Exporting RTK Points to CAD (DWG/DXF): How to Deliver Them Without Coordinate Issues

Table of Contents

• Introduction

• What is RTK?

• Basics of CAD drawings and coordinate systems

• Causes of coordinate troubles

• Points to prevent coordinate troubles

• Procedure for exporting RTK data in DXF/DWG format

• Simple surveying with LRTK

• FAQ

Introduction

Have you ever had trouble when importing coordinates of points obtained by high-precision GNSS positioning (RTK) into CAD drawings because the positions didn't match? Even if you survey with centimeter-level accuracy, if points are not displayed in the correct locations on the drawing due to coordinate shifts, it can lead to serious mistakes and rework. To prevent such "coordinate troubles", it is important to correctly understand the differences between the RTK positioning coordinates and the coordinate system of the design drawings, and to take appropriate measures. In this article, we explain the causes of coordinate shifts when exporting points obtained by RTK to CAD (DWG/DXF formats) and how to prevent those troubles. The points are organized clearly so that those involved in surveying and design can exchange data with confidence.

What is RTK?

RTK (Real Time Kinematic) is a real-time high-precision positioning technique that uses GNSS (satellite positioning). By comparing via communications the GNSS data from a fixed reference point that acts as a base station and a rover observed while moving, and instantly correcting errors, it can reduce the errors of several meters typical in standard GPS to a few centimeters. In recent years, RTK has been increasingly used across a wide range of fields such as drone surveying, as-built management at construction sites, and agriculture, and alongside traditional total station surveying it is becoming a leading method for field surveying.

Coordinates obtained by RTK positioning are fundamentally based on a global-scale geodetic datum (world geodetic system). When using network RTK services commonly used in Japan, the results are often obtained as geographic coordinates (latitude and longitude) or plane rectangular coordinates in the Japan Geodetic Datum 2011 (JGD2011), but depending on the settings they may also be output in global coordinates such as WGS84 latitude/longitude and ellipsoidal height. In any case, the values provided by RTK are Earth-referenced absolute coordinates. On the other hand, if the coordinate system used in site design drawings or CAD data differs from this, the numeric values will not match even if they indicate the same point. In the next chapter, we will look at the coordinate systems used in design drawings.

Basics of CAD Drawings and Coordinate Systems

To express positions on design drawings or CAD data, a coordinate system (reference coordinates) is established. In civil engineering and architectural drawings, a public coordinate system defined by the Geospatial Information Authority of Japan (for example: the plane rectangular coordinate system of JGD2011) may be used, but it is also common to use site-specific local coordinates (local coordinate system). For example, one corner of a site might be set as a temporary origin (0,0), and the directions of the X and Y axes chosen arbitrarily when creating drawings. In drawings created using such local coordinate systems, the values will usually differ by orders of magnitude from those expressed in a public coordinate system. In an extreme example, a point shown on a drawing as "(120.00, 50.00)" could correspond to approximately "(200000, 50000)" when represented in JGD2011 coordinates, and they would not match visually at all.

Which coordinate system the design drawings use is indicated in the drawing legend or in the annotations of the survey results. For example, it should be explicitly stated as "Coordinate system: JGD2011 plane rectangular coordinate system, zone ○" or "local coordinates with ○○ control point as the origin." If the drawings do not indicate the coordinate system, be sure to confirm with the designer or the client. Understanding the reference coordinates used in the design is the first step to preventing discrepancies in later stages.

Also, be careful about the unit system. When obtaining coordinates from CAD data, values may be in millimeters (mm (in)). Because surveying uses meters (m (ft)), overlooking this difference can cause values to be misinterpreted by a factor of 1,000. For example, if a CAD point's coordinates are "12000, 5000" (12000 mm (472.44 in), 5000 mm (196.85 in)), converting to meters yields "12.000 m (39.370 ft), 5.000 m (16.404 ft)", and confusing the units can cause large errors. Therefore, always confirm the drawing data's unit (m or mm) and convert it as necessary.

Causes of Coordinate Troubles

Now, let's organize the main causes of the "coordinate shift" that occurs when overlaying positioning data obtained by RTK onto CAD drawings. The following factors can cause situations where the same point does not match between the on-site drawing and the RTK values.

• Coordinate system differences: The main cause is that the RTK coordinate system and the coordinate system of the design drawings are different. RTK positioning results are latitude/longitude or absolute coordinates based on global geodetic systems (JGD2011, WGS84, etc.), whereas drawings may use local orthogonal coordinates (coordinates based on an arbitrary origin and orientation), so it is natural that the numerical values do not match even for the same point. Also, even if both use a public coordinate system, if the system (zone) number is different the origins will be different, causing a large offset (discrepancy).

• Differences in geodetic datum: In Japan, since 2002 the World Geodetic System (JGD2000/2011) has been the official standard. However, older drawings and cadastral maps may use the former Japanese geodetic system (Tokyo Datum), which has a different reference from the current World Geodetic System. Because there is a systematic offset on the order of several hundred meters between the old datum and the World Geodetic System depending on the region, directly comparing coordinate values based on the old datum with current RTK positioning results will produce large discrepancies.

• Unit differences: As mentioned above, if the coordinate values on the drawings use different units—meters (m) and millimeters (mm)—the numerical values can be off by a factor of 1000. Misunderstandings caused by insufficient checking of the unit system can result in positioning points being plotted in completely incorrect locations.

• Insufficient localization: Failing to perform localization (coordinate alignment) to match RTK positioning values to the site's local coordinate system can also cause offsets. For example, if GNSS-derived absolute coordinates are applied to drawings that were drafted with the site's own origin and orientation without any correction, the measured points can be plotted tens to hundreds of meters (tens to hundreds of ft) away on the drawing.

• Axis rotation and scale differences: Even if you align coordinates at a single point, if the site's coordinate axes are rotated relative to north or the scale is different, the discrepancy increases with distance. Especially for large sites or where the drawing's XY axes are tilted from true north, matching a single point is insufficient. Unless you correct angle and scale using multiple points, large positional errors will occur farther away.

As a result of the causes described above, a "coordinate mismatch" can occur in which the coordinates of points obtained by RTK do not match the design data in CAD. The next section explains key points for preventing these problems in advance and for correctly transferring positioning data.

Key points to prevent coordinate issues

To ensure RTK positioning data and the coordinates in the design drawings match correctly, keep the following points and procedures in mind.

• Pre-check of the design coordinate system: Before starting surveying, confirm the coordinate system used for the project. Determine whether the drawings are in Zone ○ of the JGD2011 plane rectangular coordinate system, in the old geodetic datum, or based on a site-specific local coordinate origin. Also confirm the units of the coordinate values (m (ft) or mm (in)) and the elevation reference (e.g., elevation above sea level). Share this information in advance with the client and the design personnel, and ensure a unified understanding on site.

• Coordinate system settings for positioning data: Review the settings of the RTK receiver and surveying software, and, where possible, obtain positioning data in a coordinate system that matches the design drawings. When using Japan's network RTK services, select the applicable plane rectangular coordinate system's zone number in the receiver's coordinate output settings, or apply transformation parameters if the old geodetic datum is required. Also, if you have your own base station, it is important to set that reference station's reference coordinates in advance to accurate public coordinate values. If you can acquire positioning data in a system close to the design coordinate system (public coordinates) in advance, subsequent correction work will be much easier.

• Localization using known points (coordinate alignment): Use known points on site (control points or boundary stakes whose accurate coordinates are known in advance) to align positioning coordinates to the site's coordinate system. Specifically, observe at least two known points (preferably three or more) with RTK, and compare the GNSS-derived coordinates with the known point coordinates on the drawings. Then calculate offsets in the east‑west and north‑south directions (planar translations), rotation angle, and, if necessary, scale differences, and apply corrections to the positioning results. This process is generally called “localization” or “site calibration (on-site correction)”. By calculating correction parameters so they fit multiple known points, all points subsequently obtained by RTK can be matched to that site coordinate system. Especially on sites using a local coordinate system, localization is the decisive process to prevent coordinate misalignment.

• Height (elevation) corrections: When including elevation (height) data in survey deliverables, pay attention to differences in vertical datums. If the elevation datum on design drawings (for example, elevations referenced to the Tokyo Bay mean sea level) differs from the datum of heights produced by RTK (ellipsoidal heights or geoid heights), the heights will not agree. A useful method is to measure a point with a known elevation—such as a benchmark—using RTK, determine the difference between the obtained ellipsoidal height and the known elevation (i.e., the local geoid difference), and apply that correction to other points. Recent GNSS receivers and software often include Japan-wide geoid models and can automatically convert to elevation, so it is advisable to make use of this functionality.

• Verification of results: After performing localization or coordinate transformation, measure other known points or validation points on site to confirm they match the coordinates on the drawings. If discrepancies of several centimeters or more appear, you need to recheck whether the accuracy of the known points used or the calculated correction parameters contain errors. If the verification shows no problems, all subsequent survey points acquired in that coordinate system will align with the design drawings with high accuracy.

• Information sharing among stakeholders: Share with not only the surveying personnel but the entire design and construction team which reference is being used to manage coordinates for this project. For example, document and communicate coordinate system conventions such as "use the ○○ control point as the origin, X-axis pointing east" or "use the ○ zone of JGD2011." This kind of information sharing helps prevent confusion when other companies or departments use the survey data in later stages.

By following the above points, you can bring the discrepancy between RTK measurement data and the coordinates on CAD drawings as close to zero as possible. In particular, coordinate alignment using multiple known points may feel cumbersome at first, but considering the prevention of construction mistakes and the effort required for later data corrections, it is an indispensable step.

Procedure for exporting RTK data in DXF/DWG format

Now, let's go over the steps for delivering point cloud data acquired by RTK to others in a CAD format. After taking the measures described above and aligning the coordinates, it will be smoother to export the data in the following order.

• Preparation of survey data: Export the coordinate data of points measured with RTK from your device or receiver. In many cases you can output it as a CSV-format coordinate list (point name, X coordinate, Y coordinate, Z coordinate, etc.). If you have aligned the coordinate system beforehand, the values in this list should already be numbers expressed in the coordinate system of the design drawings.

• Importing into CAD drawings: Import the exported coordinate list into your CAD software and plot the points. Procedures vary by CAD software, but for example you can use a dedicated point-import feature or scripts to place point objects in bulk. Set point colors, symbols, layers, etc., as needed so they can be reviewed on the drawing. It is also effective to use surveying-specific tools or GIS software to generate a DXF from the coordinate list and open that in CAD. The important thing is to plot the coordinate values faithfully without altering them. Be careful not to add offsets or change the scale during import.

• Check the unit system and drawing settings: After placing points in the CAD, check the drawing's unit system and scale settings. If the CAD software's drawing units are set to mm (in), either convert the coordinate values from meters (m (ft)) to millimeters (mm (in)), or change the unit setting to m (ft). A mismatch in units can cause the coordinates you carefully aligned to be displayed shifted again. Also double-check the drawing's overall reference point (the 0,0 position) to ensure there is no unintended origin shift.

• Save in DXF/DWG format: After placing the points correctly, save the drawing data in the specified format. Choose DXF or DWG according to the recipient's operating environment. If there is no particular specification or the recipient's CAD software is unknown, providing the file in DXF offers higher compatibility and is more reliable. If you provide it in DWG, be careful to save with version compatibility in mind. Add the site name, date, etc., to the drawing name and layer names so it’s easy to identify which data it is and keep everything well organized.

• Explicit coordinate system information: For exported CAD data, we recommend attaching a note about the coordinate system where possible. For example, annotating the drawing with "Coordinate system: JGD2011 (zone ○) / Unit: m (ft)" will reassure the recipient. Also, plot several reference or known points on the drawing and display their coordinate value labels so they can be used to verify overlays when aligning data. If you provide a coordinate list (CSV) together with the drawing data, it is helpful to specify the coordinate system, datum, and unit system within that file.

If you hand over the DXF/DWG data created by the above procedure, you should be able to share the survey results smoothly without coordinate issues. On the recipient’s side, they can open that DXF/DWG and, by overlaying it on existing design drawings or referring to the necessary locations, make use of the survey points with the correct positional relationships. If, after delivery, you receive an inquiry from the recipient that “the positions don’t match,” confirm again together whether there was a mix-up in the coordinate system or units.

Simplified Surveying with LRTK

In recent years, tools that make RTK positioning and coordinate alignment easier have emerged. For example, LRTK is a high-precision GNSS device used in conjunction with a smartphone, and it is powerful for simplified surveying on site. By simply selecting a regional public coordinate system or any local coordinate system in the dedicated smartphone app, positioning results can be converted and displayed in real time as X, Y, Z coordinates of that coordinate system, allowing coordinates to be obtained from the start according to the same reference as the design drawings. It also has a function to register known points on site in the app and perform coordinate correction (apply localization) with one touch, enabling coordinate alignment to be completed in a short time without having to think about complicated calculations. At an actual construction site, when coordinate-alignment settings were made in advance using LRTK with 2-3 reference points, subsequent as-built measurements recorded all points as aligned to the public coordinate system (design coordinate system). As a result, the obtained survey point data could be directly reflected in the final CAD drawings, eliminating the need for post-processing coordinate transformations and leading to significant efficiency gains, according to reports.

In this way, by using LRTK, anyone can easily achieve high-precision positioning and coordinate alignment even without specialized knowledge. It also offers high mobility on site, allowing a single person to carry the equipment and quickly record survey points while walking. Even those using RTK for the first time can operate it intuitively, and it will be a strong ally for experienced surveyors in shortening work time. Since you no longer have to worry about coordinate discrepancies, you can confidently import and utilize survey data in CAD. With the spread of high-precision positioning and digital construction, tools like this are expected to attract increasing attention.

FAQ

Q1. How many known points should be prepared for coordinate alignment (localization)? A. Ideally, prepare three or more known points (points whose coordinates are known). Two points can correct for planar translation and rotation, but with three points you can correct more precisely, including subtle scale differences. If you absolutely have only two or fewer known points, perform thorough post-measurement verification. Even on sites using a public coordinate system, it is reassuring to make a habit of measuring one known point before starting surveying and comparing it to the plan coordinates. If there is any discrepancy, localization should be carried out immediately to correct it.

Q2. 測った点の高さ（標高）が設計図の値と合わない場合はどう対処すれば良いですか？ A. 高さが合わない原因の多くは、RTKが楕円体高を基準にしているのに対し、設計側はジオイド高（海抜標高）を基準にしていることによるものです。この場合、地域ごとのジオイド高（ジオイド差）を調べてRTKの楕円体高から差し引くことで標高に換算できます。国土地理院が公開している日本全国のジオイドモデル（例：GSIGEO2011）を使えば高精度に変換可能です。一番確実なのは、現場の既知の水準点をRTKで測定し、その差分を他の点の高さに補正適用する方法です。例えば基準点Aの設計標高が50.000 m (164.042 ft)で、RTK測定の楕円体高が84.300 m (276.575 ft)だった場合、差し引き34.300 m (112.533 ft)がその地点のジオイド差となります。他の測点についてもRTKで得た高さから34.300 m (112.533 ft)引けば、設計と同じ基準の標高値に揃えることができます。

Q3. If I want to align new RTK survey data to coordinate data from old drawings (such as the old Japanese geodetic datum), what should I do? A. Because there is a constant offset of several hundred meters (several hundred ft) between the old datum and the current world geodetic datum (JGD2011), they will not match as-is. When using old coordinate data, you first need to perform an official coordinate transformation from the old datum to the world geodetic datum. Using the transformation parameters and software published by the Geospatial Information Authority of Japan (e.g., the “Conversion to the World Geodetic Datum” tool), you can perform high-precision batch conversions. If you cannot use the official tools, a simple alternative is to observe 1-2 known points in the old datum on site with RTK, determine the offset to the new datum, and apply that offset to all coordinates (strictly speaking, over large areas rotation and scale differences can also occur, so correction with three or more points is preferable). In any case, when handling data with different reference frames, we recommend converting them using specialized methods.

Q4. Between DXF and DWG, which format is better to provide? A. If the recipient has not specified or you do not know their software environment, it is safest to provide the files in DXF format. DXF is an exchange format that can be read by many CAD and GIS applications, so it is reliable in terms of compatibility. On the other hand, if the recipient uses a specific CAD application and the version is clear, it is acceptable to provide the files in the DWG format that corresponds to that software. DWG is a software-specific format that can fully preserve functions and data, but be careful because it may not open if the versions do not match. When in doubt, choose DXF; even when you are certain, it is advisable to confirm the recipient’s preferred format just in case.

Q5. Is it necessary to perform localization (coordinate alignment) for each project every time? A. If the coordinate system used on site or in the drawings differs between projects, we generally recommend performing localization for each project. If a site uses the same public coordinate system (e.g., a particular zone of JGD2011), you will generally be fine without localizing every time, but still make it a habit to measure one known point and compare it with the drawing coordinates before starting surveying. If there is no discrepancy you can proceed as is, but if there is any difference—even a small one—you should immediately perform localization using multiple points to correct it. Localization takes a bit of effort, but if you consider it insurance against rework caused by coordinate misalignment, it will ultimately improve overall efficiency and quality. When exchanging data between different sites in the future, always be sure to check and adjust coordinates each time.