS positioning can often place objects several meters away from their true positions. This is insufficient to meet the centimeter-level accuracy often required for mapping and civil engineering surveys. That is where GCPs (control points) become necessary.

If control points are placed appropriately, the aerial data can be corrected by aligning the model to those control points during analysis. This process can reduce original errors of several meters down to a few centimeters or less. In fact, the Ministry of Land, Infrastructure, Transport and Tourism’s public surveying manual states that “to keep 3D point cloud positional errors within 5 cm, the installation of control points is necessary.” Even strict accuracy standards that cannot be met by relying only on a drone’s GPS can be achieved by using control points.

In short, control points are an indispensable foundation for high-accuracy surveying. Without control points, generated terrain models and orthophotos can float in arbitrary coordinate systems, making it impossible to guarantee accurate dimensions or positional relationships. For beginners, using GCPs is a straightforward way to reliably improve accuracy with relatively little extra effort.

Basic procedure for surveying using GCPs

Now, step-by-step, here is the basic workflow for conducting drone surveys using GCPs (control points).

• Plan where to place control points and verification points First, survey the target area and plan where to place GCPs and verification points. As a rule, it is desirable to place several points around the perimeter of the survey area and several points distributed evenly inside the area. For a small site, an example layout is points at the four corners plus one in the center. The required number of points and spacing depends on the desired accuracy and the area, but if high accuracy is the goal, try to place points as evenly as possible across the entire area. On terrain with large elevation differences, plan to place points at the highest and lowest spots as described below.

• Place aerial targets (markers) At the planned positions, set the aerial targets that mark the control and verification points on the ground. Aerial targets are markers that make the location easy to identify in aerial photos; they are typically boards or sheets with high-contrast patterns such as black-and-white crosses or checkerboards. The size of the marker should be sufficiently large depending on the drone’s flight altitude (as a guideline, a size that occupies at least 15 pixels in the photo is recommended). When installing them, be sure to secure them firmly with nails or weights so they are not blown away by wind or shifted by vehicles.

• Survey the positions of control and verification points Next, measure the exact three-dimensional coordinates of each installed control and verification point. Specifically, use surveying instruments such as GNSS surveying equipment (high-precision GPS receivers) or total stations to determine each point’s latitude, longitude, and elevation. If very high accuracy is required, use methods appropriate to the situation such as network RTK, long static surveys, or traverse surveys from known points. This coordinate measurement work can be done on a different day than the drone flight. Measuring in advance is efficient because it reduces on-site work on the flight day (and if the flight is postponed due to bad weather, having coordinates already obtained lets you respond calmly).

• Conduct aerial photography with the drone Once marker targets are placed and their coordinates are known, fly the drone to capture aerial images of the survey area. When planning the flight, adjust altitude and course so that all control points and verification points appear in multiple photos. Generally, you will use automated flight to cover the entire site, but double-check that sufficient overlapping photos are taken around control points. In this way, collect the photographic data for the entire area.

• Process the photos and align by control points Import the captured aerial photos into analysis software (SfM software or photogrammetry tools) and generate 3D models and orthophotos. At this stage, input the coordinates of the control points you previously measured and mark the corresponding control points (aerial targets) on the photos. The software uses these known coordinates to perform alignment (bundle adjustment) of the aerial photos and georeferences the entire model to the specified coordinate system. As a result, the model’s scale (dimensions), position, and elevation become faithful to the real world, producing a high-accuracy map.

• Verify accuracy using verification points After processing, check accuracy using the verification points you placed earlier. Since verification points were not used in the processing, read their coordinates from the finished model and compare them with the actual measured coordinates. If horizontal and vertical errors at multiple verification points fall within the required accuracy (for example, within 5 cm (2.0 in)), the survey result has achieved the target accuracy. If errors are large, review whether control point marking mistakes occurred, whether the placement or quantity of points was appropriate, or whether there were problems with photo capture conditions.

• Cleanup (remove markers) After photography and data processing are complete, promptly remove the aerial targets installed on site. Especially if you placed markers on public roads or someone else’s property, be sure to restore the site to its original condition. Clean and store the recovered markers for the next survey.

These are the basic steps. It may feel a bit cumbersome at first, but using control points properly will greatly improve the accuracy of drone surveys.

Tips for placing GCPs

Here are concrete tips and points to keep in mind when placing control points (GCPs) on site. Pay attention to the following to obtain high-accuracy data.

• Even distribution around the perimeter: Place control points so they surround the survey area and are also distributed evenly within it. At minimum, securing 3–4 points around the perimeter (to cover the corners) plus at least one internal point will make results more stable. For large areas, increase the number of points so they are as evenly spaced as possible and avoid large blank zones without control points.

• Ensure the necessary number and spacing: Decide the number and spacing of control points according to the required accuracy. For high accuracy (centimeter-level), adjacent control points should ideally be within about 100 m (328.1 ft), and every location should be within about 200 m (656.2 ft) of surrounding control points. Even if a slightly looser accuracy tolerance is acceptable, keep perimeter point spacing to around 100–200 m (328.1–656.2 ft) and add internal points as appropriate. If you’re unsure whether it’s enough, err on the side of installing more points.

• Place points in areas with large elevation differences: On mountainous or undulating sites, place control points at both the highest and lowest locations. If you only place points on flat areas, vertical errors may result in height distortion in the model. Providing reference points at high and low locations stabilizes overall elevation accuracy.

• Choose open locations with no obstructions: Install control points where they are clearly visible from the air. Trees or buildings can cast shadows and hide markers in drone photos. Choose open places where markers are not obscured from above or from oblique angles. Also prefer locations with an open sky for GNSS measurements (good satellite visibility).

• Make aerial targets large with high contrast: Use aerial targets that are sufficiently large and highly visible. Typical patterns include black-and-white crosses, X marks, or checker patterns. While white and black are standard, you may combine yellow or orange if they stand out better against the surroundings. Size depends on the drone altitude and camera performance; for example, if the ground sampling distance (GSD) is about 1 cm/px (0.4 in/px), a marker of 50 cm square or larger (19.7 in square or larger) is recommended. The higher the flight altitude, the larger the marker should be; conversely, at low altitude with detailed imaging, smaller markers are acceptable. The important point is that the marker occupies enough pixels in the photo to be easily identified.

• Secure the markers firmly: If a placed aerial target moves, accurate alignment becomes impossible. Secure the four corners with nails or pegs, place weights, or use tape so the marker does not move from its designated position until imaging is complete. Even slight movement can cause errors in the final product, so don’t skimp here.

• (For laser surveys) Raise the marker off the ground: Although not applicable to photogrammetry, when using drone-mounted laser scanners you may also use control points. Because laser point cloud data lacks color information, a marker placed on the ground may not be detected. Mount the marker on a small ladder or pole to raise it slightly above the ground so the laser can detect it. Photogrammetry-focused beginners need not worry about this, but keep it in mind when using laser equipment.

• Number and record points in advance: When handling multiple control points, label each with a number or name for easy identification. For example, write “GCP1,” “GCP2,” etc., on the markers or mark them on a map and link each to its measured coordinates. This prevents mixing up which point corresponds to which coordinate during photo marking or verification. Beginners are especially prone to mismatches, so keep thorough records on site.

• Measure coordinates in advance: As noted above, measuring control point coordinates before the flight day is efficient. If possible, measure them at the site by the day before so you can focus on drone capture on the flight day. If measuring in advance is difficult, do not rush to measure coordinates after capture; it’s more reliable to measure all points calmly before flying.

Keep these points in mind and your control point placement will be solid. It may feel confusing at first, but you will be able to place them efficiently with practice.

Cautions when using GCPs (common mistakes)

Here are common mistakes beginners make or points to be careful about when using control points. To prevent errors, pay attention to the following.

• Control points do not enclose the area: If there are insufficient perimeter control points or they are biased in their placement, you cannot stably fix the entire model. This can cause distortions such as edges lifting or the center sinking in the processed 3D model. Be especially careful if there are no control points at the four corners or there are blank zones along the perimeter. Always arrange points to properly enclose the area.

• Insufficient internal control points: If the area is large but there are too few internal points or the spacing between adjacent control points is extremely large, the model’s geometry will be weak. This also causes distortion and reduced accuracy. For big areas, add internal points as appropriate to ensure stable accuracy across the whole area.

• Points biased relative to terrain variation: If the terrain has large elevation differences but points are placed only in high or only in low areas, vertical accuracy will be biased. For example, if you place points only at the foot of a mountain, errors may appear near the summit. Place points at each extreme of the terrain to stabilize vertical accuracy.

• Markers are too small or hard to see: If the marker is too small or dirty and indistinct in the photos, it becomes difficult to align precisely in the software. For instance, if the marker is blurred, you may be unsure which pixel is the true coordinate, causing deviations of several centimeters to tens of centimeters. Use sufficiently large, clear markers and ensure they are identifiable even when zoomed in.

• Incorrect control point coordinates: Recording mistakes in measured coordinates or incorrect survey instrument settings (such as different coordinate systems or units) can lead to registering wrong values for control points. In such cases, even careful placement will produce grossly shifted final results. Double-check measured coordinates and confirm each entry when inputting them into the software to avoid mistakes. Also beware of differences in geodetic systems specific to Japan (world geodetic system vs. old geodetic system) and in map projections (e.g., plane rectangular coordinate systems). Using the wrong reference system can shift positions by tens of meters, so unify the coordinate system in advance.

• Control points were not captured in photos: Sometimes a placed control point does not appear in any photos due to flight plan mistakes. Points placed right at the outer perimeter, in particular, can fall outside the imaging area depending on the flight pattern. In such cases, that point cannot be used for analysis and accuracy will degrade. When making a flight plan, ensure that all control points are definitely captured in photos by adjusting overlap and coverage. Even if you miss some points, having multiple other points can mitigate the risk.

• Control points moved: If control points (markers) shift before or after imaging, the correspondence with measured coordinates breaks down. For example, if strong wind blows a marker away during flight or someone kicks it, the result can be fatal. After installation, avoid touching the markers until the flight is complete, and inspect them periodically to ensure no abnormalities. For long shoots, make it a habit to check marker status during breaks.

Paying attention to these items will help you avoid typical failures when using control points. Careful planning and verification are the shortest path to high-accuracy results.

Latest technologies and the trend toward GCP-less workflows

Control points are key to high accuracy, but their installation takes time and effort. Recently, to address this issue, surveying technologies have emerged that can reduce or eliminate the need to install many control points. Here we touch on trends in “GCP-less surveying” driven by the latest technologies.

• RTK/PPK-equipped drones: A growing trend is drones equipped with high-precision positioning technologies known as RTK and PPK. RTK (real-time kinematic) allows a GNSS receiver onboard the drone to obtain high-precision positions in real time, while PPK (post-process kinematic) improves accuracy by correcting GNSS data after flight. These technologies correct the positions of aerial photos themselves to centimeter-level errors, reducing reliance on control points for high accuracy when aligning photos. In fact, tests by the National Institute of Advanced Industrial Science and Technology have reported that an RTK-equipped drone reduced the number of required control points from five to one while still achieving accuracy within 5 cm (2.0 in). Thanks to RTK/PPK, it is becoming possible in some cases to drastically reduce the number of control points required. However, even when reducing points to almost none, it is recommended to install verification points for accuracy checking. Also, for official public surveys, strict validation is required before accepting results without control points, so even when using RTK/PPK drones, it is common to use a small number of control points or verification points as a precaution.

• GNSS-integrated targets: A novel invention to reduce the labor of placing control points is aerial targets with built-in GNSS receivers. These markers record their own position coordinates during the capture, enabling you to derive control point coordinates from that data afterward. Traditionally, you had to place a control point and then survey it separately to obtain coordinates, but this tool allows simultaneous aerial imaging and coordinate acquisition, greatly reducing work time. Some systems can automatically detect markers in photos in the cloud, cross-reference them with observation data, and automate coordinate computation and report generation. This effectively automates a series of processes previously performed by experienced surveyors, bringing an era closer where anyone can obtain high-accuracy survey results with the push of a button.

Advances in these technologies have made the previously time-consuming task of installing control points much more efficient. However, these systems and RTK-capable drones are often expensive and require advanced operational knowledge, so they are not immediately accessible to everyone. Adoption barriers remain for beginners, but as these technologies become more widespread, we will move from an era of “lots of control points and lots of effort” to an era where “few or no control points are needed.”

Simple surveying with LRTK

Among the latest technologies, LRTK stands out as a survey solution that is especially easy for beginners to use. LRTK is an all-in-one RTK surveying system that combines a small, high-performance GNSS receiver attachable to a smartphone or tablet with a dedicated app and cloud service. The palm-sized receiver weighing only a few hundred grams contains an antenna, a high-precision GNSS chip, a battery, and a communications module; attach it to your smartphone, launch the app, and you can quickly start centimeter-level positioning (centimeter-level positioning (cm level accuracy (half-inch accuracy))). It requires no complicated initial setup or cable connections and is designed to be intuitive even for those without surveying expertise.

With LRTK, anyone can perform high-precision positioning without difficult settings or special equipment. For example, attach the LRTK receiver to a smartphone and walk the site, pressing a record button at each point to automatically capture high-precision coordinates and instantly save and share them in the cloud. Tasks that used to require experienced surveyors—such as staking out positions or checking as-built measurements—can now be performed by inexperienced personnel using LRTK’s AR navigation feature. By following on-screen guidance, the user is directed in real time (e.g., “move ○ cm more to reach the target”), allowing them to reach the target point with almost no error.

LRTK is a revolutionary system that simplifies RTK-GNSS’s high accuracy and maximizes usability. Even small construction companies and local governments without specialized survey teams can perform surveying easily by one person with LRTK. Tasks that depended on veteran craftsmen are being transformed into digital operations anyone can do, and LRTK-led “simple surveying” has the potential to greatly change on-site practices. In fact, LRTK is already being used on many sites, contributing to improved productivity and labor savings. If you’re a surveying beginner thinking “isn’t there an easier way to do high-precision positioning?”, it’s worth considering modern tools like LRTK.

FAQ

Q: How many control points should I prepare? A: The appropriate number depends on the site size and required accuracy, but as a basic rule, 3–4 points around the perimeter and at least 1 point inside is a good guideline. For small sites, about 5 points (four corners plus one center) is typical; for large sites, you may place 10 or more points evenly. Increase the number of points to improve accuracy. If your goal is 5 cm (2.0 in) accuracy, place points with spacing within about 100 m (328.1 ft) where possible. When in doubt, install a few extra points as a safety margin.

Q: Can I do drone surveying without control points? A: Recently, with the spread of RTK-equipped drones and PPK processing, there are cases where high accuracy is achieved with zero control points. However, without such high-performance equipment, surveying without control points can produce errors of several meters, making high-accuracy surveying difficult. For tasks demanding strict accuracy, it is still safer to use control points (or at least verification points). Using RTK drones without any control points requires prior validation and high-quality satellite correction data, and is not yet common practice. Therefore, at present, the most reliable method is to perform drone surveys with appropriately placed control points.

Q: How do you obtain the coordinates for control points? A: You obtain control point coordinates by conducting high-precision GNSS surveying or total station surveying. Specifically, use a network RTK GNSS receiver (rover) to receive correction data from reference stations and measure, or use a total station from a known reference point. If you don’t have professional surveying equipment, consider renting or hiring a specialist. Recently, easy surveying devices combining a smartphone and a small GNSS receiver (such as LRTK) have appeared, enabling beginners to measure centimeter-level coordinates easily. The important thing is to obtain coordinates with errors on the order of a few centimeters. Ordinary smartphone GPS or simple handheld GNSS receivers do not provide sufficient accuracy, so be cautious.

Q: What is the difference between control points and verification points? A: As mentioned, control points are reference points used to align (correct) the model during photo analysis. Verification points (checkpoints) are not used in processing and are set aside to confirm and evaluate accuracy. In short, control points make the model “correct,” while verification points measure “how correct” it is. Verification points are treated as unknowns in the software; compare the processing result’s coordinates for those points with pre-measured true coordinates to compute errors. If these errors fall within the required tolerance, the result is acceptable. For accuracy management, prepare not only control points but also appropriate verification points; a recommended practice is to allocate at least half the number of control points as verification points (for example, 4 control points and 2 verification points).

Q: How should I prepare aerial targets for control points? A: Aerial targets are commercially available, but you can also make them yourself. A sturdy vinyl sheet or plywood with a conspicuous pattern will serve as an improvised marker. Typical designs are black-and-white checker patterns or cross marks. The key is that the marker has clear contrast with the surroundings and is sufficiently large for the flight altitude. For example, a white-black marker on grass is easily visible due to contrast with green; on sand, use black and yellow combinations tailored to the environment. As a guideline, for shooting from about 100 m (328.1 ft) altitude, a size of about 50 cm–1 m square (19.7 in–3.3 ft square) is a common standard (for higher altitude use larger markers). If using fabric sheets, secure them with stakes or tape so they don’t flap in the wind. Choose waterproof vinyl material as a backup for rainy conditions, and prepare several well-made markers that can be reused for future surveys.