No GCPs Required? What Is the New Surveying Method That Smartly Evolves the Use of Ground Control Points with Smartphones

First, do you know how "ground control points (GCPs)"—commonly used to improve accuracy in drone and photogrammetry surveys—are used and why they are important? GCPs are reference points used to align survey data to accurate coordinates, but their placement and measurement are laborious. In recent years, technological advances have brought about new surveying methods to the point that people wonder, "Will GCPs become unnecessary?" This article explains how GCPs are used and what roles they play, and explores the possibility that new smartphone-based surveying methods could eliminate the need to set up GCPs.

Table of contents

• What is a ground control point (GCP)?

• How GCPs are used and their roles

• Challenges of setting GCPs

• Emergence of new surveying methods

• High-precision positioning advancing with smartphones

• Will GCPs become unnecessary?

• Smart surveying realized with LRTK

• FAQ

What is a ground control point (GCP)?

A ground control point is a reference point placed on the ground to align terrain data obtained by surveying or photogrammetry to the correct position and elevation. In aerial photogrammetry in particular, the 3D model created from captured images can drift to an arbitrary location, so multiple points with known coordinates are placed as GCPs and used to fix the entire model to a real-world coordinate system. In other words, GCPs act like anchors that tie survey data to the actual geodetic system (coordinate system and elevation).

Typically, GCPs are fitted with markers that are easy to identify from the air (called aerial markers, often cross or X marks). The GCP itself is a point on the ground, but the aerial marker makes its position easier to identify in aerial photos. Each GCP is measured in advance with high precision to obtain coordinate values (often latitude/longitude or planar rectangular coordinates, and elevation), and these are used as reference data. By using multiple ground reference points like these, camera position and attitude estimation during photogrammetry becomes more stable, lens distortion correction and model scale (dimensions) become more accurate.

How GCPs are used and their roles

So how are GCPs specifically set up and used? Here is the basic workflow.

• Selecting and installing GCPs: Choose suitable locations inside and around the survey area and place aerial markers that are easy to distinguish from above (for example, black-and-white X-mark boards). Position them as stably as possible around the corners and near the center to stabilize the model.

• Measuring coordinates: Use GNSS survey receivers (GPS receivers) or total stations to measure the precise coordinate values of each installed GCP. Obtain latitude, longitude, and elevation in a public coordinate system (such as the World Geodetic System or a planar rectangular coordinate system), and convert to local coordinates from known points if necessary.

• Input into photogrammetry: In photogrammetry software that processes photos taken by drone, mark the corresponding points on images for each GCP and enter the measured coordinate values. This allows the software to associate points on photos with real geographic coordinates.

• Model adjustment: When the photogrammetry software runs aerial triangulation (the photogrammetric computation), it performs bundle adjustment using the input GCPs as constraints. The position and scale of the entire model are adjusted to fit the GCPs, resulting in 3D point cloud data and orthophotos that accurately match the site’s geodetic coordinate system.

• Accuracy verification: If necessary, set several separate checkpoints. Checkpoints are not used in the software’s computation; instead, they are used to independently verify error by comparing coordinates of corresponding locations on the finished model. Proper use of GCPs can reduce horizontal and vertical errors to on the order of a few cm (a few in).

That is the basic use and role of GCPs. In summary, GCPs have been indispensable for "accurate positioning of survey results" and "overall accuracy improvement."

Challenges of setting GCPs

While GCPs are important for ensuring survey accuracy, their installation and measurement come with various challenges. If you conduct surveys in the field regularly, you may be familiar with issues such as the following.

• ‎Time and effort: Installing GCPs requires walking the site before surveying, placing markers at multiple locations, and measuring each with high-precision equipment. On large sites or in areas with rough terrain, this process alone can take longer than flying or scanning.

• Personnel and cost: Measuring GCPs requires expertise and equipment, so surveyors or experienced staff often handle it, increasing labor costs. Purchasing and maintaining high-precision GNSS equipment or total stations also involves expense.

• Safety issues: Installing and measuring GCPs in areas with landslides or immediately after disasters can be dangerous due to unstable footing. It can be difficult to place markers on steep slopes or at height.

• Dependence on environment and conditions: In sites heavily covered by vegetation or with few open areas visible from the air, securing suitable GCPs is difficult. Weather affects the work; like drone flights, GCP installation can be delayed by rain or strong winds.

• Limits on accuracy: Placing GCPs does not guarantee perfection. In wide-area surveys, if GCPs are far apart, accuracy in the intermediate areas can decline. If you don't place a sufficient number of GCPs in a balanced layout, you may not achieve the expected accuracy.

As described above, handling GCPs in traditional surveying involves much work, lowering overall surveying efficiency. Against this background, demand has grown for "an easier way to conduct high-precision surveying."

Emergence of new surveying methods

In recent years, new technologies have emerged in surveying to address these issues. Representative advances include equipping surveying drones with high-precision GNSS and improving the positional accuracy of imaging equipment itself. With RTK/PPK-capable drones, the position of each photo can be recorded at the centimeter level, making it increasingly possible to generate high-accuracy models without relying on GCPs.

For example, on the latest RTK-enabled drones, the onboard GNSS receiver receives real-time correction information during flight, significantly reducing positional error of captured photos. Under favorable site conditions, it is said that mapping is possible even with zero GCPs, and there have been reports of horizontal accuracy around 1–2 cm (0.4–0.8 in) and vertical accuracy around 5–6 cm (2.0–2.4 in). Traditionally, vertical (Z) accuracy in aerial photogrammetry was weak, but RTK has improved it to some extent. However, in terrains with large elevation differences, slight vertical misalignments can still occur, so fully omitting GCPs requires careful validation.

Alongside advances in drone surveying, new smartphone-based surveying methods are gaining attention. Smartphones may seem simple, but modern devices include high-performance GNSS receivers and sensors that, with clever use, can achieve professional-grade positioning. Trials are already underway to use smartphones alone for high-precision positioning and 3D scanning, making the once-unthinkable "GCP-free" surveying a realistic possibility.

High-precision positioning advancing with smartphones

What makes smartphone-based surveying revolutionary? The key is the recent high-precision positioning technologies built into smartphones.

First, current high-end smartphones (for example, the latest iPhone models) include chips that support multiple satellite positioning systems (GLONASS, Galileo, Michibiki (QZSS), etc.) in addition to GPS. They can receive multiple frequency bands such as L1 and L5, making it easier to remove ionospheric errors and mitigate multipath effects. As a result, the GNSS observation accuracy on smartphones themselves has improved dramatically.

Next, the use of augmentation signals and networks. In Japan, by using the centimeter-level augmentation service (CLAS) provided via the quasi-zenith satellite system Michibiki, smartphones can apply corrections to received GNSS signals and achieve positioning to within a few cm (a few in). Smartphones can also connect to network RTK services (so-called Ntrip) over the Internet to obtain real-time correction data. By incorporating these augmentations, smartphones can achieve position accuracy comparable to dedicated surveying equipment.

In addition, smartphones pack IMUs (inertial measurement units), cameras, and LiDAR sensors for distance measurement by light. For example, recent iPhones include LiDAR that can scan surrounding structures as point-cloud data. Combining that with high-precision positional information enables a single smartphone to complete on-site 3D surveying.

In practice, the potential of smartphone surveying has begun to be demonstrated in various places. For example, Fukui City in Fukui Prefecture introduced an iPhone-based high-precision positioning system for disaster site assessment, initiating rapid, low-cost 3D surveying of damaged areas. Where GCP installation would previously have been required, being able to survey with a smartphone in hand has major benefits and contributes to more efficient administrative disaster response.

Thus, smartphone surveying is revolutionary in that it enables high-precision surveying using a ubiquitous general-purpose device. If dedicated equipment becomes unnecessary, the barrier to surveying drops dramatically, and an era when surveys can be performed whenever needed may be at hand.

Will GCPs become unnecessary?

As seen so far, technological progress is certainly reducing dependence on GCPs. Does that mean we can say, "GCPs will no longer be needed" in the future?

The conclusion at present is: "In many situations the burden of placing GCPs can be eliminated, but for some cases it is safer to use a minimal number of reference points for verification." Latest RTK drones and smartphone positioning make it possible to keep survey results within a few cm (a few in). Especially for small-scale surveys and rapid situational assessments, there are more cases where practical accuracy can be achieved without GCPs. This can dramatically improve surveying efficiency.

However, for surveys that require strict accuracy control or for official public surveys where deliverables are submitted, it is still recommended to set one or two known points as checkpoints for verification. The National Institute for Land and Infrastructure Management’s reports indicate that while RTK-equipped UAVs can reduce the number of GCPs, they do not assume completely eliminating them; appropriate reference points and accuracy verification are prerequisites to meet required accuracy. The same applies to smartphone surveying: although high-precision coordinates can be obtained in real time, it is prudent to measure one existing known point on site (for example, an existing triangulation point or bench mark) and use it to validate and adjust survey results.

In other words, the stance is: "With the latest technologies, GCP-less workflows are OK in most cases, but for important projects use one-point GCPs for double-checking." Put differently, where dozens of GCPs used to be necessary, in some cases none or only one to two may suffice. In the field, this means the process bottleneck of installing GCPs could be largely removed.

Smart surveying realized with LRTK

One of the new technologies opening the way to a "GCP-free" era is a smartphone-based surveying solution called "LRTK." LRTK is a system composed of lightweight equipment and a smartphone app that uses iPhones and similar smartphones to achieve centimeter-level positioning. It features enabling cm level accuracy (half-inch accuracy) that previously required expensive surveying equipment with just a palm-sized device and an app.

With LRTK, the following kinds of smart surveying can be achieved on site:

• Simple workflow: Attach the smartphone to a dedicated holder and point it at the sky to obtain high-precision position coordinates. No heavy tripods or complex initial setup are required, and one person can complete the surveying task. In effect, your smartphone becomes a "mobile reference point."

• High-precision position information: In addition to multi-frequency GNSS reception, combining Michibiki (QZSS) augmentation signals and proprietary cloud correction technology enables positioning accuracy of about horizontal 1–2 cm (0.4–0.8 in) and vertical about 3 cm (1.2 in). Validation comparable to first-order bench marks has been performed, confirming coordinate accuracy on par with dedicated GNSS receivers.

• 3D scanning and instant sharing: Scanning the site with the smartphone camera or LiDAR provides high-precision 3D point-cloud data. Obtained data are saved and shared in the cloud immediately, allowing on-site 2D/3D display, distance and area measurement, and even remote verification of point-cloud models from the office via the cloud.

• AR-assisted staking: Measured data can be used not only for mapping but also for AR display on the smartphone screen at the site. For example, overlaying design drawings or BIM models in their correct on-site positions and using that for stakeout or as-built verification can be completed with a single smartphone. AR display requires high-precision positioning to avoid misalignment, and LRTK enables such construction-management efficiency improvements.

• Operable in diverse environments: The portability of a smartphone and small antenna makes it easy to bring into mountain areas or disaster sites where vehicles cannot enter. Even in areas without mobile coverage, receiving Michibiki’s CLAS signal directly allows positioning to continue, so it performs well in areas outside communication range. There are cases where stable positioning was achieved in mildly tree-covered environments by combining multi-GNSS and correction data, enabling reliable surveying under conditions that caused large errors with traditional equipment.

LRTK thus greatly simplifies traditional surveying processes and supports the realization of "smart surveying" that anyone can perform quickly anywhere. Because it does not require pre-installing GCPs or large-scale equipment, you can begin surveying on-site as soon as you decide to do so. Future surveying will shift toward a digital approach that combines the expertise of seasoned professionals with the computing power and networks of smartphones.

Those accustomed to traditional methods may be skeptical, but field introductions have begun and reports indicate tangible improvements in both efficiency and outcomes. If you are troubled by the effort of setting GCPs or by survey delays, consider trying such smart surveying technologies. Utilizing LRTK could make surveying faster and smarter than ever.

FAQ

Q: Are ground control points (GCPs) and aerial markers the same thing? A: Strictly speaking, they are slightly different. A GCP refers to the ground point whose coordinate values are known, while a aerial marker is the physical sign placed at that point so it appears in aerial photos. In practice, people use aerial markers to make a point a GCP by measuring its coordinates, so they are treated almost synonymously. In short, the place where you install an aerial marker and measure its coordinates becomes a GCP.

Q: Can a smartphone really achieve centimeter-level surveying accuracy? A: Yes. The latest smartphones have high-performance GNSS chips supporting multiple satellite systems and multiple frequencies. By using correction information (for example, Michibiki’s CLAS signal or network RTK), accuracy approaching that of dedicated surveying equipment is achievable. Demonstration tests have even reported differences between smartphone-measured positions and high-precision GNSS equipment on the order of several millimeters (≈0.1–0.2 in). However, to consistently achieve centimeter-level accuracy (cm level accuracy (half-inch accuracy)), conditions such as a clear sky view and proper use of correction services are required.

Q: Is smartphone surveying possible even in remote mountains with no mobile coverage? A: Yes, depending on conditions. Systems like LRTK can directly receive Michibiki’s centimeter-level augmentation signal, so real-time corrections can continue even where cellular coverage is absent. Also, if communication is unavailable, PPK (post-processing kinematic) methods allow high-precision results to be obtained later. Therefore, smartphone surveying in remote areas is entirely feasible.

Q: What is required to use LRTK? A: Basically, a compatible iPhone or iPad and the LRTK app are sufficient to get started. The dedicated hardware is limited to a small antenna and holder that attach to the smartphone, and they are lightweight. The app guides operation, so no specialized surveying expertise is strictly necessary to begin. Of course, some surveying knowledge helps in setting coordinate systems and utilizing data more smoothly, but obtaining high-precision coordinates on site does not require special qualifications.

Q: Is it really safe to proceed with no GCPs at all? A: In most cases, yes, but for critical projects it is recommended to record one or two known points for verification. There are increasing situations in which routine work can be done without GCPs thanks to high-precision smartphone and drone surveying. However, when deliverables require strict assurance—such as surveys submitted for official inspections—checking against known points on site provides a safeguard to detect and correct potential errors or coordinate shifts. Think of this as insurance: for projects where you want to avoid any extra work, proceed GCP-less; if you want confirmation, use a small number of GCPs as needed.