You can do this with a smartphone! The secrets to improving photogrammetry accuracy and how to use RTK

Can you believe you can turn site terrain and structures into detailed 3D models with just a smartphone? Three-dimensional measurement, which traditionally required expensive surveying equipment and specialized knowledge, can now be done easily with a smartphone and photographs. The core technology enabling this is photogrammetry.

This article explores the appeal of smartphone-based photogrammetry and the secrets to improving accuracy. In particular, it explains in detail the practical benefits of RTK (Real Time Kinematic), the key to ensuring coordinate accuracy. From the low barriers to entry and the main points for improving accuracy to the simplification of work procedures, we unravel smartphone × photogrammetry + RTK as a new surveying method that supports on-site DX. At the end of the article we introduce an easy surveying solution that uses RTK, LRTK, offering tips to boost your motivation to implement it on site.

What is photogrammetry? 3D measurement made possible by smartphones

Photogrammetry is a technique for reconstructing the three-dimensional shape of an object from multiple photographs. By matching common feature points among numerous photos taken by drones or single-lens reflex cameras and calculating their spatial positions, point clouds and 3D models are generated. Recently, high-performance smartphone cameras and processing power, together with dedicated apps and cloud services, have made photogrammetry easy to use.

The biggest advantages of smartphone photogrammetry are its convenience and immediacy. Without bringing special equipment, you can capture the site with a familiar smartphone and later obtain detailed 3D models. For example, if you photograph the surrounding terrain and structures at a construction site with a smartphone, you can create point cloud data that digitally replicates the current state. Photo-based models reproduce textures (color) and are visually intuitive, making it easier to grasp complex shapes that are difficult to understand from plans or photographs alone.

Photogrammetry is also attractive because it can be introduced at low cost. Traditionally, 3D measurement with terrestrial laser scanners or mobile mapping systems required equipment investments on the order of millions of yen, but smartphone photogrammetry can be started with existing smartphones and general-purpose software. This greatly lowers the barrier for ordinary construction managers to handle 3D data in their daily work. The next chapter looks at the points to improve accuracy when performing photogrammetry with a smartphone.

Points to improve photogrammetry accuracy

To obtain highly accurate results with smartphone photogrammetry, careful consideration of shooting methods and processing steps is essential. The main points are summarized below.

• Capture the entire target without omissions: Take a sufficient number of photos from above, at oblique angles, and around the site so that the entire area to be reconstructed has no blind spots. Missing shots can cause holes or shape loss in the model.

• Ensure photo overlap: Shoot so adjacent photos overlap most of the subject (as a guideline, 70% or more). With sufficient overlap and parallax, alignment between photos is stable and a precise point cloud can be obtained.

• Prevent blur and out-of-focus images: Photos affected by camera shake or poor focus cannot be used for processing. Hold the smartphone steady with both hands and use a tripod or mount if necessary. Use a self-timer or remote shutter to suppress vibration at the moment of capture.

• Appropriate focus and exposure: Aim to have the entire subject in focus. Photos with deep depth of field (overall sharpness) are preferable to those where only the background is sharp. Large contrasts in brightness also reduce accuracy, so compensate exposure or choose overcast or shaded conditions to shoot under uniform lighting.

• Avoid moving subjects: If people or heavy machinery move into the frame during shooting, processing errors may occur. Whenever possible, shoot when the subject and surroundings are still.

• Use known dimensions and control points: To correctly set model scale and coordinates, it is effective to place rulers or markers (control points) that serve as scales in the scene and include them in the photos. By scaling the model during post-processing to those known dimensions, you can obtain more reliable measurements.

By following these points during shooting and processing, the accuracy and quality of photogrammetry models will increase dramatically. If you take plenty of photos carefully and perform additional shooting or reprocessing when necessary, you can obtain highly accurate 3D data suitable for practical use even with a smartphone.

Dramatic improvement in coordinate accuracy with RTK

Another important factor in improving photogrammetry accuracy is ensuring the correctness of coordinates. No matter how detailed the shape reconstructed from photos is, if the model’s absolute position in the real world is off, it is insufficient as surveying deliverables. Standalone smartphone GPS can have position errors of several meters, so a 3D model created may be offset by several meters when overlaid on drawing coordinates. This is where the high-precision positioning technology called RTK (Real Time Kinematic) proves powerful.

RTK uses two GNSS receivers—a base station and a rover—and sends correction information from the base to the rover to cancel out errors in real time, dramatically improving positioning accuracy. Normally, positioning with built-in smartphone GPS generates errors on the order of 5-10 m (16.4-32.8 ft), but using RTK can reduce that error by about 1/100 to a few centimeters (a few cm (≈1 in)). In practice, if horizontal accuracy of about 2-3 cm (0.8-1.2 in) and vertical accuracy of about 3-4 cm (1.2-1.6 in) can be obtained, the quality required for civil engineering surveying and as-built management is sufficiently met.

Recently in Japan, services for corrections—such as the Geospatial Information Authority’s permanent GNSS reference station network and carrier-provided VRS (Virtual Reference Station)—have been established, enabling RTK correction information to be obtained over the Internet without dedicated radio equipment. By connecting a small RTK-capable antenna to a smartphone, you can measure centimeter-level positions anywhere on site in real time. If high-precision position information is attached to each photo, the models generated by photogrammetry will be aligned to correct coordinates from the start, greatly reducing the need to adjust later to control points.

The main benefits of RTK introduction are summarized as follows:

• Centimeter-level absolute accuracy: It greatly reduces the error of conventional smartphone GPS (several meters), allowing you to obtain the model’s position and dimensions with near-measured accuracy. This lets photo-based surveying data be used directly for precision-demanding applications such as as-built inspection and displacement measurement.

• Reduction of ground control points: Because high-precision coordinate tags are attached to the photos themselves, the need to place and survey numerous ground control points (GCPs) when composing the model is reduced. The number of reference points to install on site can be minimized, saving preparation effort and time.

• Prevention of distortion during wide-area scans: Even when walking long distances while shooting and scanning with a smartphone, RTK continuously corrects the rover position, preventing positional drift and scale distortion across the whole model. You can acquire consistently high-accuracy point clouds for large fill sites or long roads.

• Instant measurement and verification: With high-precision position-tagged 3D models, you can measure dimensions and volumes on site and compare them to design values for verification. Detecting even small discrepancies on site prevents rework and improves quality control accuracy.

• Easy integration with CIM/BIM data: Since generated point clouds and models conform to public coordinate systems (such as the World Geodetic System or plane rectangular coordinate systems), they can be smoothly overlaid with BIM models and drawing data from design. This facilitates visualizing differences between design and reality and smooth data handover to subsequent processes.

By combining RTK, photogrammetry outputs evolve from mere 3D models into high-precision digital data that are acceptable as surveying deliverables. So how does the combination of smartphone photogrammetry and RTK change on-site workflows? The next section looks at those implementation effects.

Easy 3D surveying realized by smartphone × RTK

Photogrammetry using smartphones and RTK brings dramatic efficiency gains and labor savings compared to traditional surveying methods. First, the required equipment is minimal—just a smartphone and a small antenna—so the burden of transporting and setting up equipment on site is greatly reduced. There is no need to set up a total station on a tripod or transport/install a large laser scanner. The convenience of being able to start surveying and point cloud acquisition immediately is a major advantage on site.

In terms of operability, smartphone surveying is also intuitive and simple. By following guides in dedicated apps and pressing buttons, photo capture and positioning proceed, with complex settings automated. Designed to be usable without advanced specialist knowledge, it is suitable not only for engineers but also for site workers. For example, tasks that previously required requesting a surveyor to confirm as-built dimensions can be handled by photographing with a smartphone and immediately modeling it to check results yourself. As more tasks can be completed by a single person, personnel coordination and wait times are reduced, speeding up site operations.

Moreover, workflows that use smartphones and the cloud enable instant data sharing. Point cloud data and survey point information obtained in the field can be uploaded to the cloud on the spot and viewed from office PCs. Unlike the past—when survey results were brought back on paper drawings or USB drives—everyone can check and use the same 3D information in real time, accelerating decision-making. As digital linking of survey data advances, construction management processes themselves will be transformed, lowering the barriers between site and office.

Thus, the new 3D surveying method using smartphone × RTK is significantly lowering introduction costs, simplifying work procedures, and becoming a tool that anyone on site can master. When high-accuracy point clouds and survey data can be acquired and shared routinely, reviewing construction plans and verifying as-built conditions become much easier, accelerating on-site DX.

High-precision 3D data required in the BIM/CIM era

In the construction industry, the full adoption of BIM/CIM has led to a strong demand for high-precision 3D data utilization. From fiscal 2023 the Ministry of Land, Infrastructure, Transport and Tourism began principle application of BIM/CIM to direct-managed projects, accelerating the industry-wide movement to capture sites in three dimensions. Coupled with severe labor shortages and the 2024 issues (working-style reforms that limit working hours), improving productivity and ensuring safety are urgent challenges. Point cloud data that can fully digitize the current state are viewed as a key to promoting DX.

Against this backdrop, case studies of drone photogrammetry and laser scanner point cloud measurement are increasing. For example, there are reports of earthwork volume surveys that used to take two days by hand being completed in 0.5 days with drone photogrammetry, demonstrating that 3D technology can dramatically improve surveying productivity. Using a smartphone makes onsite 3D scanning even easier, enabling small- to medium-sized sites to routinely acquire 3D data for quality control and as-built records.

For promoting BIM/CIM, high-precision point clouds obtained by photogrammetry + RTK are a powerful asset. Overlaying the design BIM model with the post-construction point cloud makes even slight as-built deviations visually apparent, helping prevent rework and enhancing quality inspection. Point cloud data acquired at completion can also be used later for detecting deterioration in maintenance and for planning future renovation work. Accumulating site history in 3D is also beneficial for passing on engineers’ know-how, and point cloud data will serve as foundational data supporting future construction DX.

Thus, utilizing high-precision 3D data obtained by photogrammetry is an indispensable trend in construction management and surveying in the BIM/CIM era. Still, you might worry, “Is it difficult to operate in-house?” To address that concern, we introduce an easy surveying solution using smartphones—LRTK—that enables anyone to perform centimeter-accurate surveying.

Start easy smartphone surveying with LRTK

Supporting the use of photogrammetry on smartphones even further is the smartphone RTK solution called LRTK. LRTK attaches an ultra-compact RTK-GNSS receiver to a smartphone (e.g., iPhone or iPad), and by simply launching a dedicated app, turns the smartphone into a centimeter-class surveying instrument. It fits in a pocket, weighs about 125 g, and has a built-in battery, making it easy to carry and use on site.

Using LRTK greatly improves the accuracy of position records at the time of photo capture, so it has a large effect on improving photogrammetry accuracy. All captured photos receive cm-level position tags, so the 3D models generated later align with the surveying coordinate system from the start and achieve high absolute accuracy even without control points. In addition to photogrammetry, LRTK functions as a versatile all-in-one surveying tool. For example, to obtain the coordinates of a point, you simply hold the LRTK receiver attached to the smartphone over that point and press a button; latitude, longitude, and height are instantly recorded. What used to be single-point surveying with surveying instruments and levels can now be completed with one touch.

Coordinates and point cloud scans acquired on site are displayed on the smartphone screen immediately. You can measure distances and height differences between two points on the dedicated app, or cut cross-sections of acquired point clouds to check dimensions instantly, enabling a speed of “measure on site and verify on site” that was previously unattainable. Recorded data can be uploaded to the cloud with one click and shared in real time with colleagues in the office. The labor of taking notes in a field notebook and bringing them back is no longer necessary, making information transfer between site and office seamless.

LRTK is offered at a reasonable price range, so it is revolutionary in that you can start without purchasing expensive dedicated equipment. Because initial investment is low, equipping each worker with a device becomes realistic—“one device per person” on site is now achievable. In practice, site managers and workers can carry a smartphone + LRTK and perform measuring and recording themselves whenever needed, which has great potential to improve site productivity. If site personnel can measure and decide in real time without relying on a specialist surveying team, quick checks and as-built inspections during construction can be done speedily.

Thus, the easy surveying enabled by smartphones and LRTK makes surveying—once regarded as high-precision but difficult—suddenly accessible. While creating 3D models with photogrammetry, you can measure and verify necessary points on the spot, dramatically improving the accuracy and speed of on-site decision-making. LRTK is truly a reliable partner that supports on-site DX from the ground up.

Now that smartphones can do so much, adopting a new surveying style that leverages high-precision photogrammetry and RTK positioning will directly strengthen competitiveness. 3D technology that used to be high-barrier can now be practiced with a smartphone and affordable devices. Take this opportunity to introduce the latest tools to your sites. Make the secrets of accuracy improvement with smartphone × photogrammetry + RTK your ally, and evolve your construction sites to the next stage.