Get altitude data from Google Earth
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Google provides heights API for requests up to 25k per day: https://developers.google.com/maps/documentation/elevation/
There are other options discussed here: How to search for elevation data using lat / lng
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If you have spreadsheet data, you can enter it in the GPS document ( http://www.gpsvisualizer.com/convert_input ) and select "Add elevation DEM data" and it will come back with a text file with elevation data in the column.
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