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Global Tides Google Earth Model

Most places have two high tides a day, one on the side nearest the Moon due to the gravitational pull of the Moon, and one on the side farthest from the Moon because the Earth is actually rotating about the Earth-Moon barycentre. Which begs the question how does all this water circle the Earth when there's so much land in the way! The answer is it doesn't, it just sloshes around the oceans like water in a rocking bathtub.

Nasa's Goddard Space Flight Center have mapped the tides from Space. Using six years of data from the TOPEX/Poseidon satellite, they derived a 16-day set of predictive data, with movies showing a synthetic view of how the tides move around the world's oceans.

To extend the visualisation of this data (and because their projection splits the UK) I extracted some frames and ported them to a Google Earth model for a three dimensional view. I reduced the animation to 7 days rather than 16, partly to reduce the file size, but mainly because otherwise it plays far too quickly in Goorgle Earth even when thespeed is set to the slowest setting.

To view this animation first install Google Earth if you haven't aleady, it's available for Windows/Mac/Linux, then open the kmz file. The default speed setting in Google Earth will need to be reduced, click on the animation controller key at the top of the Google Earth window as shown below to bring up the dialog to change the speed. To start, click the play icon on the animation controller. The animation controller also allows you to animate frame by frame. Unfortunately there is no way to change the field of view to show the whole Earth.

Global Tides, Google Earth model, kmz file 1.3MB.

How to make a Google Earth model

Assemble the required frames as png files, here I extracted them from full.mov on the TOPEX/Poseidon Flat Earth Tide Height Model web site from the NASA/Goddard Space Flight Center Scientific Visualization Studio. I extracted the frames using the free RAD Video Tools, if you're starting from bitmaps you can skip this step. I reduced the palette to 32 colours to reduce the file size using the following dos command, forfiles /M *.png /C "cmd /c convert @file +dither -map ..\palette1.png -type palette -depth 8 @file" which launches the convert program from the excellent free ImageMagick suite, and uses the sample palette file palette1.png: which is simply a 32x1 pixel image of the desired colours.
Create a kml file to control the animation, this is just a text based xml file; as this animation is synthesised, I arbitrarily set the dates to be in January 1970.
Adjust the north/east/south/west settings such that the maps line up with the Google Earth, as the coastlines are superimposed this is fairly straightforward.
Zip all the kml and png files into a zip file, and rename this file to have a .kmz extension. (To reverse engineer any .kmz file, simply rename to .zip and open it.)