How to obtain coordinates indoors? A practical guide to acquiring high-precision location information without using GPS

Table of Contents

• The necessity and challenges of obtaining coordinates indoors

• Traditional indoor surveying method: total station

• Why GPS doesn't work indoors

• Indoor positioning using Wi-Fi

• Indoor positioning using BLE beacons

• Indoor positioning using UWB (ultra-wideband radio)

• Indoor positioning using RFID tags

• Indoor positioning using Pedestrian Dead Reckoning (PDR)

• Positioning using IMES (Indoor Messaging System)

• Indoor positioning using geomagnetism

• Indoor positioning using ultrasound

• Indoor positioning using visible light

• Indoor positioning using cameras and AR technology

• Points for selecting indoor positioning technologies

• Simplified surveying using LRTK (a new smartphone + RTK method)

• FAQ

The Necessity and Challenges of Acquiring Coordinates Indoors

In construction sites and in the management of large facilities, the demand for obtaining high-precision positional coordinates within indoor spaces is growing. For example, to accurately measure the installation positions of equipment inside a building or to arrange interior layouts according to drawings, it is necessary to know the coordinates of each point. In factories and warehouses, tracking the real-time locations of workers and materials can also improve operational efficiency and safety. However, indoors the radio signals of satellite positioning systems such as GPS (GNSS) are difficult to reach, making it impossible to obtain location information as easily with smartphones or dedicated devices as you can outdoors.

Because high-precision positioning indoors is difficult, up until now people have measured distances with tape measures or laser distance meters based on drawings, or manually calculated coordinates using reference points established inside buildings. These methods are time-consuming and labor-intensive and also carry the risk of human error. In recent years, digital transformation (DX) has progressed in the fields of construction and facilities management, raising expectations for technologies that can efficiently and accurately acquire position coordinates indoors.

Traditional Indoor Surveying Method: Total Station

One conventional method for obtaining high-precision coordinates indoors is surveying with a total station (TS). A total station is a surveying instrument with an integrated telescope that measures the horizontal angle, vertical angle, and distance from known reference points to a target point, and computes the 3D coordinates of the target point using the principles of triangulation. It is a versatile technique that can obtain survey point coordinates with millimeter-level accuracy (mm (0.04 in)), but there are several challenges in its operation.

First, there are issues of manpower and cost. Conventional TS surveying requires a two-person team: one person operates the instrument while the other holds the prism at a remote survey point. When measuring many points inside a large building, it is not uncommon for the team to spend an entire day moving around. A high-end auto-tracking TS can be operated by a single person, but it requires equipment acquisition costs on the order of several million yen. In addition, regular calibration and maintenance to preserve accuracy require specialized knowledge and expense. Furthermore, administrative tasks such as plotting survey results on drawings and preparing reports also arise, making it difficult to immediately apply coordinates measured on site to construction and management.

As described above, surveying with total stations achieves high accuracy but imposes significant burdens in terms of personnel, time, and cost, so more efficient methods for obtaining coordinates indoors have been sought. In the next chapter, we will explain the main technologies for acquiring indoor location information without using GPS, along with their advantages and disadvantages.

Why GPS Can't Be Used Indoors

Radio signals from GPS satellites are designed to travel long distances to the Earth's surface, but they have difficulty reaching building interiors. One reason is radio wave shielding. Indoors, surrounded by concrete or metal, the satellites' weak radio signals are blocked by walls and ceilings. Also, indoors or underground, because the satellites are not in direct line of sight the signals are reflected (multipath) by walls and floors, so the receiver only gets indirect signals, causing errors in position calculations. Furthermore, GPS assumes reception from four or more satellites, but indoors it is often impossible to secure a sufficient number of satellites. The same applies in urban canyons between tall buildings and in underground shopping areas: in environments where the sky is not visible, GPS accuracy can drop drastically or positioning may fail completely.

For these reasons, satellite-based positioning systems such as GPS are often impractical indoors. To address this problem, various technologies for determining position in indoor environments without relying on satellites have been researched and put into practical use. Below, let's look in order at the main methods for obtaining coordinates indoors without using GPS.

Indoor Positioning Using Wi-Fi

This is a method for estimating position using Wi-Fi access points installed inside a building. A smartphone or dedicated device calculates approximate distances based on received signal strength and time-of-arrival differences from multiple surrounding Wi-Fi access points, and performs triangulation. Because it can leverage existing Wi-Fi infrastructure, it has the advantage of being relatively easy to deploy without adding new equipment.

However, the accuracy of Wi‑Fi positioning generally remains on the order of several meters (several ft). Signal strength is prone to fluctuation due to walls and people, and in environments where access points are not densely deployed, such as offices or commercial facilities, errors tend to be larger. Also, achieving stable positioning may require creating radio signal strength maps for each location in advance (fingerprint method), so initial setup can be time-consuming. While it offers the convenience of immediate use, it is a field where further technological improvements are expected to enhance accuracy.

Indoor positioning using BLE beacons

BLE beacon (Bluetooth Low Energy beacon) is a small wireless device that emits a low-power Bluetooth radio signal at regular intervals. Beacons are installed at various points within a building, and a device such as a smartphone receives their signals to estimate position. Methods include estimating distance from received signal strength and trilateration using signals from multiple beacons.

The advantage of BLE beacon positioning is that beacon devices are compact and low-power, making them easy to install and to replace batteries. Because their signals are specialized for shorter ranges than Wi‑Fi, when placed appropriately they can determine position with relatively high accuracy within a few meters (within a few ft). They are also used in location-based services, such as marketing that links with smartphone apps to send push notifications in specific areas.

On the other hand, the challenges are that the range over which radio waves reach is narrow (typically a radius of several meters (several m (several ft))), so many beacons must be installed to provide coverage, and that accuracy can become unstable due to radio interference and reflections. Even so, because improved accuracy can be expected compared with Wi‑Fi positioning, adoption of beacons for indoor navigation and asset management in commercial facilities, exhibition halls, hospitals, and similar venues is increasing.

Indoor positioning using UWB (Ultra Wideband radio)

UWB (Ultra Wideband) is a communication technology that uses radio over an extremely wide frequency band and can achieve very high positioning accuracy on the order of tens of cm (tens of in). In UWB-based indoor positioning, two-way communication is performed between fixed stations (anchors) installed indoors at regular intervals (for example, about 30 m (98.4 ft) apart) and mobile stations (tags) attached to people or objects, and positions are calculated by measuring differences in radio signal arrival time and angle. Because radio timing can be measured at the nanosecond order, a characteristic of UWB is its very high theoretical ranging accuracy.

The advantage of UWB positioning is that it can achieve top-class accuracy among the indoor positioning technologies currently available. Experiments have demonstrated highly accurate tracking that stays within approximately 15 cm (5.9 in) of error, and applications are beginning in areas such as position control of AGVs (automated guided vehicles) within factories, motion analysis of athletes, and worker safety management at construction sites. UWB is also more resistant to interference than other wireless technologies, which provides the benefit of stable positioning.

The drawbacks include the relatively high cost of deploying dedicated tags and anchor devices, and the effort involved in building the installation environment. To cover an entire large floor, multiple anchors must be attached to the ceiling or similar structures, and initial calibration is also required. For this reason, UWB tends to be adopted only in cases where high accuracy is essential (for example, when tracking people or objects with errors within tens of centimeters (cm-level accuracy (half-inch accuracy))).

Indoor Positioning Using RFID Tags

RFID (Radio Frequency ID) technology is also used indoors to determine the whereabouts of objects and people. RFID is a system in which small IC tags carry identification information and are read wirelessly by a reader. Rather than "measuring" position, it is primarily used to detect where tagged items have passed and in which area they are located.

For example, by installing RFID readers on shelves or at entrances and exits in a warehouse and reading RFID tags attached to inventory, you can record that an item has passed that location. The read range is limited to about a few cm (a few in) to a few m (a few ft), but it is effective for "area detection" and "entry/exit management" where precise positioning is not required. RFID tags themselves are inexpensive and can come as sticker-like labels, and they are widely used for asset management and logistics tracking.

Positioning with RFID is closer to detection events than to real-time coordinate measurements, so it is not suitable as a means of continuously obtaining high-precision coordinates. However, because tags can store various information and require no batteries—allowing them to be used semi-permanently—they are important as a complement to other positioning technologies.

Positioning by Pedestrian Dead Reckoning (PDR)

PDR (Pedestrian Dead Reckoning) is a method for estimating relative position using accelerometers and gyroscopes built into smartphones and similar devices. It sequentially measures the acceleration and rotation while a person carrying a smartphone moves, accumulates traveled distance and direction, and calculates the current position. For example, if the starting point is known, it’s like updating the coordinates relatively: "you moved 5 m (16.4 ft) east from there, then 3 m (9.8 ft) north."

The advantage of PDR is that it can be completed solely on the device without relying on external infrastructure. Since it can update its position using only sensor measurements even inside buildings where GPS is unavailable, it is applied to navigation in underground shopping areas and inside buildings. Moreover, by combining altitude sensors and barometers, movement between floors (use of elevators or stairs) can also be detected.

However, PDR has the problem of cumulative error. Because accelerometers always contain tiny errors, the estimated position gradually drifts over time. When walking long distances, it is not uncommon for the drift to reach several meters or more (several ft or more), and at present it is difficult to use PDR as a completely standalone method. Therefore, in practice it is common to combine it with other methods, periodically calibrating (resetting) the position. For example, one might track movement with PDR and correct the current position when beacons or QR codes placed at key points are read.

Positioning with IMES (Indoor Messaging System)

IMES (Indoor Messaging System) is a unique indoor positioning technology originating in Japan. It aims to provide positioning information indoors where radio signals from satellites do not reach, using a mechanism equivalent to GPS. Specifically, IMES transmitters installed indoors, such as on ceilings, emit pseudo-signals in the same format as GPS satellites. These signals encode information such as the transmitter’s latitude, longitude, altitude, and floor, and when a compatible receiver (such as a smartphone) receives them, it can determine its position as if it had received a GPS satellite.

The advantage of IMES is that it allows users to handle outdoor GPS positioning and indoor positioning seamlessly. In theory, the same device can continue to obtain position even after entering a building, and the position representation is unified in latitude and longitude. However, a current challenge is that general smartphones cannot receive IMES signals by default. Special receiver modules are required, and because compatible devices are extremely limited, adoption has not progressed. Also, installing transmitters entails costs and the need to secure power, so in reality there are few deployments outside large-scale facilities.

Indoor positioning using geomagnetism

There is also a method that estimates position by using fluctuations in the building's geomagnetic field (magnetic field). Concrete structures incorporate rebar and steel frames, which slightly disturb the Earth's magnetic field, causing distinctive magnetic field strengths and patterns at different locations. In geomagnetic positioning, magnetic field data are measured in advance at various points throughout a facility to create a map, and the current magnetic field information captured by a smartphone's magnetic sensor is matched against that map to infer the location with a similar pattern.

The advantage of this method is that it can utilize building-specific magnetic field data without installing new equipment. Once a detailed magnetic field strength map has been created, stable positioning is possible provided the environment does not change significantly. In experiments, there are cases in which magnetic field data were acquired in real time with a smartphone and the current location within a floor could be identified with an accuracy within 2 m (6.6 ft). It can be considered a relatively low-cost approach that can be realized using only existing smartphone sensors.

One drawback is that it is easily affected by changes in the surrounding environment. When a large vehicle passes nearby or an elevator moves, the magnetic field can be disturbed, temporarily reducing accuracy. In addition, initial work is required to measure and database the magnetic field patterns for each building, which can be time-consuming in large facilities. Nevertheless, as a unique method that can achieve relatively good accuracy without using radio waves, geomagnetic positioning is also being put into practical use.

Indoor positioning using **ultrasound**

ultrasound-based indoor positioning has also been studied. ID information and other data are encoded onto high-frequency sound waves that are inaudible to humans; these are transmitted from ceiling speakers and received by microphones on smartphones and similar devices to determine position. Although sound waves are easily blocked by walls, they can accurately cover a small area when the space is divided into rooms.

An advantage of ultrasonic positioning is that, because smartphones are always equipped with microphones, a dedicated receiver is unnecessary. On the transmitting side, only a speaker needs to be installed, and, unlike lighting or Wi-Fi, no radio-use license is required. With proper system configuration, position detection can reportedly be achieved with an error of about 1-2 m (3.3-6.6 ft).

However, as drawbacks, it may be subject to interference in environments with other noise or devices using ultrasound, and because sound waves cannot reach beyond walls or shelves, it can only perform positioning within line of sight. In addition, because speakers require a power supply, securing installation locations and wiring is also a challenge. Currently, it is mainly adopted experimentally for specific uses, but when the environment is properly prepared it is a method that can deliver stable accuracy.

Indoor positioning using visible light

Visible Light Communication technology can also be applied to determine position. This works by making LED lights used for illumination blink at high speed (appearing as continuous light to humans) to transmit signals, and by reading those light patterns with a smartphone camera or a dedicated sensor to figure out one's location. Each light is given a light pattern embedding a unique ID or location information, and the receiver determines "which light it is under".

The advantage of visible light positioning is that it can leverage the existing lighting infrastructure that is always used in buildings. Although lighting fixtures need to be replaced with compatible LEDs, once installed you can obtain a position using only a smartphone within the area illuminated by the lights, without any other equipment. Because light does not pass through walls, it allows clear location determination for each area, and positioning error can also be kept relatively small, on the order of tens of centimeters (tens of in) to about 1 m (3.3 ft).

The challenges are the cost of replacing existing lighting with dedicated LED fixtures and the fact that positioning becomes impossible in shaded areas where light does not reach. Also, because the smartphone must have its camera activated, there are hurdles in terms of power consumption and usability. Even so, because it does not use radio waves, it experiences less interference and is attracting attention as one of the means to achieve high-precision indoor floor navigation.

Positioning Using Cameras and AR Technology

In recent years, the advancing AR (augmented reality) technology is being used to estimate self-position from smartphone camera images, and this method is also revolutionizing indoor positioning. Smartphone AR capabilities (ARKit, ARCore, etc.) can track the device's movement in space while analyzing features of the surroundings captured by the camera. This is a technology similar to what is known in robotics as Visual SLAM (simultaneous localization and mapping), and it can accurately capture the device's relative movement by detecting floor and wall patterns and objects without markers.

The strength of AR positioning is that it enables smooth real-time position tracking indoors using only a camera and IMU sensors. For example, once you calibrate your current position at a known point (by scanning a QR code, matching an indoor map with camera images, etc.), you can thereafter simply walk with the device and it will continuously update position coordinates as if there were a virtual GPS. Because AR technology can overlay virtual objects on a smartphone, it is also used for visually intuitive position guidance, such as "AR-assisted stakeout and layout marking" that aligns positions on blueprints with the real environment, and indoor navigation features that overlay arrows indicating the direction of travel.

However, there are caveats to camera-based self-position estimation. First, because sufficient ambient light and features are required, rooms with only plain white walls or very dark locations do not provide good accuracy. Also, like PDR, this tracks relative position, so errors gradually accumulate during long-duration or long-distance movement. Therefore, it is desirable to periodically reset (recalibrate) the AR space at known points or to use it in combination with other sensor information. Even so, the fact that relative accuracy on the order of several centimeters to several tens of centimeters (several in to several tens of in) can be achieved with just a smartphone and without special equipment is a major attraction, and experimental introduction of AR surveying using tablet devices has begun at cutting-edge construction sites.

Points for Selecting Indoor Positioning Technologies

Above, we introduced representative methods of indoor positioning that do not rely on GPS. Each method has its own strengths and weaknesses, and the most suitable technology depends on the application and environment. When obtaining high-accuracy position information indoors, it is important to select and combine methods while considering the following points.

• 必要な精度: The choice of technology depends on whether errors of a few meters (several ft) are acceptable or whether accuracy of tens of centimeters or less (tens of in or less) is required. For navigation purposes, Wi‑Fi or beacons may be sufficient, but construction accuracy verification will likely require high‑precision methods such as UWB or AR + ranging.

• インフラ設置の可否: Consider whether the environment allows installing many devices like beacons or UWB, or whether you want a solution that works with existing infrastructure only. In places where you cannot freely place equipment, such as museums or commercial facilities, methods that require no installation—like smartphone sensors or geomagnetic positioning—are advantageous.

• リアルタイム性: The appropriate method also differs depending on whether you need to track people or objects dynamically or measure the coordinates of specific points at particular moments. For the former, continuously locating solutions such as PDR + corrections or UWB systems are suitable. For the latter (point surveying), methods that measure one point at a time with high accuracy—such as total stations or smartphone + RTK—are options.

• コスト・運用: Budget for deployment and the operational effort cannot be ignored. Dedicated systems can be high‑precision but costly and labor‑intensive. For small sites, it is worth considering low‑cost, easy‑to‑manage solutions such as smartphone apps. If you combine multiple methods, you will also need a mechanism to integrate the data from each.

At present, there is no indoor positioning technology that is perfect on its own, so in practice it is common to combine multiple methods according to environmental conditions. For example, you might use beacon signals to obtain a rough position most of the time, and use AR for precise alignment when more detail is required; by using them complementarily in this way you can improve both reliability and accuracy. Choosing the optimal method based on your objectives and constraints is the most efficient way to obtain high-precision coordinates indoors.

Simple surveying with LRTK (smartphone + RTK: a new method)

Finally, as an innovative positioning method that has appeared in recent years using smartphones, we introduce LRTK-based simple surveying. LRTK is a solution that makes it easy to use high-precision GNSS positioning in the real-time kinematic (RTK) mode on smartphones. By attaching a small RTK-capable GNSS receiver to a smartphone or tablet, centimeter-level positioning that previously required dedicated equipment can be achieved on mobile devices.

An external LRTK receiver that can be attached to an iPhone is a device weighing only a few hundred grams that connects to a smartphone via Bluetooth or a cable. It performs RTK positioning outdoors in clear weather and in locations such as by a building’s window or on a rooftop where satellite signals can be received to some extent, allowing you to obtain the latitude, longitude, and elevation of that point in real time on your smartphone screen. Its accuracy is within a few centimeters (a few cm (a few in)) horizontally, which is sufficient for public surveying. Because the acquired coordinates can be instantly saved and shared to the cloud, the time lag of taking field-measured data back to the office for processing can be greatly reduced.

The advantage of LRTK is that surveying work can be easily completed by a single person without using an expensive total station (surveying work can be easily completed by a single person). Verifying survey points indoors and outdoors, which traditionally required two people, can now be finished simply by tapping points with a smartphone in hand. Also, since the latest smartphones are equipped with LiDAR scanners and high-performance cameras, when combined with LRTK they can capture 3D point cloud data of the surroundings on site and use AR features to project positions from design plans into real space for verification. In other words, the smartphone itself becomes a high-precision positioning device and scanner (the smartphone itself becomes a high-precision positioning device and scanner), dramatically improving the efficiency of on-site data measurement and positioning work.

However, it should be noted that RTK-GNSS technologies, including LRTK, fundamentally rely on satellite signals from above, so positioning becomes difficult if you go deep indoors. That said, even inside buildings, in places where the sky is partially open—such as near windows or on rooftop floors—it may be possible to maintain positioning by receiving augmentation signals from the Japanese Quasi-Zenith Satellite System (Michibiki). Also, by first measuring a reference point outdoors and then linking relative positions indoors using a smartphone’s AR function, it is possible to continue surveying from the reference point even deep inside buildings. In this way, LRTK has the potential to dramatically simplify the traditionally laborious "indoor coordinate acquisition," and it is attracting attention at various sites, especially in the construction industry.

FAQ

Q1: Why is GPS hardly usable indoors? A1: Because radio signals from GPS satellites are blocked or reflected by building walls and ceilings. Indoors, you cannot directly receive a sufficient number of satellite signals to obtain the information needed for position calculation. Even if signals are received weakly, multipath errors are large and an accurate position cannot be determined. As a result, GPS alone cannot provide practical accuracy indoors.

Q2: Which method provides the highest accuracy for indoor positioning? A2: It depends on conditions, but with dedicated equipment, UWB can achieve very high accuracy of tens of cm (tens of in). Also, using a total station makes it possible to measure coordinates with millimeter-level precision (mm; 0.04 in). However, considering operational difficulty, approaches like a smartphone + RTK can achieve accuracy on the order of a few cm (a few in), which is practically sufficiently high. Choose the method that meets the required accuracy depending on the intended use.

Q3: Can you accurately determine indoor position using only a smartphone's sensors? A3: There is PDR technology that estimates relative movement using built-in smartphone accelerometers and gyros, but used alone errors gradually accumulate. For that reason, it is difficult to determine position for long periods and with high accuracy using only smartphone sensors, and it is necessary to periodically correct the current location with other methods. On the other hand, if you combine a smartphone camera with AR technology, you can track indoor movement quite accurately for short periods. The point is that by skillfully combining smartphone sensors so they complement each other, you can ensure practical accuracy.

Q4: Can RTK-GNSS be used indoors? A4: In deep indoor areas completely covered by a roof, even RTK cannot receive GNSS signals, so it is basically unusable. However, inside buildings, locations near windows or openings may be able to see satellites, allowing RTK positioning there. In Japan, satellite augmentation signals (QZSS CLAS) are also available, and correction information can be received directly even when outside communication coverage. Therefore, if conditions are right, RTK can be used in parts of indoor spaces. If that is difficult, you can maintain high accuracy by switching to relative positioning indoors (e.g., AR or laser ranging) based on reference points obtained outdoors.

Q5: What method do you recommend for easily measuring indoor coordinates? A5: If ease of use is the priority, I recommend leveraging smartphone apps that do not require special infrastructure. For example, using a smartphone AR surveying app or LRTK-like solutions that combine an external GNSS receiver can allow you to obtain fairly accurate coordinates while working alone. In small indoor spaces, simple surveying is possible by first setting a reference point with your smartphone, then using the AR display to guide you to the points you want to measure and recording them. Since you can get started without expensive equipment or extensive preparation typical of conventional methods, it’s a good idea to consider these approaches first.