Hello dear readers!
This post is a small synopsis of my experiences so far as a student in this years Summer of Code in Space, where I shall recount the whole adventure of integrating Sentinel-2 data into Marble Virtual Globe.
So what exactly is this data, and why is it important to us?Well, Copernicus is the worlds largest single earth observation programme directed by the European Commission in partnership with the European Space Agency (ESA). ESA is currently developing seven missions under the Sentinel programme. Among these is Sentinel-2, which provides high-resolution optical images, which is on one hand of interest to the users of Marble and the scientific community as a whole.
Our goal with this years SOCIS was to adapt this data into Marble. Since Marble has quite the track record of being used in third party applications with great success, this would essentially be a gateway for many developers to get easy access to high quality images through the Marble library.
So first order of business? Adapt the world. The summer has begun to get exciting.
First Acquaintance with Marble
Of course, nothing can happen so quickly, and the first task was obviously on a smaller scale. In order to familiarize me with the inner workings of Marble, my mentor gave me a task to adapt an already available dataset into a map theme for Marble. This is how I came to know TopOSM.
The TopOSM maptheme in Marble.
This task came with its own fair share of challenges, from getting Marble to display the legend icons correctly, to creating a suitable level 0 tile, but in the end it did give an insight into exactly how the creation of a map theme, from the first steps to uploading goes. At this point, the challenge was underway, and so began the real part of our ambitious project to tackle the whole world through Sentinel-2's lens and integrate it into Marble.
Sentinel-2 - From drawing board to tilerendering
After many discussions with my mentor, regarding ideas on how to make the data suitable for use in Marble, we finally came up with a plan. That plan would let us use the currently available Data Hub as a source for our images (since we don’t have a simple server we could just get the data from, as in the case of TopOSM). At that point, we just have to edit these images into a suitable format, and everything will be fine. A three step process:
Step 1. Download some data.
Step 2. Edit it.
Step 3. Use it in Marble.
As you may have guessed, step 2. was to be troublesome. Around this time my mentor came up with the first iteration of the guide for this “three-step” process. We also found an application that would suit our needs for the editing, this was to be QGIS, but we would also be using GDAL.
The now mostly finalized guide can be found here, however here are the original steps:
Step 1. Find some suitable (has few clouds, isn’t very dark, etc.) data on the Data Hub, and download it.
This step was to be the least troublesome, since what do you need? A good internet connection (check?), and hard drive space, since each dataset we download is about 4-7 gigabytes (also check). The only problem was the downloads seemed to fail, without warning or rhyme, or reason. One could move the mouse constantly, and it might fail, one could leave the computer unattended and the same thing would happen, even though the last 5 datasets were downloaded without fail.
It was quite a mystery, but thankfully the browser could restart the download (after refreshing the page and logging in again to make sure). Another helpful site was an archive, where some of the more recent datasets were uploaded. These could be easily downloaded with wget without any issues, so the troublesome downloading (the Data Hub only allows two concurrent downloads at a time) was more or less solved.
Step 2. Edit the data.
Here's how a few tilesets look when loaded, after you applied the styles.
As any reader may have felt profound despair at this point, so did I. My mentor most likely as well, as we both, very rightly, felt that there had to be a better, faster, more efficient way.
Who knew that the whole solution for all this would be a script?
So from then out, I was knee deep in the documentation to find exactly which classes store file-saving settings in QGis (hint: it’s this one). Applying the style was something I’ve seen used in plugins, so that was a good stepping stone.
A second great discovery was the fact that QGIS provides an easy way to generate the query window (so I didn’t have to meddle with the appearance, just finding the relevant settings in the documentation) through the processing toolbox.
Soon, the first version of the script was ready: Load the vrts in QGIS, open the script window, select where you want to save it, and which styles you want to use, and presto. An hour later you might be done (with that dataset. Onto the next!). My mentor was quite happy with the fact that you didn’t have to sit there and apply settings every 1 to 4 minutes, instead just once every half an hour or so (to load the next batch up). The sky, or in this case processing power of your computer, was the limit.
The last step in editing involves the creation of the actual slippy map tiles. Thankfully, that was already available in QGIS plugin (QTiles), so we didn’t have to find another way to make that. Tile creation however is a very slow process (it takes more than a day to process about 10 datasets), so this step is still a bit problematic. Even splitting the project into smaller sections doesn’t do much speedwise, but it’s fairly reasonable: there are 15 levels we need to create, with level 0 being a single image of the entire Earth, level 1 being four images, level 2 being those four split into 4 again, and you can soon see that at level 14, there are many tiles being generated. Such as it is.
Step 3. Upload and use the tiles in Marble.
This step is fairly obvious, you need to upload your freshly generated tiles onto the Marble servers, and soon you will be able the see the fruits of your labour with your own eyes. As of this post, more than 70 tilesets have been generated anduploaded, but there’s still a long way to go.
For now I’m just glad to say that I’ve had a wonderful experience this summer here at Marble, by having helpful mentors around who welcomed me into the community, heard all my issues and tried to support me whenever I got stuck. Overall I’m really happy that I took part, because I learned a lot about communication, project management, problem solving, both on my own, and with help. Of course, this is just the beginning of everything, and I hope to become much more productive and helpful in the future. As for everyone, I wish you all a great summer and many great experiences to you all.