Import Google Earth Surface

In addition to providing a graphical method for displaying feature-rich data located anywhere on the globe, Google Earth also provides the ability for software applications to extract its underlying terrain data. While the elevational accuracy of the Google Earth surface should be considered extremely coarse, it might be suitable for large-scale watershed modeling studies, preliminary land-planning studies or "proof-of-concept" preliminary designs.

When extracting terrain data from Google Earth, it is important to keep "diminishing returns" in mind. As an example, a land surveyor might perform a traditional grid-based topographic survey by sampling the land every 50 feet. Although a 25 foot grid spacing would yield more accurate results than a 50 foot grid, it would typically take at least twice as long to survey. Harvesting terrain data from Google Earth operates in a similar fashion:

1. The Google Earth "project area" is identified and the limits of the site are calculated
2. Horizontal and vertical "sample" intervals are established
3. Terrain data is gathered at each identified sample location and used to form a surface model

Consider the following example. Based on the physical screen size of the Google Earth application and the "zoom" (or "view") resolution of a project site, the following values (summarized at the bottom of the dialog box) were returned:

In the sample above, the total area is calculated and displayed (0.1 mi²) along with the desired "projection" system for our project site. Although it might be desired to sample every pixel in this project... 1,107,270 = (1366+1)*(809+1), in all, the "point of diminishing return" would be quickly reached and could clog Google servers with extraneous terrain requests; see the NOTE section below.

Spatial Reference: Displays the spatial reference coordinate projection system of the current drawing. The projection can be set using the Drawing Setup command.

Extent - Current Google Earth View: Gets the overall dimensions of the Google Earth session and displays the results in both pixels and the appropriate units of measure.

Extent - Current Drawing View: Gets the overall dimensions of the current CAD view and displays the results in both pixels and the appropriate units of measure.

Extent - Select from Drawing: Sets the overall dimensions of the Google Earth session to conform with a drawing window from CAD and displays the results in both pixels and the appropriate units of measure.

Pixel Sampling Interval: Allows the ability to indicate how often a pixel row or column should be sampled for terrain elevation. Smaller intervals result in higher total samples and longer processing time.

Consider the following "sample" diagram:

Referring back to our horizontal and vertical samples shown in the dialog box above, we are requesting:

The resulting total samples 4968 = (92)*(54) and it is recommended that this value be at or below the Google Earth session threshold of 5000.

Note:

• In an effort to protect their servers from abuse, Google will rapidly return 5,000 sample requests per Google Earth session and then "throttle down" the remaining sample requests to about 1 per second. In the example above but with a sample interval of 1H and 1V, the terrain surface would be completed in a little over 12 days, 18 hours. For this reason, it is strongly suggested that the horizontal and vertical sampling intervals be set so that the sample result is at or below the 5000 sample threshold.
• The Import Google Earth Surface routine fetches terrain data in real-time from the Google servers and requires an Internet connection to proceed. In the event that an Internet connection is not available, the following error message may be displayed: "Failed to initialize Google Earth. Please ensure Google Earth client software is functional and online"
• It bears repeating that the terrain data returned by Google Earth should only be used for illustrative or proof-of-concept purposes only!

See Also:

• To import a Google Earth image into your drawing, use the Place Google Earth Image command.
• To import KML content into your drawing, use the Import Google Earth File command.
• To export content from your drawing to a KML file, use the Export Google Earth File command.

Prompts

Identify first corner: Identify one corner of a drawing window that should be used to set the Google Earth display
Identify opposite corner: Identify the opposite corner of a drawing window that should be used to set the Google Earth display

Pulldown Menu Location: Civil > Surface > Import/Export Surface, Survey > Surface > Import/Export Surface, Takeoff > Tools > Import/Export, Construction > Import/Export
Keyboard Command: gesurface
Prerequisite: Coordinate projection system, Functioning version of Google Earth with Terrain enabled, Internet connection