2

I have trouble bringing land cover data from the USGS into a "decent" coordinate system.

I obtained an .img file of the "Global Land Cover Characteristics Data Base Version 2.0" from http://edc2.usgs.gov/glcc/tabgeo_globe.php. I renamed the file to *.bsq and created an .hdr file according to the following description on the USGS website:

9.2 Geographic Projection Parameters

The data dimensions of the Geographic projection for the global land cover characteristics data set are 21,600 lines (rows) and 43,200 samples (columns) resulting in a data set size of approximately 933 megabytes for 8-bit (byte) images. The following is a summary of the map projection parameters used for the Geographic projection:

Projection Type: Geographic

Units of measure: arc seconds

Pixel Size: 30 arc seconds

Radius of sphere: 6370997 m.

XY corner coordinates (center of pixel) in projection units (arc seconds):

    Lower left: (-647985, -323985)

    Upper left: (-647985, 323985)

    Upper right: (647985, 323985)

    Lower right: (647985, -323985)

So I have created the following .hdr file:

nrows   21600
ncols   43200
nbands  1
nbits   8
layout  bsq
skipbytes   0
ulxmap  -647985
ulymap  323985
xdim    30
ydim    30

When I now load the file into QGIS, it looks fine. But to convert the file from the original CRS to one of my choosing, I would have to know the original CRS in the first place. And this is where I am stuck: What is the CRS of the "Global Land Cover Characteristics Data Base Version 2.0" database? Can someone tell me from the aforementioned projection infos? I tried EPSG:4326 and EPSG:3857, but they do not seem to be correct.

Improve this question
  • You would have to make a custom definition because none of the existing ones use arc-seconds. It would be based upon NAD83 using GRS80 ellipsoid. I found an older issue in Proj.4 about adding the ability to set the angular unit, so I'm not sure if it's supported. – mkennedy Jul 16 '15 at 16:53
1

Thank you @mkennedy for your input.

I am posting an answer to my own question in case anybody stumbles across this issue.

From this website here, I was able to download the same dataset, but as .tif with pre-defined CRS. Go to the EarthExplorer from this aforementioned website, choose "Land Cover / GLCC" and take the .tif file. Registration (free) is required beforehand.

I guess that's as simple as it gets.

Improve this answer
  • Can you add the CRS definition that QGIS uses after loading the file? – AndreJ Jul 20 '15 at 10:59
  • The .tif file comes with the standard WGS84 (EPSG:4326) out of the box. But as I mentioned in the question at the top: Applying this CRS to the .bsq file did not give any meaningful results. So I assume the USGS people have done the transformation internally, before offering the .tif as WGS84 to the public. – Bob3k Jul 20 '15 at 11:53
0

Converting the units to degrees, you can use this header file:

nrows   21600
ncols   43200
nbands  1
nbits   8
layout  bsq
skipbytes   0
ulxmap  -179.996
ulymap  89.996
xdim    1
ydim    1

to perform a simple gdal translation with: 

gdal_translate -a_srs "+proj=latlong +r=6370997" -a_ullr -179.996 89.996 179.996 -89.996 goge2.bsq destfile.tif

to get the data into a GIS compatible format:

enter image description here

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.