It is possible to use Google Earth to obtain contour information to use in landscape design work. There are many resources on the net that show how it is done. They all rely on extracting elevation data from Google Earth and creating a DXF data exchange file containing polylines labelled with the contour level. Most, but not all, methods use labelled 2D polylines to hold contour data even though AutoCAD (and gCADPlus) allows for storage of 3D information in polylines. Many beginners think about contours formed from polylines with elevation data and get confused. 2D polylines on appropriately labelled layers are all you need.

gCADPlus has the ability to import (load) DXF information so (as some suggest), you do not need the Pro version of SketchUp to get contours. Once you have generated a DXF using some tools described below, the contours can be imported using the DXFIN option on the File menu. Tip: After importing a DXF, you may well need to simplify layer names as each contour is often placed on its own weirdly named layer.

This YouTube movie explains the process rather well, but there are many alternative approaches:
https://m.youtube.com/watch?v=pbs7bhv7HgQ

You will need to download and install the following application software

1. TCX Converter

TCX converter

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2. QuikGrid

Quikgrid

As step by step guide to extracting contours from Google Earth

Start Google Earth Pro, fly to location

Fly to site

Use the path tool and set up a zone around your site. Make sure you leave the add path dialog box open while you do that.

AddPath

Next save path as KML

Save as KML

Now start TCX converter, change the language to suit and load the exported KML file.

TCX converter

Go to Track Modify, select Update Altitude

Export the result to CSV file, making sure that you use the comma as separator.

CSV

Open the resultant CSV file in Excel, delete the following columns - UNIX time, TIME, Dist, hr, cad, temp, power

TCXInExcel

Delete record 1

Now rearrange latitude and longitude columns to get them in the correct order. Move column A to the right one column as shown below. Tip: You may have to insert a blank column in order to do this.

REarrange latitude and longitude

Save the cleaned up and rearranged Excel file (still as CSV)

QuikGrid01

Change contour interval if needed.

Change interval

Go to Edit Data output options and select export polylines as DXF

Output

Save to DXF and using File >DXFin, import to gCADPlus. Zoom extents. The figure below shows a typical contour file.

In gCADPlus

If required, change the background color and color of layers for major and minor contours

contours

 

Contours from Google Earth