Point Coordinate Transformations (CAD Clinic: Civil 3D Tutorial)

30 Nov, 2008 By: Phillip Zimmerman

Use Point File Formats to import and export points to assigned coordinate systems.

Editor's Note: This tutorial courtesy of Imaginit.

Reading external ASCII files through a Point File Format creates cogo_points. Civil 3D has several pre-defined file formats. When using a Point File Format, it assumes the coordinate system or local coordinates match between file and drawing. However, there are times when the file and the drawing have different coordinates systems, i.e. NAD27 coordinates in the file and NAD83 for the drawing's coordinate system. If the file and drawing's coordinate systems differ and you want to transform the coordinates on import, then the Point File Format must have an assigned coordinate system. A coordinate system is assigned in the file-format definition panel's middle left. See Figure 1.

A file format with a coordinate system.

The second place coordinate transforms can occur when importing points directly into a surface. The point files process is exactly the same as importing points from an ASCII file to the drawing, but the resulting points do not become cogo_points, rather they become surface points.

The last place where point coordinate transformations may occur is when inserting survey points into a drawing. If the survey and drawing have different systems, the point coordinates transform when inserting them into the drawing. Also, when a survey's units are U.S. survey foot and the drawing uses international feet, when inserting the survey points into the drawing, a transformation occurs. Defining a Point File Format with a Coordinate System There are two places to define a Point File Format. The first is the Setting panel's Point branch, and as the second is a modification or copy of a format in the Import/Export Points dialog box.

When in the Import/Export dialog box, clicking the Point File Formats icon (upper right) displays the Point File Formats dialog box. It is here that the user defines new, copies, or modifies an existing format. See Figure 2.

Point File Format dialog box.

After clicking New, Copy, or Modify, the Point File Format dialog box displays. The Point File Format dialog box Coordinate zone transform toggle allows the format to have an assigned system. When this toggle is active, the globe icon calls the Select a coordinate zone dialog box. The systems are the same as those listed in Edit Drawing Settings for a Civil 3D drawing. See Figure 3.

Select a coordinate zone dialog box.

After assigning a coordinate system to the format, you should change the Northing and Easting entries to Grid Northing and Grid Easting. To do this, in the Point Format dialog box (Figure 1), click the Northing heading and in the Select Column Name dialog box, click the drop list arrow, and, from the list, select Grid Northing. Repeat this for the Easting heading. After making these modifications, your dialog box should look similar to Figure 1.

Transferring Point File Formats
If you develop several Point File Formats, they are available only to their drawing. You transfer the formats to other drawings or the template file through the drag-and-drop method used to transfer styles.

To transfer the format to a template, open the template file while still in the drawing containing the new point file formats, and in the Windows menu select Tile Vertically. This allows you to view both the drawing and the template at the same time. In the drawing with the new Point File Format, expand Setting's Point branch until viewing the Point File Format list. From the list, select the new Point File Format, and while holding down the left mouse button, drag it to the template file. When lifting up on the left mouse button, the file format is added to the template.

Using a Transformation Point File Format
Selecting a point file format with an assigned coordinate system is only the first step to importing and transforming points. After selecting the format and the file to import, you must toggle on Do coordinate transformation if possible. See Figure 4.

Import Points dialog box.

After importing the file, you should review the original coordinates to the new coordinates in the drawing. See Figure 5.

Original and transformed coordinates.

Exporting Points
Having an assigned coordinate system is also available as an export points format. If you want to export NAD 83 point coordinates to NAD 27, in the Export points dialog box, you would assign a NAD 27 format, toggle on Do coordinate transformation if possible, and when exported, the resulting file contains NAD 27 coordinates.

Surface Definition -- Point Files
When adding point data to a surface, they can come from either a point group or directly from a point file. When directly importing points to a surface, the points appear only as surface points and not cogo_points. The interface for importing points to a surface is the same as the Import points command. If you use a point file format that has an assigned coordinate system and wanting to transform the points to the drawing's system, following the same steps mentioned above for importing points will transform them to the drawing's system.

Transformations When Inserting Points From Survey
Related to point file formats with differing coordinate systems, is collecting survey data in one coordinate system and inserting the Survey points into a drawing with a different system. For example, conducting a survey in NAD 83 and inserting its points into a NAD 27 drawing, causes the point coordinates to transform from NAD 83 to NAD 27. The Survey command, to create network control points from drawing points, also transforms points from the drawing coordinate system to the survey system.

In Survey, you do not need coordinate systems to cause a transformation. If collecting a survey in U.S. foot and inserting survey points into a drawing using international feet will cause the point coordinates to change. You must make every effort to keep the survey and drawing units the same.

Point File Formats allow you to import and export points to assigned coordinate systems. A point format file with an assigned coordinate system different from the drawing's coordinate system, transforms the point's coordinates to the drawings coordinate system values. These same formats can be used to export points coordinates held in one system to a different system assigned to a Point File Format.

The same Point File Formats import points directly to a surface. This process is the same as importing points to a drawing. The difference is importing to a surface does not create cogo_points, but surface points.

The last place transformations occur is when inserting points from a survey into a drawing. If the survey is NAD83 and the drawing NAD27, inserting the points from the survey transforms their coordinates to match their coordinates in the drawing's coordinate system. Even if using local survey coordinates, if the survey units do not match the drawing units, the survey point coordinates with transform to match the drawing units. This occurs when surveying in U.S. foot units and inserting them into a drawing set to international foot units.

Related Content:AEC, GIS

About the Author: Phillip Zimmerman

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