Blue Marble
The classic Blue_Marble photo of Earth was taken by the crew of Apollo 17 on their way to the moon in 1972.
In 2002 NASA released an updated version suitable for GIS created by a mosaic of satellite images with the clouds removed. In 2005 this was rereleased, with a separate image provided for each month of the year.
This page deals with importing the latter into GRASS.
2002
Data source: NASA's Visible Earth website.
- From this site you can also get the well known "Earth at night" image, and a version of the Blue Marble including clouds.
The entire world image is 43200x21600, 1/2 arc-minute (~1km per pixel) resolution.
Download
To download the maps you have a few choices:
- The new maps can be found here:
21600x21600 East Hemisphere (401mb)
21600x21600 West Hemisphere (241mb)
The Wget program is nice for downloading these. For example:
wget ftp://veftp.gsfc.nasa.gov/bluemarble/land_shallow_topo_east.tif wget ftp://veftp.gsfc.nasa.gov/bluemarble/land_shallow_topo_west.tif
- From the original site (only gzip compressed)
Goto http://mirrors.arsc.edu/nasa/Blue_Marble_2002/
If you are west, download
- MOD09A1.W.interpol.cyl.retouched.topo.3x21600x21600.gz
if you are east, download
- MOD09A1.E.interpol.cyl.retouched.topo.3x21600x21600.gz
If you are in England or West Europe, you will need both files, because they have to overlap.
- Download them via ED2k Link (bzip2 compressed):
ed2k://|file|top_nasamap_east.raw.bz2|379972653|7312437945bd47ccf0b2a0c3452d5836|/ ed2k://|file|top_nasamap_west.raw.bz2|226307352|13ab6e8a6e014fa23bb83db25855bb71|/
This files have a uncompressed size of 1.4GB each and are in raw RGB format. The bzip2 compressed files are 363MB (east) and 216MB (west) in size.
- Check if the files are available from geotorrent.org
Import
- Create a lat/lon location. I used WGS84 as the datum, but as typical datum differences are much less than the pixel resolution I'm not sure if it makes a difference which one you chose. The only problematic choice might be sphere (the GDAL "null nadgrids" problem). You can overlay a world coastline, like admin98.shp, to test how you've done.
- Alternatively you can import into a raw XY location, and reset the map bounds to 90N,S 180E,W with the r.region module. You would then need to convert the location to lat/lon with g.setproj. If all went well r.info should report the cell resolution as exactly 0:00:30 (i.e. 30 arc-second).
- Create "world files" for the raw .tif images (thus making them into GeoTiffs)
- land_shallow_topo_east.wld
0.008333333333333 0.000000000000000 0.000000000000000 -0.008333333333333 0.00416666666666665 89.99583333333334
- land_shallow_topo_west.wld
0.008333333333333 0.000000000000000 0.000000000000000 -0.008333333333333 -179.9958333333333 89.99583333333334
- Import into a lat/lon location with r.in.gdal. Bypass the projection check with the -o flag.
- This will create three images, one for red, green, and blue.
- Zoom to the extents of both images (or just one) with:
g.region rast=BlueMarble_2002_east,BlueMarble_2002_west
- Display with d.rgb
- Merge east and west images by color bands with r.patch
- If you wish to combine R,G,B bands into a single raster map do so with r.composite. Only do this if you must, as you'll lose detail.
Issues
- If you want to down sample, try r.resamp.interp with the cubic method.
- I've had good results exported at natural resolution, and then cubic rescale in the GIMP to my desired size. I find the images a bit dark, brighten them in GIMP with Tools-> Color Tools-> Brightness-Contrast. Increase both brightness contrast together to around +40.
- Some of the pixels are NULL. If any of the R,G,B map pixels are NULL, d.rgb will show white for that cell, which may stick out badly. To mitigate the problem, try erasing the display to black first with d.erase.
2005 monthly
aka Blue Marble next generation, available at 8km, 2km, and 500m per pixel resolution. (i.e. 4 arc-minute, 1 minute, and 1/2 arc-minute*)
- [*] They claim 500m but from the filenames I think that may really mean 1/2min, which at the equator is 926m. They might be able to argue about cells covering less area a higher latitudes if the 4' and 1' versions used the same method. (I haven't actually downloaded the 1.36gb dataset, maybe it is really 15sec resolution!)
Download
Again you can download the entire dataset, but it may be preferable to only download the information for your local area. To do this you can use a WMS (Web_Map_Service) server and associated modules:
- The 500m dataset is available by BitTorrent. The tracker file is given at the NASA website. Each month is 1.36gb.
- [ktorrent] is a nice BitTorrent client for Linux