Contour plan preparation of desired area using Google earth and QGIS software

How to Generate Topographic Contour Plans as PDF or AutoCAD DWG/DXF

Generating accurate topographic contour plans is an essential step for civil engineers, site planners, landscape architects, and GIS analysts. While ground-truth land surveys remain the gold standard, open-access spatial tools allow you to generate high-resolution surface elevation models and contours quickly without expensive proprietary equipment.

This guide walks you through a complete end-to-end workflow using Google Earth Pro, GPS Visualizer, and QGIS to create professional vector contours ready for CAD editing or PDF publication.

Required Software & Tools

• Google Earth Pro (Free Desktop Edition): Used to define your site boundary and capture coordinate path data.
• GPS Visualizer (Web Tool): Accesses public DEM (Digital Elevation Model) datasets to assign elevation values (Z-coordinates) to geographic coordinates (X/Y).
• QGIS (Free & Open Source GIS): Used for spatial data interpolation, raster modeling, vector contour generation, and map labeling.
• AutoCAD or Civil 3D (Optional): Required only if you intend to import, edit, or incorporate the exported DXF vectors into engineering drawings.

Step-by-Step Technical Guide

1. Launch and Setup Google Earth Pro
   Open Google Earth Pro on your desktop. Ensure that your view is set to look straight down (press shortcut R to reset orientation and tilt to true north).
2. Locate Your Project Area
   Enter geographic coordinates (Latitude/Longitude) or search for your site by address or location name in the top-left search pane. Zoom into your specific parcel of land.
3. Draw a Dense Path Grid & Save as KMZ
   Select the Add Path tool from the top toolbar. Draw a dense grid or zig-zag pattern across your entire target area.
   Pro Tip: The accuracy of your interpolated contours directly depends on point density. Place points close together across elevation changes, slopes, and ridges.
   Once completed, right-click your newly created path under the Places pane, select Save Place As..., and save it as a .KMZ file.
4. Attach Elevation Data via GPS Visualizer
   Navigate to gpsvisualizer.com/elevation. Upload your saved KMZ file. Ensure the output format is set to Plain Text, then click Convert & add elevation. This tool cross-references your 2D points with satellite DEM sources (SRTM/ASTER) to append accurate Z-elevation values.
5. Download the Processed Data File
   Click the link provided by GPS Visualizer to download the converted plain text file (commonly .txt or .csv format containing columns for Latitude, Longitude, and Altitude/Elevation).
6. Initialize a New QGIS Project
   Launch QGIS and click Project → New (or press Ctrl+N) to set up a clean workspace. Ensure your project's Coordinate Reference System (CRS) is set correctly (e.g., WGS 84 / EPSG:4326 or your local UTM Zone).
7. Import Elevation Data into QGIS
   Navigate to Layer → Add Layer → Add Delimited Text Layer. Select your downloaded text file. Under Geometry Definition:
   • Set X field to Longitude
   • Set Y field to Latitude
   • Set Z field to Altitude/Elevation
   • Assign Geometry CRS to EPSG:4326 - WGS 84
   
   Your points will render in the QGIS canvas as a point cloud representing your surveyed path:
8. Generate a Continuous Elevation Surface (Interpolation)
   To draw contours, QGIS needs a raster grid Surface (DEM). Open the Processing Toolbox (Ctrl+Alt+T) and search for Natural Neighbor or TIN Interpolation (SAGA algorithms).
   Set your input layer to the point layer, choose the elevation attribute as the target field, and adjust the cell resolution (e.g., cell size 0.0005 depending on site scale) to compute the interpolated surface.
9. Extract Vector Contour Lines
   Go to top menu: Raster → Extraction → Contour.
   • Input Layer: Select your interpolated raster surface.
   • Interval between contour lines: Define your desired vertical distance (e.g., 1m, 2m, or 5m intervals based on terrain steepness).
   • Attribute name: Keep as ELEV (stores height for each polyline).
   
   Click Run to generate your 2D contour polylines:
10. Apply Contour Labels & Styling
    Right-click your Contour layer in the Layers panel and choose Properties → Labels. Change the dropdown from No Labels to Single Labels, select ELEV as the label value, and adjust font size, color, and placement along line paths for legibility.

Exporting Options:

• For AutoCAD / Civil 3D (DXF Export): Right-click the contour vector layer → Export → Save Features As.... Select AutoCAD DXF as the format. Ensure the target CRS matches your local projected grid (e.g., UTM) so lines retain real-world scale and 3D elevation attributes when opened in CAD software.
• For Printable PDF Map: Open Project → New Print Layout. Add a map canvas, scale bar, north arrow, title block, and legend, then click Export as PDF.

By leveraging this open-source workflow, engineers and designers can rapidly establish accurate baseline terrain models for conceptual earthwork, drainage assessments, and site planning prior to field surveys.

Yogendra Gopal Borse

Civil Engineer | Assistant Engineer Grade-I, Maharashtra PWD

B.Tech (Civil) from VJTI Mumbai. Experienced in bridge design, road works, estimation, project monitoring and digital engineering tools. Creator of YogiPWD – practical technical resources for civil engineers.

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