RTK coordinate systems used in construction: Differences between State Plane / UTM / Local grid

Table of Contents

• Introduction

• What is RTK

• What is a coordinate system

• State Plane coordinate system

• UTM coordinate system

• Local grid coordinate system

• How to choose a coordinate system on construction sites

• Simple surveying with LRTK

• FAQ

Introduction

In recent years, ICT construction and the digitalization of surveying have advanced in the construction industry, and high-precision positioning using RTK (real-time kinematic) is increasingly being used on sites. With RTK-GNSS, positions can be measured with errors of a few centimeters (a few inches), dramatically improving efficiency and accuracy in staking out batter boards, as-built control, and machine guidance for heavy equipment. However, to use positioning data obtained by RTK correctly, it is important to understand “which coordinate system is used to represent positions.” Even measurements of the same point can shift by several meters (several ft) to tens of meters (tens of ft) depending on the coordinate system used, which may cause mismatches with design drawings or other survey data.

In construction projects, public surveying coordinate systems defined by national or local governments (for example, Japan’s plane rectangular coordinate system based on JGD2011) or the globally used UTM coordinate system may be used. In cases where the relationship to surrounding locations is not important, it is also common to use a local grid (local coordinate system) with an arbitrary origin and orientation. This article explains the characteristics and differences of the three coordinate systems frequently discussed in RTK surveying for construction—the State Plane coordinate system, the UTM coordinate system, and the local grid coordinate system. It also provides guidance on which coordinate system to choose on site, and at the end introduces a simple surveying method using the latest RTK tool LRTK.

What is RTK

RTK stands for Real-Time Kinematic and is a high-precision positioning technique using GNSS (global navigation satellite systems). While standalone GPS can have errors of several meters (several ft), RTK uses two receivers—a base station (reference) and a rover—and compares their observations in real time to cancel errors and obtain positions with centimeter-level (half-inch accuracy). Specifically, the base station is set up at a point with known coordinates, the GNSS signal errors at that location are calculated and sent to the rover as correction information, and the rover uses this to correct its own positioning results. Whereas conventional smartphone GPS can have errors on the order of 5–10 m (16.4–32.8 ft), RTK can improve accuracy to about 1–2 cm (0.4–0.8 in).

Such high-precision positioning is becoming indispensable for accurate setting-out and as-built control of ground and structures in civil surveying and construction management. For example, RTK surveying is useful for foundation stake-out (batter boards), road alignment surveys, and checking pavement thickness. RTK technology is also being applied widely in ICT construction (machine guidance/machine control) that equips excavators and bulldozers with RTK-GNSS for automated control, drone photogrammetry, and precision agriculture such as auto-steering tractors.

What is a coordinate system

A coordinate system is a reference used to express positions on the Earth numerically. Coordinate systems range from global to highly local. Fundamentally, a coordinate system defines “which reference surface and reference point are used as the origin, and which units and axes are used to express position.”

For example, the world geodetic system adopted by GPS (WGS84) expresses position by longitude, latitude (degrees), and height referenced to the center of the Earth. This is called a geographic coordinate system (spherical coordinate system) and can uniformly represent any point on Earth, but because it uses angular measures it is not convenient for distance measurement on maps or for design drawings. Therefore, projected coordinate systems (planar coordinate systems) that transform latitude/longitude into planar XY coordinates are used.

In a projected coordinate system, a projection surface covering a region is set and X (east–west) and Y (north–south) coordinate values are defined in units of true length such as meters. Distortion due to the Earth’s curvature occurs, but by limiting the area the distortion is kept as small as possible. Examples designed to minimize distortion by establishing projection origins and axes for each region include Japan’s plane rectangular coordinate system defined by the Ministry of Land, Infrastructure, Transport and Tourism and the United States’ State Plane Coordinate System. Conversely, the global UTM coordinate system divides the Earth into 60 zones to cover the whole world. The finer the zone division, the higher the accuracy within each zone, but the less universally applicable the system becomes.

For very limited coverage such as a construction site, it is also common to ignore the Earth’s curvature and treat the site as a true plane, using a local coordinate system with an arbitrary origin. Strictly speaking this is not based on a geodetic datum, but for small sites it can be used with practically sufficient accuracy. Thus, choosing an appropriate coordinate system according to purpose and coverage is important for effective use of survey data.

State Plane coordinate system

The State Plane Coordinate System is a type of planar coordinate system mainly used in the United States. The U.S. is divided into more than 130 zones by state (and in some cases subdivided within a state), and each zone is designed to minimize projection distortion within that zone. Specifically, each zone has its own projection method and parameters; for example, Lambert conformal conic is used for states that are wide east–west, and transverse Mercator is used for states that are long north–south. The reference datum is NAD83 (the North American Datum of 1983) or similar, and many zones are adjusted so that the difference between planar distance and ground distance is less than 1/10,000.

The State Plane coordinate system is an official coordinate system used by states and local governments for surveying and civil engineering, and it is widely used for land parcel surveys and urban infrastructure design. Coordinates are usually expressed as large values unique within a state (for example, on the order of hundreds of thousands to millions of meters (hundreds of thousands to millions of ft)), and in the U.S. coordinates may be provided in feet due to the influence of the imperial system. While Japan does not use this system, conceptually it is similar to Japan’s plane rectangular coordinate system (Japan divides the country into 19 zones, whereas the U.S. divides by state and often further subdivides). Using the State Plane system allows high-precision, standardized management of survey results within each state, but when crossing state boundaries you must perform coordinate transformations between adjacent zones.

UTM coordinate system

The UTM coordinate system is a universal projected coordinate system covering the whole world. UTM stands for Universal Transverse Mercator, and as the name implies it is based on the transverse Mercator projection. The Earth is divided into 60 longitudinal zones each 6 degrees wide, and each zone uses a transverse Mercator projection centered on its central meridian. Zones are numbered 1–60; for example, Honshu in Japan is largely in UTM zone 54, and Hokkaido is in zone 53. The datum used for the projection can be WGS84 or national datums (in Japan, JGD2011, etc.).

UTM defines coordinates relative to the equator and the central meridian of each zone, expressing X (easting) and Y (northing) in meters. The scale factor at each zone’s central meridian is set to 0.9996, so accuracy is high near the zone center but distortion increases toward the zone edges (although for typical map use this is not problematic). Because UTM is standardized worldwide, it is widely used for international geographic data exchange and continental-scale mapping. For example, the U.S. military’s MGRS grid is based on UTM, and UTM is a standard coordinate reference in the GIS field.

At the construction site level, UTM is not often used directly even for public works, but it is adopted for wide-area infrastructure planning and surveying. Also, specifying UTM as the coordinate system for drone photogrammetry so that results align with global geodetic maps can make later data use easier. The important point is to know which UTM zone and datum your data are expressed in and to be able to convert to other coordinate systems (for example, plane rectangular coordinates or local coordinates) as needed.

Local grid coordinate system

A local grid coordinate system (local coordinate system) is an arbitrary coordinate system that is valid only within a particular site or project. Unlike public coordinate systems, it is not necessarily based on a geodetic datum, so it cannot be used elsewhere, but it has the advantage of being highly convenient within the site. In most cases, a convenient point on the site (for example, a corner or the center of the site) is chosen as the origin and given provisional coordinate values (e.g., X=10000 m, Y=10000 m), and the coordinate axes are oriented to align with major site directions (property boundaries or building orientation). Thereafter, survey data are expressed as planar rectangular coordinates along those axes. With a local coordinate system the coordinate values on site remain relatively small and simple, making it intuitive to understand spatial relationships on drawings.

Local coordinate systems are often used for works that do not need to consider relationships to surrounding public control points (for example, building work within a site that does not connect to existing infrastructure). In large public works, it is also common to plan initially in local coordinates and then convert to a public coordinate system when preparing submission documents. The biggest advantage of a local coordinate system is that site coordinates are simple and easy to handle, and distances measured on the ground and distances between coordinates on drawings almost match. In normal projected coordinate systems there is a slight scale error—for example, two points 100.000 m (328.084 ft) apart on the ground might appear as 99.996 m (328.071 ft) in coordinates—but if the site is treated as a plane in a local coordinate system you do not need to worry about such differences.

However, because a local grid coordinate system is site-specific, it is not compatible with other coordinate systems. If you need to overlay site-obtained coordinates with public maps or other survey data, you must perform coordinate transformation (localization) to relate them. Specifically, survey several points on site whose public coordinates (for example, plane rectangular coordinates) are known, then compute the translation, rotation, and scale differences relative to the local coordinates. Modern RTK-GNSS receivers and surveying software often include a “site calibration” (localization) function, allowing easy one-touch calibration of the local coordinate system using known points. By reassigning site-specific local coordinate values to global coordinates such as latitude/longitude obtained from RTK positioning, you can align survey results with design drawings and other survey data at high accuracy.

How to choose a coordinate system on construction sites

Finally, here are points to consider when choosing an appropriate coordinate system on a construction site.

Consistency with existing references: At project start, check which coordinate system design drawings and documents for government submissions are based on. For example, public works in Japan often specify the plane rectangular coordinate system (zone ◯), while U.S. projects may use the State Plane coordinate system. If design drawings or control points are already provided in a specific coordinate system, it is generally safest to follow that system.

Site scale and location: If the site covers a very wide area or spans multiple zones, a single local coordinate system may be inadequate. In such cases it is better to use a public coordinate system (plane rectangular coordinates or UTM) to unify the area. Conversely, for a small site such as a single building lot where the relationship with surroundings is not critical, a local coordinate system may simplify site work.

Accuracy and distortion: The required accuracy also affects the choice. Local coordinate systems are excellent for relative accuracy within a site but may include small offsets in absolute position or orientation relative to geodetic datums. If strict conformity to public surveying standards is required (for example, connecting to adjacent works or comparing precisely with satellite positioning data), measure in a public coordinate system from the start or work on the assumption that localization will be performed later.

Convenience and work efficiency: Consider which coordinate system site surveyors and construction managers are accustomed to. If everyone understands plane rectangular coordinates, standardizing on that system is fine. If large coordinate values are cumbersome, showing local coordinates on drawings in addition can help. The key is to use a consistent coordinate reference within the project and clearly define procedures for converting between different coordinate systems. When exchanging survey data, always share which coordinate system the data use, whether units are metric or feet, and which control points were used. This prevents major mistakes from coordinate mix-ups.

With these points in mind, conduct RTK surveying on site after selecting and setting a coordinate system that matches the project purpose. Fortunately, recent positioning equipment and software have extensive coordinate transformation functions, and modern tools like LRTK make it easy to switch coordinate systems on site and localize using known points.

Simple surveying with LRTK

Even advanced RTK technology only shows its full value when coordinate systems are properly understood and configured. However, on actual sites many worry that RTK equipment is difficult to operate or that coordinate transformations are complex and require expertise. A notable solution is LRTK, a simple surveying solution provided by Lefixea.

LRTK is a groundbreaking tool that allows anyone to easily use RTK positioning simply by attaching a small, high-precision GNSS receiver to a smartphone or tablet. The pocket-sized dedicated device (LRTK Phone) weighs only a few hundred grams and obtains RTK correction information via the internet from regional reference station networks and satellite augmentation signals, displaying centimeter-level (half-inch accuracy) positioning results in real time on a smartphone app. Even without special training as a surveyor, users can measure their current position by pressing a single button in the app, dramatically streamlining tasks like staking and leveling.

The LRTK app also strongly supports handling coordinate systems. It automatically converts the measured latitude/longitude and ellipsoidal height to XY coordinates in a selected coordinate system (for example, JGD2011 plane rectangular coordinates in Japan or UTM coordinates in the global geodetic system) and displays them. It also automatically computes orthometric heights considering geoid correction, so you do not need to worry about cumbersome height corrections on site. Measurement data can be uploaded to the cloud with one button, allowing office staff to instantly check site survey points from a web browser and overlay them on design drawings to verify as-built conditions.

By using LRTK, advanced RTK surveying becomes accessible through intuitive operation. You can quickly measure necessary points and mark-outs on site without being troubled by complex coordinate system differences. Especially on small sites or in construction management where surveying occurs frequently, LRTK can be a strong ally as a “one-measuring-unit-per-person” tool. Modern technology has made it possible to utilize high-precision position information without relying on complex coordinate transformations or specialist knowledge.

FAQ

Q: If coordinates on design drawings differ from coordinates measured with RTK, what are the main causes? A: The most likely cause is that the coordinate systems used by the drawing and the survey differ. For example, if the drawings are based on a public coordinate system (plane rectangular coordinates) but RTK results are compared in raw geographic coordinates (latitude/longitude), large position discrepancies will occur. Differences in datums (for example, WGS84 vs. JGD2011) or unit systems (mixing feet and meters) also cause offsets. As a countermeasure, first confirm the coordinate system used for the drawings, then transform the RTK measurements to that same coordinate system (or output in that system directly during RTK measurement). Using known control points and performing localization is also an effective method.

Q: Can overseas coordinate systems such as State Plane or UTM be used within Japan? A: Mathematically they can be used. UTM is a global standard, so if applied in Japan, Honshu would fall in UTM zone 54N and Hokkaido in zone 53N, and UTM grids sometimes appear on maps from the Geospatial Information Authority of Japan. However, public surveys in Japan typically use JGD2011 plane rectangular coordinates (zones 1–19), so UTM is not commonly used in domestic practice. The State Plane coordinate system is specific to the U.S. and is not directly relevant to surveying in Japan. You are unlikely to encounter the term “State Plane” domestically; plane rectangular coordinates or geographic coordinates are generally used. That said, if participating in overseas projects or handling foreign survey data, knowledge of those foreign coordinate systems and skills to convert them to Japanese systems are required.

Q: What should be noted when using a local coordinate system on site? A: First, clearly document how the local coordinate system is defined. Share with the team which point on site is the origin, which directions are the X and Y axes, and what coordinate values were assigned to the origin. Also, to prepare for later alignment with public coordinate data, secure at least 2–3 known control points on site whose public coordinates are known and record their local coordinates. This allows accurate localization to link local and public coordinates when needed. When providing drawings or data to others, state clearly that “these coordinates are in a site-specific local coordinate system” to avoid confusion with public coordinates.

Q: I want to start RTK surveying—can beginners handle it? A: Yes. In recent years, RTK equipment and services that are easy for beginners to use have increased. Previously, expensive RTK equipment and self-installed base stations were required, but now regional reference station networks and satellite augmentation signals make high-precision positioning accessible. Smartphone-linked solutions like LRTK allow intuitive app operation to complete positioning and data management, making it easier for those with limited expertise to get started. However, we recommend learning basic concepts of coordinate systems and surveying safety measures. Because RTK is highly accurate, configuration mistakes can lead directly to errors. Use the coordinate system concepts explained in this article, start with simple sites, and gain practical experience—beginners can effectively make use of RTK surveying.