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AutoCAD

Harness Google Earth Tools in Civil 3D (CAD Clinic: Civil 3D Tutorial)2008, Part 2.

31 Jul, 2007 By: Mike Choquette

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An overview of Civil 3D coordinate systems and the Publish to Google Earth feature.

Google's massive database of geospatial data includes street mapping, satellite and aerial photographs, digital terrain model data, and more. This incredible resource is available for browsing through a free utility called Google Earth (downloadable from earth.google.com). Imagery and terrain models accessible through Google Earth can be imported directly into Civil 3D with a minimum of fuss, making for an extremely useful feature set. In last month's "CAD Clinic" column, we introduced this topic by describing the data available, as well as the steps required to import those images and terrain models. This month, we conclude our review of using Google Earth and Civil 3D by considering coordinate system issues and by looking at the Publish to Google Earth tool.

How to Apply Coordinate Systems to Civil 3D Drawings
If you intend to have imported Google data overlay your drawing data, it's best to have your drawing assigned to a real-world coordinate system. If you're working with surveyed data, you may want to look for coordinate information in the drawing or talk to the surveyor or mapping company to be sure which one to use. If you know what system the drawing is in, you can identify that system through Civil 3D's Drawing Settings. To do so, right-click on the drawing name and choose Edit Drawing Settings in the Settings tab of the Civil 3D Workspace. Once there, assign the correct coordinate system under the Zone tab. If a survey was left in a local coordinate system (such as one based on the coordinate values 5000, 5000) the surveyor or mapping firm are often the most qualified people to translate the drawing to a real-world coordinate system.

On the other hand, if your drawing contains only rough, schematic linework at incorrect coordinates, Google Earth can help you manually georeference the drawing. Assign the desired coordinate system as mentioned above and then import a Google Earth image as described in the previous article. Because the image will be automatically located at the correct coordinates in the assigned system, you can now use AutoCAD commands such as Move, Rotate, Stretch, and Scale to locate your drawing objects over the image. Don't forget that the AutoCAD Align command can do all of the above steps at one time.

The key is to identify known points in your drawing such as street intersections or building corners and then manually align those with the same points in the image file. Congratulations, your drawing is now located at approximate real-world coordinates! Again, we only recommend this scenario for schematic or sketched drawing data -- translating surveyed data accurately is a science unto itself that must take into account the curvature of the Earth and other major concerns. Surveys and design drawings based on them should be translated to a coordinate system only by licensed surveyors and other professionals specifically trained for the task.

Publish Civil 3D Data to Google Earth
One way to create a 3D, interactive model of your Civil 3D design that can be viewed outside of AutoCAD is to publish it to the Google Earth utility. Although there are other ways to create 3D viewable files (such as 3D DWF), one advantage to publishing to Google Earth is that your model becomes framed by surrounding Google Earth imagery, giving it some context. Don't let the name of this feature fool you, though, your model is not going to be published as part of Google's online database for everyone to see. Instead this routine creates a KML file that contains the data. A person who wants to view your model must have this file (or a compressed version of it in the KMZ format) and must have Google Earth installed.

The Publish to Google Earth command is located under the File menu. The command launches a wizard that helps you create your Google Earth file. In the first step, you enter a name of your site, a description, and an optional hyperlink. Later, in Google Earth, you can locate your project through a push-pin place icon that carries this information. The wizard also lets you select what you'd like to include in the file. To speed up the process and cut down on clutter in general, you should exclude text and 2D geometry. We recommend assigning surfaces to first have a style that displays their triangles. Also, when working with corridors, include only the corridor surfaces and not the corridor itself to cut down on linework.

Next, you can either use the drawing's current coordinate system or define one based on points in the drawing. We highly recommend using the first option if possible. The Nudge step of the wizard lets you adjust the coordinates if you find it necessary to tweak the position of the final product. Here you can also identify whether the elevations are relative to sea level or Google Earth's ground elevations. If your drawing uses any kind of surveyed vertical datum then use the sea level option. This step also lets you choose if you'd rather drape all objects over Google Earth's ground elevation as well. This choice often isn't the best for any kind of Civil 3D models. The last step lets you choose a filename. Note that the file format defaults to the compressed Google Earth file type (KMZ). If you have any trouble with the file you could always push the ellipsis (?) button, then choose the uncompressed (KML) version instead from the Save as Type drop-down list.

After publishing is complete, you can view the file immediately with the View button. Here's an example of a 3D view of a Civil 3D project from within Google Earth.

If the Google Earth terrain is overlapping your own and causing some visual problems, you can try a few adjustments. If you brought in your own existing ground terrain model, try turning off Google Earth's Terrain layer, which shows Google's imagery at elevation 0. If the ground appears too far away after doing this, leave the Google terrain on and nudge your surfaces up slightly through the Publish wizard until the overlap is resolved.

Summary
Google Earth provides a free, very easy-to-use way to import aerial and satellite images into Civil 3D drawings, as well as approximate (non-survey grade) terrain models. Although the quality of the imported data may not be as high as what you can download from GIS Web sites, the ease of which it can be used makes these tools extremely valuable. They can be particularly helpful in the early phases of design when there isn't much other data to be found. Once Civil 3D design models are underway, you can publish them to a format that can be viewed in the Google Earth utility. Google Earth allows others -- even non-AutoCAD users -- to easily view and interact with your design in 3D. These are very useful features that every Civil 3D 2008 user should be familiar with.