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The GRACE Plotter Pt. 2 – Engineering a fluid solution to a water problem

By 3 March 2021February 1st, 2024Data visualization

All that water has a mass

Spirit, conception and ignition

If you haven’t read the two previous articles yet, Fish, tools and investment, and Da Vinci, Saint-Exupéry and the old Irish distillery, we suggest that you start by reading them first. They will give you an idea of why the GRACE Plotter was created. And why other tools could be created in the same spirit. So after the ins and outs of this grandiose and ethereal vision, we are now coming back to earthly problems, such as engineering the software and implementing the code. What would be a mind without hands and arms to execute on the vision anyway? As Alan Watts says, you need to have the skills “to bring Heaven down to Earth”.

The initial problem

Da Vinci and Saint-Exupéry imagined and piloted flying machines to travel the skies. Now we are observing Earth from space, and our machines are called satellites. We calculate Earth’s gravity field from the GRACE and GRACE Follow-On satellite missions. So the initial problem for Stellar was to improve our process for gravity field analysis. We routinely had to extract time series of gravitational data on specific regions of the Earth. But it was a repetitive and cumbersome process. Think about recompiling code every time you need to change the latitude or longitude by one degree. North, South, East and West. Thing about manually setting titles, axes, legends for images. Think about waiting between 30 seconds and one minute until the image was generated. And then having to gather several numerical time series for comparison purposes, and then having to manually include them together in a new plot. Count 30 minutes every time you have a new idea or a new zone to explore. How far will it take you at this pace? Factor in the boredom and the exasperation from repetitive tasks, and what do you get? After you’ve explored 5 zones, you will call it a day, and leave your analysis stalling there, until you find the courage to get your hands dirty again. Next week? Maybe.


So, wouldn’t it be nice if you could choose the point on a map with a mouse, drag it quickly and easily to another zone, see what you’re doing in real time, plot several time-series of several groups together and see the results in less than 5 seconds? Then get another idea and get your new result in another 5 seconds? Then download the graphs for your presentations with just another click? Then share the results with your colleagues all over the world with just another click? Just imagine. You could wake up at 2 am in the morning with a burning question and you would get your answer in the blink of an eye. And have fun with a lovely easy to use interface? What would be the time-improvement factor? And the happiness factor? That surely looked very seductive to us. Wouldn’t you like one for yourself?

Imagining the pieces of the puzzle and bringing them together

Earth’s (changing) gravity field

As we explained in the article Gravity field and the water cycle, the GRACE mission allows us to monitor the variations of Earth’s gravity field, which in turn allow us to monitor the movements of water at the surface of the Earth: lakes, rivers, monsoons, polar caps, etc. So, in an era where climate change is all over the news from breakfast to dinner, every citizen of the world should know everything about the GRACE and GRACE Follow-On missions. And about the GRACE Plotter! (we would be rock stars then!).

Ice water

No water

Watery areas

From stacks of zeroes and ones to an interactive website

So, how does it work? We receive the telemetry data from GRACE, basically stacks of zeroes and ones sent from the satellite to the Earth. Then we convert all this data into monthly maps of water contents at the surface of the Earth. In concrete terms, the products we deliver are monthly 1°x1° grids of Equivalent Water Heights differences between the gravity field model of a given month and an average gravity field model over 20 years. Typically our graphic scale goes from -35 cm to 35 cm. Once you have computed all your monthly grids (we produce them since 2002), you can extract time-series over a given area. It could be a single grid point, a square, a rectangle, a specific shape, a area of interest, you name it… For example, we like to study water cycles on dedicated areas, such as hydrological basins (Amazon, Congo, etc.), places where monsoons are important (India), in the polar caps (Antarctic, Greenland, etc.). With just a few clicks, your graph will appear automatically. Before the GRACE Plotter, we had to perform the tedious work on Linux machines, using Fortran, Bash, and a plotting software amusingly named GRACE (or XMGRACE, but there was no relation whatsoever with the satellite). Now, we do it with with a mouse and a few clicks through an interactive website. It feels just like Steve Jobs and Steve Wozniak walking out of the Xerox park.

How does it work?

The best thing is to try it for yourself. We will share the link below. But let’s present the basic features and share a few screen captures. Here are the principles. There are 4 sections: 

  • A form section,
  • A map section,
  • A graph section,
  • A data section.

There are also two small additional help sections, inside the top menu and at the bottom of the page (which stand as a replacement for any nightmarish 100-pages user’s manual we mentioned earlier). Let’s share a few screen captures.

The landing page

Here is the landing page and the logo of the website.

There is a historical reason for the “Marvellous” nickname, which we will reveal one day in a future post.

The form section

The form section allows you to choose the gravitational data: the production center, the data release, the gravity functional (gravity anomalies, geoid heights, or equivalent water heights), and the geographical region on which you want to extract time-series (a point, a rectangle, a custom polygon, or a predefined hydrological basin).

Form section

The map section

The map section allows you to interactively choose the area of data extraction: it can be a point, a rectangle, a custom shape or a predefined hydrological basin. You can fill in exact coordinates, or interactively set the area with your mouse. You can drag the point anywhere you want or draw a polygon on the map.

Map section

The graph section

The graph section is where the time-series will appear. It shows the result of the extraction over the area. If you choose equivalent water heights, you get the conversion of the measured gravity field into an equivalent layer of water at the surface of the Earth. This is what you choose if you want to compare it to altimetry measurements, for example to get the level of lakes, the extent of the ice melt in Greenland, or the variations of water content in Amazonia over the years.

Graph section

The data section

The data section will provide you with the numerical values. You can read and download the data in the numerical data section. There is a lot of information available: the full list and coordinates of grid points used for computation, the weights, as well as different formats and units for the data.

Data section

Try it by yourself

« A short sketch is better than a long speech », said Napoléon Bonaparte. So, enough theory for today, now is time for practice. We’ll give you a few themes to play with:

  • What is the amplitude of water level change during monsoons in Amazon and India?
  • How did the Iceland ice cap melt over the past 15 years?
  • What is happening to African lakes? 
  • Was the Sumatra Earthquake felt by gravity?
Legacy version Modern version

What next?

In the next post, we’ll give you tricks and tips about a few cool features in the plotter. How to change the background maps, how to save and share your work, how to plot regressions and analyze the data, and much more. Stay tuned.