The Importance of Surveying in Disaster Recovery

When heavy rains or earthquakes cause levee breaches or landslides, the first priority is an accurate assessment of current conditions. Without rapidly surveying the affected area and any changes in topography, it is impossible to draft an effective restoration plan. If riverbank revetments collapse in a flood, engineers must quickly measure the extent and degree of erosion to determine the scope and method of repair. Likewise, when a sabo facility is buried by a debris flow, calculating the volume of collapsed material and mapping the altered terrain are essential for formulating a safe recovery strategy.

Immediately after a disaster, however, sites are strewn with rubble and fallen trees, footing is poor, and hazards abound. Traditional methods—total-station (TS) or level surveys—require clear lines of sight and careful point setups, making rapid response difficult. In fact, after a 2004 typhoon damaged farmland in Shikoku, debris and driftwood blocked the line of sight for TS measurements, forcing a rethink of survey techniques to speed recovery. Relying solely on conventional tools risks inflating manpower and timelines, delaying reconstruction.

RTK positioning has therefore come to the fore. By providing real-time, centimeter-level GNSS coordinates, RTK proves powerful on disaster-recovery sites. In the Shikoku case, a network RTK service enabled two workers—one surveyor and one assistant—to complete a survey of roughly 60 ha of damaged farmland perimeter in about one week. Using TS alone would have taken an estimated four to five times longer, demonstrating substantial efficiency gains.

Because disaster-recovery surveying demands both safety and speed, RTK adoption is accelerating. Conventional GNSS methods (static or standalone) offer only meter-level accuracy, insufficient for restoration design. RTK, by contrast, trims errors to a few centimeters, capturing fine topographic changes. The height of a breached levee or depth of scouring can be measured on site with RTK, providing data that feeds directly into swift recovery design. From initial response through project completion, high-precision RTK surveying forms a critical foundation for disaster-recovery operations.

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Basics of RTK Positioning and Its Use on Disaster Sites
RTK, short for Real-Time Kinematic, is a method that achieves centimetre-level accuracy by having two GNSS receivers—a base station with known coordinates and a mobile rover—track satellite signals simultaneously. The base computes the errors in real time and transmits corrections to the rover, which then outputs a highly accurate position. Whereas stand-alone GPS typically drifts 5–10 m, RTK reduces that error to just a few centimetres.

Because of this precision, RTK is now used in tasks that once exceeded ordinary GPS capability—civil-works surveys, machine guidance for earth-moving equipment, and autonomous drones, for example. RTK can be run in two main ways. Local RTK requires you to set your own base station on site and broadcast corrections by radio. Network RTK—the emerging standard—leverages national or commercial reference-station networks; the rover receives corrections over the internet, so no base need be set up and high accuracy can be maintained over longer baselines.

On disaster-recovery sites, RTK’s chief advantage is speed. Because centimetre-level coordinates are available instantly, surveyors can update maps and drawings as they walk. After a levee breach, for instance, carrying an RTK rover around the site allows a topographic map of the flooded area to be produced the same day. In one Hokkaidō flood, network RTK was used to survey 27 km of farm-drain pipes—about 2,700 points—in only six weeks. Although mobile coverage was patchy in the mountains, satellite geometry and sky visibility were adequate and work proceeded smoothly.

RTK’s accuracy rivals that of total-station or levelling surveys, enabling precise comparisons with pre-disaster drawings and design values. Because positions are calculated on site, extra points can be shot immediately if gaps are found—eliminating the back-and-forth inherent in traditional methods.

RTK also saves labour. A TS survey normally needs at least two people (operator and prism holder); RTK needs only one person carrying a pole. There is no requirement for line-of-sight, so the surveyor can weave around rubble and vegetation—an enormous advantage amid disaster debris. In the Hokkaidō case, dense bamboo would have demanded time-consuming clearing for TS, but RTK required only point-by-point shots, cutting the schedule to roughly one-third of the conventional estimate and far exceeding efficiency expectations.

As a result, the work was finished in roughly one-third of the time required by conventional methods, delivering far greater efficiency than expected. In this way, RTK positioning proves highly effective on disaster sites, combining speed and accuracy to give powerful support throughout the recovery process.

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[Case Studies] How RTK Enabled Rapid Disaster Recovery

Below are three real-world examples in which RTK positioning smoothed and accelerated disaster-recovery projects for river-control and erosion-control works.

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Case 1: RTK in Levee-Restoration after River Flooding

Case: Heavy rainfall caused several small and mid-sized rivers to overflow, damaging farmland on the floodplain and sections of the levee. Before the revetment could be rebuilt, the entire affected area had to be surveyed quickly.
Challenge: Traditionally, crews used a total station and staff rod to survey river cross-sections and determine inundation depth and erosion in the flooded zone. After the overflow, however, sediment and debris were scattered widely, making it impossible to secure clear sight lines. The large number of required points meant that a manual survey would likely take several weeks.

RTK solution: A network-based RTK-GNSS survey was proposed. After a preliminary accuracy check and the client’s approval, the team began work with a single rover and three personnel. The receiver pulled correction data over the existing cellular network, and the crew walked from point to point. About 2,700 locations—including the positions and elevations of 27 km of farm-drain ditches and culvert pipes—were surveyed in roughly one and a half months.

Effect: Although this was the first use of network RTK on the site, the survey finished trouble-free and far sooner than expected. Even in dense stands of dwarf bamboo, there was no need—as with a total station—to clear vegetation to create straight sight lines, so the workload was greatly reduced. Compared with the traditional TS + level method, which was estimated to take more than three times longer, RTK delivered a dramatic efficiency gain. The data enabled early design of the revetment repairs, allowing the project to move from temporary to permanent reconstruction shortly after the flood. The client praised RTK-GNSS for providing wide-area, high-precision data that were essential to the recovery plan.

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Case 2: Surveying & Volume Calculations at a Landslide Site

Case: A slope collapsed after torrential rain, burying a downstream road with debris. The restoration project had to estimate the volume of fallen material and plan countermeasures to prevent secondary disasters.
Challenge: The landslide slope was unstable, making detailed ground surveys dangerous. Traditionally, crews surveyed only the perimeter of the slide with a total station (TS) and then estimated the debris volume, but such estimates were often inaccurate, risking under- or over-haul of material and overlooking additional countermeasures.
RTK solution: The team combined RTK-assisted UAV photogrammetry with ground RTK. Drone imagery was geotagged with RTK coordinates to generate an orthophoto and dense point cloud, producing a 3-D model of the slide. Areas the drone could not capture—under forest canopy or at the toe of the slope—were supplemented with additional RTK shots taken on the ground. This hybrid approach yielded a highly accurate debris-volume calculation.
Effect: UAV + RTK surveying provided comprehensive coverage of the slide in a short time while keeping field exposure to a minimum, improving both safety and efficiency. The precise point cloud allowed the team to calculate the exact volume to be removed and to size dump-truck trips and stockpiles appropriately. It also sped design of retaining walls and other countermeasures. According to one surveying firm, RTK-based UAV surveys have repeatedly enabled rapid landslide assessment and fed directly into design and construction planning. Thus, RTK offers a new solution for landslide investigation, improving both volume estimates and overall recovery planning.

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Case 3: RTK-Based Construction Control for an Emergency Sabo Dam

Case: An emergency project called for the rapid construction of a concrete sabo (debris-control) dam in a mountain stream to mitigate future debris flows. The work had to be completed within a very tight schedule.
Challenge: From excavating the bedrock foundation to placing the dam body, the project demanded highly accurate control of both position and elevation. Traditionally this meant setting control points, checking as-built dimensions with a total station, and assigning extra personnel for construction surveying. In the winding, mountainous terrain, however, sight lines were poor, and establishing survey baselines along the twisting stream was difficult. Frequent surveying also placed a heavy burden on the crew and slowed overall progress.

RTK solution: The project adopted RTK positioning alongside the latest ICT-construction techniques. First, a set of known control points was established, creating a site-wide coordinate system; throughout construction, a rover RTK setup allowed continuous surveying. Excavator operators and site managers carried tablet devices fitted with RTK-GNSS receivers, enabling them to measure as-built coordinates instantly whenever needed. During foundation excavation, the finished surface was scanned with RTK and compared with the design model, preventing over-cutting and streamlining as-built control. While the dam body was being placed, pour elevations were checked on the fly with RTK, and tasks such as center-line layout—previously done separately with chalk lines—were performed quickly using RTK-linked equipment.

Effect: Integrating real-time RTK positioning into construction management delivered benefits for both quality and schedule control. Site supervisors could check required coordinates and elevations on their own tablets without waiting for specialist surveyors, reducing idle time. During concrete placement, for example, they verified on the spot whether the pour had reached the specified crest elevation and immediately fine-tuned the volume. Because the as-built data were stored digitally, they could be used directly for inspection documents and 3-D completion records. Ultimately, the project finished sooner, and fewer survey personnel were able to oversee multiple sites, yielding cost savings as well.

This project also embraced Japan’s i-Construction initiative, implementing “smart construction” by combining 3-D design data with RTK positioning. Even in the sight-line–poor environment of a mountain valley, RTK provided high-precision measurements wherever the sky was open enough for satellite reception. In fact, the system worked flawlessly under these difficult conditions, and the sabo dam was completed exactly to design tolerances. Similar RTK-based construction control is now expected to enhance future dam and slope-protection projects.

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Concrete Steps for Introducing RTK

What preparations and procedures are required to apply RTK positioning to disaster-recovery work? The sections below outline the specific steps for deploying RTK and the key points to keep in mind.

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Step 1: Gather the Required Equipment
To perform RTK surveying you need at least one GNSS receiver (antenna-integrated models are acceptable). A typical setup uses two units—a base station and a rover—but if you rely on an existing correction-data service you can work with a single rover receiver. Choose a multi-GNSS model that tracks not only GPS but also GLONASS, QZSS (Michibiki), and other constellations; a dual-frequency unit (L1/L2) is preferable for higher accuracy.

You must also provide a communication link so the rover can receive corrections. If you run your own base station, you may use a low-power or UHF radio to transmit corrections to the rover. If you use a network RTK service, equip the rover with internet access—either a built-in SIM module or a smartphone that can tether—to stay online in the field.

Finally, you need a field controller (dedicated handheld or tablet) to configure the GNSS receiver and view data. Many modern receivers can be operated directly from a smartphone or tablet.

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Step 2: Create the Survey Plan
With the equipment assembled, draw up an RTK survey plan. In disaster-recovery work, it is crucial to collect all required data efficiently within a limited timeframe. First decide how you will secure control points. Even when using network RTK, it is wise to establish at least one known point near the site in the public coordinate system; this lets you verify RTK accuracy and serves as a backup if communications fail.

For large areas, consider dividing the site into sectors so multiple crews can survey in parallel. Set the point density (spacing) and decide which parameters to observe (horizontal position only, or elevations as well) according to the deliverables. For example, if you need a topographic map, place points every few metres; if you need a volume calculation, capture the perimeter and cross-sections of the slide mass. Because RTK yields immediate results, you can identify gaps on site and add points as needed. Finally, plan the post-processing workflow—such as CAD drafting or point-cloud analysis—so the survey data can hand off smoothly to the restoration design phase.

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Step 3: Operation and Field Survey

Carry out the RTK survey on site according to the plan. Before starting, set up and power on the base station—or log in to the correction service—and make sure the rover is receiving the corrections correctly. Avoid periods when GNSS satellite geometry (high GDOP) or ionospheric effects are poor; observe during times that give the best accuracy. Throughout the survey, continuously monitor the current reception status and solution. Centimetre-level accuracy is guaranteed only when the RTK solution is in “FIX” (integer ambiguity resolved). Check repeatedly that the rover shows FIX; if it drops back to FLOAT, wait a moment or re-initialize the receiver until FIX is regained.

At each point, allow enough observation time and, if possible, use an averaging function to improve accuracy. When the survey is finished, review the data on site for gaps or outliers; if anything is missing, measure those points again before leaving.

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Step 4: Using and Sharing Survey Data
Data captured with RTK can be put to work immediately. A field controller can display quick contour lines or cross-section plots so the team can review conditions right on site. Back in the office, the data are imported into CAD or GIS software for full drafting and analysis. In disaster recovery, the survey enables comparison of pre- and post-event topography and informs restoration methods. For example, when rebuilding a river levee, engineers use RTK elevations at the breach to calculate the missing fill by comparing them with intact sections. In sabo planning, longitudinal gradients and cross-sections of the stream are extracted to decide dam height and storage capacity. RTK data can also be stored in formats suited to electronic deliverables, adding value for long-term maintenance. Increasingly, survey results are shared in the cloud so designers and clients can view them in real time.

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Step 5: Verifying Costs and Benefits
Although RTK equipment requires an initial investment, its cost must be weighed against the gains. High-precision GNSS receivers once cost hundreds of thousands of yen, but prices and sizes have dropped. A subscription to a commercial correction service may run only tens of thousands of yen per year, which often offsets the labor and setup saved by skipping a local base station. One approach is to roll out RTK on a small site first and measure its impact: How much survey time was cut? How many crew hours were freed for other tasks? Did the accuracy meet requirements? If, as in the examples above, RTK reduces survey time to one-third or even one-fifth of the traditional method, the return on investment is clear. Once benefits are proven, extend RTK to other projects and branches and establish it as the standard practice.

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Cautions When Introducing RTK

To keep RTK running smoothly on-site, pay close attention to several points.

Communication environment – Network RTK relies on internet corrections, so connection quality directly affects accuracy and stability. If the rover’s link is unstable, correction data arrive intermittently, the solution may drop back to FLOAT, or positioning can stop altogether. Always use the strongest available signal. In cities, 4G/5G is fine, but in mountains or underground a phone hotspot or pocket Wi-Fi may lose coverage. Prepare countermeasures such as switching temporarily to your own base station or using Japan’s QZSS CLAS satellite corrections, which work even without internet. CLAS-capable receivers let you maintain centimetre-level accuracy in disaster zones that lack cell service.

Satellite reception – RTK needs clear sky view. Tall buildings or trees can block signals or cause multipath reflections, making fixes unstable. Place antennas in the open and, if necessary, mask low-elevation satellites in the receiver settings to cut noise. In rain, droplets on the antenna attenuate signals; use a weather cover or wipe it periodically. Because GNSS draws power continuously, bring spare batteries or chargers for long shifts.

Operator proficiency – Although setup is not difficult, unfamiliar users may stumble at first. Provide training so every team member can configure the gear and troubleshoot quickly; that readiness ensures RTK can be deployed smoothly whenever it is needed.

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Introducing LRTK

As interest in RTK solutions for disaster-recovery work grows, one product drawing particular attention is LRTK, a pocket-sized RTK positioning unit. Developed by Lefixea—a startup spun out of the Tokyo Institute of Technology—LRTK is an RTK-GNSS receiver that snaps onto a smartphone or tablet. Weighing just 125 g and only 13 mm thick, it contains its own battery, instantly turning a handheld device into a centimetre-level, all-purpose survey instrument. Paired with a dedicated app and cloud service, LRTK lets anyone bring high-precision positioning to the field with minimal effort.

Easy to carry even on disaster-recovery sites, LRTK lets you start surveying the moment it’s needed.
Key features: its greatest strength is the combination of convenience and high accuracy. Whereas conventional RTK equipment conjures images of receivers mounted on tripods, LRTK slips into a pocket and snaps onto a phone in an instant, delivering centimeter-level positioning right away. Coordinates appear on the phone screen in real time and can be used for point-cloud capture or layout staking. Because all data and point clouds upload to the cloud immediately, information flows seamlessly from the field to the office—for example, measurements taken at a disaster site can be sent on the spot and reviewed by designers back at headquarters.

Despite this functionality, LRTK is priced very affordably, making “one device per worker” a realistic way to boost productivity dramatically. Its accuracy is also dependable: LRTK supports Japan’s Quasi-Zenith Satellite System “Michibiki” CLAS service, so it can receive correction data directly from satellites and maintain centimeter precision even in remote mountain valleys or at disaster scenes where ground communications are down.

This capability is extremely valuable during large-scale disasters, giving users the confidence that surveying can continue even when mobile networks are down. The unit supports multiple constellations—GPS, GLONASS, Galileo, and Michibiki—so it is easier to maintain a stable FIX solution even in urban canyons. Field tests show that average horizontal accuracy improved from about 12 cm in standalone mode to roughly 8 mm when using LRTK with averaging, delivering performance on par with conventional professional instruments.

Deployment on job sites: LRTK is already being adopted on many civil-engineering projects. On one road-construction site, layout staking that used to require a total station was completed by a single worker in a short time using LRTK and a smartphone app. On a bridge-inspection job, crews measured pier settlement with LRTK from an aerial-work platform and saw inspection efficiency rise because they could view results instantly. In disaster-recovery scenarios, LRTK has been employed for elevation checks on damaged residential lots and for alignment surveys on temporary detour roads, helping speed critical decisions. Above all, its “just use your phone” simplicity appeals to field personnel and drives adoption, since the system is intuitive and requires little specialized training.