on December 27, 2022

QGIS – How to globe-rotate a raster around the Greenwich Meridian?

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Globe-Rotating a Raster in QGIS: No Spinning Globe Button? No Problem!

So, you’re working with spatial data in QGIS and need to give your raster a little spin, huh? Specifically, you want to center it around the Greenwich Meridian. Maybe you’re tired of seeing the Pacific Ocean split down the middle of your world map. I get it. While QGIS doesn’t have a dedicated “globe-rotate” button that magically spins your map, don’t worry! There are definitely ways to get the job done. It’s all about understanding how GIS handles these things.

Think of it this way: raster datasets live in a specific coordinate system, a sort of spatial address system. This system dictates how the data is projected onto a flat surface. When a raster crosses the Greenwich Meridian, longitudes are typically expressed from -180° to +180°. Globe-rotating, in essence, means shifting the whole thing so that 0° longitude (Greenwich) becomes the new center. This might mean your longitudes end up ranging from 0° to 360°, or maybe they’re re-centered back to -180° to +180° but with the data rearranged. Tricky, but totally doable. Let’s dive in.

Method 1: Warp It Like a Pro (Using GDAL, of Course)

The “Warp (Reproject)” tool is your best friend here. It’s like the Swiss Army knife of raster transformations, powered by the mighty GDAL (Geospatial Data Abstraction Library). This tool lets you reproject your raster into a new coordinate system or tweak the existing one, effectively sliding it around the globe.

Here’s the lowdown:

  • Fire up QGIS: Pretty self-explanatory, right?

  • Load ‘er up: Drag and drop your raster file into QGIS, or use the “Add Raster Layer” button.

  • Warp Speed: Go to Raster > Projections > Warp (Reproject). Alternatively, just type “warp” or “reproject” into the Processing Toolbox search bar.

  • Configure the beast: This is where you tell the tool what to do.

    • Input Layer: Pick the raster you want to transform.
    • Source CRS (Optional): If QGIS doesn’t know the raster’s current projection, tell it here.
    • Target CRS: The key to the whole operation! You might not need to change the actual CRS, but you’ll need to play with the “georeferenced extents” to make the shift happen. For instance, if your data is already in WGS 84 (EPSG:4326), stick with it, but adjust those extents.

  • Georeferenced extents: Define the spatial boundaries of your output raster. This is where you tell QGIS how much to shift the data.

  • Tweak to perfection: Mess with the other settings, like the resampling method (how pixel values are interpolated). “Nearest Neighbour” keeps the original values intact, but “Bilinear” or “Cubic” can give you smoother results. Your call, depending on what you’re after.

  • Hit “Run” and cross your fingers! (Okay, it’s usually pretty reliable).

  • Behold! A new layer should appear. Check it out and make sure the shift worked as expected. If not, tweak those parameters and try again.

    • Method 2: Clip, Shift, and Stitch (The DIY Approach)

    This method is a bit more hands-on, but it works in a pinch. It involves cutting the raster into pieces, moving one piece, and then gluing them back together.

    Here’s the gist:

  • Slice and dice: Use the “Clip Raster by Extent” tool to create two raster files. One covering -180 to 0 longitude, the other from 0 to +180.
  • Shift that sucker: Use the rasmover function to shift the part from 180 to 360 longitude with -360.
  • Frankenstein it: Use the “Merge” tool (Raster > Miscellaneous > Merge) to combine the two back into one.

    • Things to Keep in Mind

    • CRS is king: Know your coordinate systems! Choosing the right one is crucial.
    • Data loss is a possibility: Reprojecting can mess with your data a tiny bit due to resampling. Pick a resampling method that minimizes this.
    • File size can balloon: Reprojecting can sometimes make your files bigger. Consider using compression (like GeoTIFF with DEFLATE) to keep things manageable.

    Final Thoughts

    Globe-rotating a raster in QGIS might not be as simple as clicking a button, but with the “Warp (Reproject)” tool, it’s definitely within reach. By understanding how coordinate systems work and playing around with the settings, you can wrangle your raster data into exactly the view you need. And hey, if you’re feeling adventurous, the clip-shift-stitch method is always there as a backup! While the GDAL Warp tool is generally preferred for its efficiency and control over the reprojection process, remember that the best method depends on your specific needs and data. Happy mapping!

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