How to Reflect Coordinates Acquired by RTK into a DXF? Conversion Steps and Precautions

Table of Contents

• Formats of coordinate data obtained by RTK positioning

• What is the DXF format?

• Differences between CAD drawing coordinate systems and RTK coordinates

• General procedure for converting RTK coordinates to DXF

• Summary

Have you ever experienced that high-precision coordinate data acquired by the RTK (Real-Time Kinematic) method did not align when imported into a DXF drawing in CAD software? Even if you achieve centimeter-level accuracy (cm level accuracy (half-inch accuracy)), if the coordinate reference is not matched, points will appear shifted on the drawing, which can lead to serious errors or rework. In addition, in civil engineering construction and GIS analysis contexts, there is a growing need to reflect survey points obtained by RTK in design drawings or to import them into maps in geographic information systems. In such cases as well, it is important to transfer data in DXF format without coordinate conversion errors. To prevent such troubles and to correctly utilize survey data in CAD drawings and GIS systems, it is important to convert RTK-acquired coordinates into DXF format following appropriate procedures. This article explains in detail the specific conversion procedures for reflecting RTK positioning data in DXF and the precautions to keep in mind when doing so.

Coordinate data formats obtainable by RTK positioning

In RTK positioning, high-precision position coordinates can be obtained in real time using a GNSS receiver or a dedicated app. The acquired coordinate data can be recorded and output in various formats. Representative examples include NMEA-format logs, CSV-format coordinate lists, or general-purpose GIS formats such as GeoJSON.

NMEA format: A standard text format output by GNSS receivers, recorded in real time as sentences (e.g., the GGA sentence) that include longitude and latitude, altitude, time information, and so on. Extracting coordinate values from NMEA logs requires parsing, but it is available as a logging option on many positioning devices and apps.

CSV format: The positioning results are saved as comma-separated text (tabular format) and contain columns for each point such as ID and coordinate values (X, Y, Z). Many instruments can export a coordinate list as a CSV file after measurement. CSV is highly versatile and can be imported by most CAD and GIS software, so it is widely used as an intermediate format. However, while CSV is convenient for simply saving a list of coordinate points, it cannot directly include geometric information such as lines connecting points or area polygons. Therefore, when using it as a drawing, you need to either draw lines in CAD software from the CSV or convert it to another format such as DXF.

GeoJSON format: A format for representing geographic information in JSON that can describe points, lines, and polygons including coordinates and attributes. Some smartphone surveying apps and cloud GIS services can output positioning data in GeoJSON. GeoJSON is convenient for integration with GIS software, and conversion to DXF can be carried out relatively easily using compatible software. Because GeoJSON can preserve the geometric structure of features in addition to coordinate data, it is suitable for visualization and analysis on maps. However, since it is difficult to handle directly in CAD software, it is common to convert it to formats such as DXF via dedicated tools or GIS software for use.

In this way, coordinate data acquired by RTK can take many forms—from text logs to tabular forms and GIS data formats—but at its core it is "a set of position coordinates based on a specific coordinate system." What is important is to understand which coordinate reference system those coordinate values are expressed in.

What is the DXF format?

DXF (Drawing Exchange Format) is a file format widely used for CAD data exchange. It can store information such as points, lines, and shapes from drawings in a text-based form, and is used when transferring data between different CAD software. In the construction and civil engineering fields, it is often specified as a shared format for survey results and design drawings, and is characterized by high versatility that does not depend on any particular software. Because DXF can record not only two-dimensional drawings but also three-dimensional coordinate values, it is suitable for data integration that includes location information (for example, importing into GIS). For these reasons, converting coordinates obtained by RTK to DXF format is effective for making RTK coordinates useful for multiple purposes.

Differences Between CAD Drawing Coordinate Systems and RTK Coordinates

Coordinates obtained by RTK positioning are basically based on absolute coordinate systems defined at the global scale (global geodetic reference frame). For example, when using the commonly used network-type RTK services in Japan, positioning results are often obtained as latitudes/longitudes and plane rectangular coordinates of the Japan Geodetic Datum 2011 (JGD2011), but depending on settings they can also be output as WGS84 latitude/longitude and ellipsoidal height (a globally common geodetic reference). In any case, RTK values follow absolute reference coordinates on the Earth.

On the other hand, CAD drawings and design plans used in practice may use different coordinate systems for each project. For public works, the country's official public coordinate system (e.g., the plane rectangular coordinate system of JGD2011) may be adopted as the drawing standard, but in many cases drawings are created in a site-specific local coordinate system (coordinates with an arbitrarily defined origin and orientation). For example, a case where an arbitrary point within the site of a construction project is set as the origin (0,0), an X axis is defined in a certain direction, and all coordinates on the drawings are established accordingly. In this case, the numeric values for the same location in the local coordinate system and in the world geodetic system can differ greatly. In extreme cases, a point that is "(100.00, 50.00)" in the local coordinates can become a value of a different order of magnitude like "(200000.00, 50000.00)" in the world geodetic coordinates, and they will not match when overlaid on the drawings as-is.

Even if both adopt the same public coordinate system (such as the plane rectangular coordinate system), a large offset can occur if the system number (zone) differs and thus the origins are different. For example, if the design drawings are in the ○ system and the positioning data are in the △ system, the coordinates of the two can be offset by hundreds of kilometers (hundreds of mi), so caution is required. Furthermore, older drawing data may use an old geodetic datum (Tokyo Datum) rather than the currently mainstream world geodetic system. Because there is a constant difference of hundreds of meters (hundreds of ft) between the new and old geodetic datums in some regions, the positions will not match as-is in such cases.

Also, in addition to coordinate systems, you need to be aware of differences in unit systems. If the numeric units in a CAD drawing are millimeters (mm (in)) rather than meters (m (ft)), the coordinate values will differ by a factor of 1,000. For example, a point "(12000, 5000)" in CAD, if recorded in millimeters, becomes "(12.000, 5.000)" when converted to meters — i.e. (12000 mm (472.44 in), 5000 mm (196.85 in)) → (12.000 m (39.370 ft), 5.000 m (16.404 ft)). If surveying data (meters (m (ft))) is imported as-is into a drawing in millimeter units (mm (in)), the values will be treated as points 1,000 times smaller and will not be displayed at the intended positions.

In this way, the main reason RTK positioning data and the coordinate values in CAD drawings do not match is that the reference coordinate system or units differ. Therefore, before reflecting coordinates obtained by RTK into a DXF drawing, you need to check and adjust the following points.

Check the coordinate reference of the drawings: Determine in advance which coordinate system the design drawings to be handed over or the ones you will use were created in. Confirm from the drawing legend or notes whether they use a public coordinate system (which system/zone) or a local coordinate system with a specific on-site point as the origin. Also check the units in the drawing data (m (ft) or mm (in)) and the elevation reference (elevation above sea level or a project-specific datum).

Align the coordinate system of RTK data: Align the output settings of positioning data, as much as possible, with the same reference used in the design drawings. In the GNSS receiver or app settings, for example, specify the zone number of the plane rectangular coordinate system for the relevant area, and, if necessary, apply transformation parameters to the previous geodetic datum. Also, if you set up your own base station for RTK positioning, it is important to accurately configure the base station’s known coordinate values to the official public coordinate values. If you can acquire data from the start in a system close to the drawing’s coordinate system, the effort required to transform it later will be greatly reduced.

Localization using known points: If the site drawings are created in a local coordinate system, make use of any known-coordinate reference points on site (for example boundary stakes or survey monuments). Measure those known points with RTK, and by comparing the obtained coordinates with the coordinates on the drawings, calculate the offset (planar translation) and rotation angle and apply corrections to the entire positioning dataset. With two or more known points you can align more accurately, and this process (localization) enables all point clouds acquired with RTK to be matched to the local coordinate system.

Adjustment of height (elevation): In addition to aligning the horizontal plane, confirm the vertical datum as well. RTK heights are based on GNSS ellipsoidal height, so as-is they may not match site elevations (height above sea level, etc.). Look up the local geoid height (geoid separation) and correct the ellipsoidal height to obtain elevation, or measure a point of known elevation at a local benchmark and apply the same offset to the heights of other points to adjust them. Recent GNSS equipment and software often include built-in geoid models that automatically convert to elevation values, so it is useful to take advantage of those features.

Notes on accuracy and formatting: After performing coordinate transformations or localization, verify with a separate validation point that any positional discrepancies have been resolved. Also, when handling coordinate data in CSV or similar formats, pay attention to limits on the number of decimal places and rounding errors. To avoid problems such as values being converted to scientific notation when opened in spreadsheet software, process the data using dedicated tools or a text editor where possible so as to preserve numeric precision.

By carrying out the above preparations and adjustments, you can align the coordinate systems of the RTK positioning data and the CAD drawing so they match and no shift occurs when the data are overlaid. Now, let’s look at the procedure to convert the coordinates acquired with RTK into a DXF file and reflect them on the drawing.

General Procedure for Converting RTK Coordinates to DXF

Exporting positioning data: First, export the coordinate data of points observed with RTK from the GNSS receiver or surveying app. If you performed standalone positioning in the field, you can also extract coordinates from the NMEA log recorded in real time, but in general it is more reliable to output a list of measured points as a file from the device after the survey is finished. In many cases you will obtain a coordinate list in CSV format with columns such as "point name, X coordinate, Y coordinate, Z coordinate…". If you have already completed the coordinate system adjustment in a preceding step, the numbers in this list should already be expressed in the same reference system as the design drawings.

Importing into CAD software: Next, import the exported coordinate list into your CAD software. Common CAD packages have functions to place point objects in bulk from an external coordinate list, or methods to plot points with simple scripts. For example, you can use a feature that reads a coordinate file in text format and automatically places points, or a function that creates points by copying & pasting coordinate values. If direct import into the CAD software is difficult, you can first load the coordinate data into GIS software and generate a DXF drawing from the point data to open in CAD. There are various approaches, but in all cases it is essential to plot the coordinates faithfully without altering the coordinate values. Take care not to accidentally apply an offset (translation) during import or change the scale due to scale settings. If the coordinate systems match, the output point cloud should align precisely with the intended positions on the drawing.

Check drawing units and settings: After placing points, check the drawing unit system, origin position, and other settings in the CAD. If the CAD drawing units are set to millimeters (mm, in), inconsistencies with the coordinate values can occur. If necessary, change the drawing units to meters (m, ft) or appropriately convert the coordinates of the imported points. Also check whether there is any unintended shift in the drawing's base point (the position (0,0)). Normally, if the coordinate systems are aligned, the origin should not shift, but if the coordinate values are overall offset, a setting error may be the cause.

Save as DXF: Once the coordinate points are correctly placed, save the drawing data as a DXF file. DXF is a generic format that does not depend on the recipient’s CAD environment, so it is a safe choice for file exchange. DXF can be read by many CAD and GIS programs, which reduces the likelihood of problems caused by format differences. On the other hand, if the recipient’s environment is clear and you know the specific version of their CAD software, you may provide the file in DWG format. However, DWG is a software-specific format, so if the versions do not match, the file may not open. If you are unsure which format to use, choose DXF, and even when certain, it is a good idea to confirm the preferred format with the recipient as a precaution. When saving, include information such as the site name and measurement date in the filename so the data can be identified later. Also, if you have three-dimensional coordinate data, DXF will preserve Z coordinate (height) information when saved, so choose whether to output a 2D or 3D drawing as required.

Clearly stating coordinate system information: Finally, it is helpful to add information about the coordinate system and references to the exported DXF data. For example, add an annotation in the DXF drawing such as "Coordinate system: JGD2011 ○ system / Unit: m (ft)" or list the coordinates of survey control points in the legend so the recipient can confidently use the data. Also, plotting several reference or known points within the drawing and displaying their coordinate value labels will serve as a check when overlaying data. If you provide a coordinate list (CSV file) together with the drawing data, explicitly state in the file header or an accompanying sheet items like "Geodetic datum: World Geodetic System ○○ (using geoid model ○○) / Vertical datum: TP (Tokyo Bay Mean Sea Level)" to prevent misunderstandings.

By creating and sharing the DXF following the above steps, you can smoothly apply the results obtained from RTK surveying to CAD drawings. The recipient can simply open the provided DXF file and overlay it onto their drawing to confirm the correct relative positions of the survey points. If the other party contacts you saying "the coordinates don't match," it is important to double-check with each other that there are no misunderstandings about the coordinate system or units.

Summary

When reflecting coordinates obtained by RTK in a DXF file, it is important to resolve beforehand issues such as differences in coordinate systems and adjustments to units, and to perform data conversion and import using appropriate procedures. In other words, if you address those points, you can use high‑precision positioning data directly in design drawings.

In recent years, tools that simplify the RTK positioning and coordinate transformation process have also appeared. For example, the LRTK is a high-precision GNSS device that can be attached to an iPhone and, on its dedicated app, by simply selecting a public coordinate system or any local coordinate system, can convert and display positioning results obtained in the field in real time as coordinate values (X, Y, Z) in that coordinate system. If reference point positions are pre-registered in the app, it also includes a one-touch coordinate correction (localization) feature, enabling field survey coordinates to be aligned with the drawing reference in a short time without having to consider complicated calculations.

In fact, at one construction site, when LRTK was used to perform a pre-setup coordinate alignment using control points, all subsequently measured points were recorded in the same public coordinate system as the design drawings. As a result, the acquired survey point data could be reflected directly into the final CAD drawings, eliminating the need for coordinate conversion during post-processing and reportedly greatly improving efficiency. In this way, by utilizing LRTK you can significantly reduce the effort required from positioning through DXF conversion.

With the spread of high-precision GNSS and digital construction, these kinds of tools will increasingly attract attention and become indispensable for integrating surveying with CAD drawings. By mastering such new devices, anyone—even without specialist surveying knowledge—will be able to easily acquire and share highly accurate positional information. Of course, for experienced surveying technicians, LRTK will also be a strong ally in shortening work time. You will be able to import survey data into CAD drawings with confidence, without worrying about coordinate shifts. To make effective use of the valuable data obtained with RTK, please apply the procedures and key points explained in this article to ensure reliable DXF conversion and drawing reflection.