3
$\begingroup$

The official data description of the recently updated GIMMS NDVI3g record describes the grid parameters of the dataset as follows.

grid-name: Geographic Lat/Lon
pixel-size: 1/12=0.0833 degrees

size-x: 4320
size-y: 2160

upper-left-lat: 90.0-1/24
upper-left-lon: -180.0+1/24
lower-right-lat: -90.0+1/24
lower-right-lon: 180.0-1/24

This means (at least to my understanding) that in x-direction, the cell size equals

> (180-1/24) * 2 / 4320
[1] 0.08331404

whereas in y-direction, the cell size equals

> (90-1/24) * 2 / 2160
[1] 0.08329475

I.e., the single grid cells are not entirely square-shaped. Am I missing something in my calculation or does the officially described pixel size of "1/12=0.0833 degrees" simply result from rounding effects of the manually calculated edge lengths in x- and y-direction?

$\endgroup$
4
$\begingroup$

Your understanding is inaccurate. The shifts by $\frac{1^\circ}{24}$ are only relevant for figuring out, where the pixel centers are located and where to start when you are looking to find the right grid index for a certain point. As for the resolution, it is much simpler. We have $4320$ pixels covering $360^\circ$ in longitude direction, i.e. a resolution of $\frac{1^\circ}{12}$ and similarly for the latitudinal resolution.

As for the square-nature of these pixels, they are only squares in lat-lon coordinates, not in terms of length on the ground.

$\endgroup$
  • $\begingroup$ Great, thanks! I wasn't entirely sure what the annotation "coordinates located UL corner of pixel" in the README was supposed to mean. $\endgroup$ – fdetsch Jan 27 '16 at 14:33

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.