The Earth is not flat, and it is certainly not round!

One year ago my friend went on a journey on the 'Oosterschelde', he sailed from New Zealand to the Falkland Islands crossing the South Pacific and rounded Cape Horn. Together with other sailing enthusiasts, he endured the elements of the ocean and enjoyed the many exciting times he had onboard. As you may have read in my previous post, I gave him a GPS receiver for an experiment I wanted to do. He placed it on board in good view of the sky and turned it on. Despite several reception losses (storms or other effects), the data clearly shows the path of the journey.

They started in Auckland, New Zealand, where they departed on the 30th of October 2013, to set sail to Chatham Island. This small island is governed by New Zealand and is closely situated to the International Date Line. Lets say the day starts on Chatham Island. After a small visit on the interesting island, the 'Oosterschelde' and its travelers started the long crossing of the South Pacific. The crossing went without any large problems, despite the large storms, which made life onboard the 'driemaster' a bit more difficult. However, after many days Cape Horn was insight, which meant a few more days of sailing and my friend would arrive at the Falkland Islands. He could look back at a great adventure! And I had my data for the experiment. 

As I explained (here), the Earth is not flat, but it is also not round. Due to the rotation of the Earth and the fact that it can deform, the shape of the Earth is more elliptic. The poles are pressed inward and the equatorial areas are bulging outwards. So the Earth looks a bit like a volleybal after somebody has sat on it for a while. But this is not the complete story. 

The Earth has mountains and trenches, bulges and holes, which affect the gravity field and the geoid. Inside the Earth there is not a homogenous mass distribution, due to plate motion, mantle convection, post-glacial rebound and effects we don't even know. These mass anomalies are responsible for a 'potato' shaped geoid (see Figure in previous post). The oceans of the Earth follow this 'potato' shape and is what we call level (waterpass in dutch). This sea level height is defined by us as 0 meter height. This enables us to say that Mount Everest is the highest mountain on Earth, despite the fact that the top of Mount Chimborazo is further away from the center of the Earth. It becomes even weirder, when I tell people that 0 meter height can vary over almost 200 meter depending on your location. So I wanted to show this with an experiment and gave my friend a GPS receiver.

The GPS receiver should show 0 meter height (or in our case 4 meter, because the GPS receiver was attached on the deck of the ship), during the complete journey of the Oosterschelde, because it was sailing at sea level, right? This is what it observed:

Sea level height with respect to the WGS84 ellipsoid. Red line is the EGM2006 model of the geoid. Blue dots are measurements of the GARMIN GPSmap 60 CSx during the voyage of the Oosterschelde.

During the complete trip the clipper and his onboard travellers made a vertical motion of around 60 meters. I know there were storms, but I did not hear of that kind of waves. Also this would give high-frequency motion, and that is not what we see. This 60 meter drop happened in 20 days, before they went up 40 meters again. This proves that the Earth is not flat, and certainly not round ;). I know it is not the maximum 200 meters drop, but than you have to sail from the center of Indonesia towards 2000 km south of India. If somebody out there is planning of doing this, please take a GPS receiver onboard and send me the data!!!