User:Ff5722/Using Sentinel-2 imagery
Sentinel-2 satellite data is licenced suitably to be used as background for tracing or any other purpose for OSM (licence). Best resolution available is 10 m, this is enough to trace major roads, rivers, lakes, railways etc (preview). All of China (and the rest of the world) is covered, and imagery is very recent (several captures per month, as recent as last week).
I made a video guide, but it is only complementary to the process described below. https://www.youtube.com/watch?v=mNqnLOgEM_Y
2. Download data from scihub.copernicus.eu. You need to register a (free) account to download data. Select an area on the map and search. The results with the green tag S2A MSI are the ones you will want. Download the zip package (usually around 700 MB).
You can use remotepixel.ca/projects/satellitesearch.html for larger previews and easier search.
3. Process data in SNAP (tutorial). Unzip the compressed folder, and open the XML file in the top level. Once it's visible in the left pane, right-click to open RGB view, the three colour bands will be pre-selected. Simply click OK and wait for the RGB image to be generated. Before exporting, use the colour manipulation tool in the bottom left pane to ensure good contrast of roads etc.
4. Export processed image from SNAP. Right-click on the image, export. Export full scene and full resolution as geoTIFF. This will take several minutes to finish.
Use image as background
A way to get the geoTIFFs as a convenient background layer is to set up upload and set up WMS server at nextgis.com, or use Mapbox Studio to set up a custom style (recommended). You can also use the PicLayer plugin to use exported imagery directly in JOSM, but then you have to calibrate it manually each time. In the latter case, feel free to export from SNAP as JPG, this will save you a lot of disk space. Another option is using the ImportImagePlugin, which can use the location data of GeoTIFF files. However I found that it is very slow and the alignment is not good.
- After signing up for Mapbox, go to tilesets > New tileset > Upload > upload your geoTIFF
- Then go to Styles > new style > pick any ('satellite', 'empty', or 'black' work best).
- Go to your new style > edit > new layer > under 'source' select tileset > select your uploaded image.
- Go back to your style, on the right click Share, develop & use and you will find WMTS (for JOSM) and TMS (for iD) endpoints.
Thanks to 7thgrade for guiding me through the processing method
This takes a few minutes to set up, but it's faster and it saves you the hassle of uploading the large files to Mapbox.
Should you want to trace a specific region, but you can't be bothered to process the imagery yourself, then send me a message and I will process it for you when I have time and send you the link to use as background layer.
A significant part of China is already processed.
- For exporting full resolution images in SNAP you will probably need 8 GB of RAM on your system.
- Use the url scheme for 'Leaflet'