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Experiments with overlaying time-series satellite imagery on a map

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### Videomap.earth

Experiments with overlaying time-series satellite imagery as video over maps.

### Frontend Setup

The frontend fetches a GeoJSON FeatureCollection representing the video files that need to be displayed alongwith their bounding boxes.

The default location for the video file is in [videos/videos.geojson](videos/videos.geojson). If you want to fetch from somewhere else, you can pass a custom `url` parameter in the querystring for the page.

So, if you are running the site at http://localhost:9000 and have your GeoJSON served at http://example.com/myvideos.geojson, you can go to http://localhost:9000/?url=http://example.com/myvideos.geojson . You will need to ensure that your GeoJSON file as well as all videos are being served with headers to enable CORS.

The GeoJSON feature collection has the following top-level properties:

- `base_url`: The Base URL for the video files. You can omit this or leave as empty string if specifying absolute URLs for videos.
 - `center`: REQUIRED: The point the map should be centered on when loading
 - `zoom`: REQUIRED: The zoom level of the map when loading
 - `frames`: Optional: Object containing metadata to be displayed per frame / timecode. See `videos.geojson` for an example.

The FeatureCollection must also contain a property called `features` which is an array of GeoJSON features. Each feature MUST:

- Be of the type `Polygon`
 - The coordinates of the Polygon must be an array, with a single array inside it, which has 5 members, each of which is an array of [lng, lat]. See the example GeoJSON file.
 - Each feature must have a property called `url` which is the path to the video file. This can be a path relative to the `base_url` or an absolute URL. If it is an absolute URL coming from another domain, it will need to be served with CORS headers.

See [videos/videos.geojson](videos/videos.geojson) for an example.

If you are running this yourself, use your own Mapbox token by editing the `mapboxgl.accessToken` value in [index.js](js/index.js).

To run, use your favourite method to serve a local web-server in this folder. With `python3` this is:

```
    python3 -m http.server 9000
```

Then, go to http://localhost:9000 in your browser.

### Video creation
The `stac2video` command-line utility can be used to prepare videos from Sentinel-2
imagery. The utility will create videos from imagery over a time range at fixed
time intervals, as specified by the user. Every frame of the videos is a
composite image. The latest non-cloudy pixel is used as compositing criteria,
using the cloud and cloud shadow masks from the Sentinel-2 SCL layer.

The key input the user needs to specify is location, a date range, and the
number of tiles to create at a specified zoom level around the center point.
The output of the script will be written to a local folder that is also 
specified by the user.

In addition to the video files, the utility also creates a valid geojson file
called `videos.geojson`. This file will be used by the frontend application
to place the video tiles on the map and to track some metadata about the
imagery used for the videomap creation.

To install the command line utility, checkout the repo and then use pip
to install the script into your python environment.

```
pip install -e .
```

The following example will create a videomap with 5x3 tiles at zoom level 12
with monthly image composites.

```
stac2video --dst=/path/to/existing/folder/videos --coordx=-9.15032 --coordy=38.72595 --start=2023-01-01 --end=2023-05-01 --zoom=12 --width=5 --height=3 --interval=1M
```

Use the help function to obtain a full list of the input parameters

```
>>> stac2video --help
Usage: stac2video.py [OPTIONS]

Options:
  --dst PATH        Directory to which output will be written.  [required]
  --coordx FLOAT    Longitude coordinate to selectthe central TMS tile
                    [required]
  --coordy FLOAT    Latitude coordinate to select the central TMS tile
                    [required]
  --zoom INTEGER    Zoom level of the target TMS tiles  [required]
  --start TEXT      Start date for video in YYYY-MM-DD  [required]
  --end TEXT        End date for video in YYYY-MM-DD  [required]
  --height INTEGER  How many tiles to include in Y direction
  --width INTEGER   How many tiles to include in X direction
  --interval TEXT   Time interval to use for compositing imagery. Defaults to
                    14D. This will be passed to the `resample` function on the
                    time dimension from xarray. See https://docs.xarray.dev/en
                    /stable/generated/xarray.DataArray.resample.html
  --up INTEGER      Use higher zoom level to determine resolution of output
                    videos. The default resolution is 256x256 pixels. Up works
                    as multiplier. For instance, for up=1, the resolution of
                    the video will be 512x512 pixels, and up=2 it will be
                    1024x1024 pixels
  --images          Output each frame as png image as well
  --keep-mp4        Keep intermediary mp4 files
  --help            Show this message and exit.
```

### Deploying the data
The only thing needed to show the videos online is to make the output of the `stac2video.py` utility accessible through the web.

Once the folder is accessible, you can point videomap.earth to the location of the automatically generated `videos.geojson` file in the folder.

https://videomap.earth/?url=https://s3.eu-west-2.amazonaws.com/videomap.earth/videos/videos.geojson

videomap.earth

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