<p>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.</p>

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

CC BY-SA 3.0
2016-01-27T14:33:26.363Z
Great, thanks! I wasn't entirely sure what the annotation "coordinates located UL corner of pixel" in the README was supposed to mean.
fdetsch
CC BY-SA 3.0
2016-01-27T14:09:02.867Z
2016-01-27T14:09:02.867Z
Christoph
<p>The <a href="https://nex.nasa.gov/nex/projects/1349/wiki/general_data_description_and_access/" rel="nofollow">official data description</a> of the recently updated GIMMS NDVI3g record describes the grid parameters of the dataset as follows. </p>

<pre><code>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
</code></pre>

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

<pre><code>&gt; (180-1/24) * 2 / 4320
[1] 0.08331404
</code></pre>

<p>whereas in y-direction, the cell size equals</p>

<pre><code>&gt; (90-1/24) * 2 / 2160
[1] 0.08329475
</code></pre>

<p>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?</p>

<remote-sensing><data-formats><cartography><geospatial><vegetation>
GIMMS NDVI3g grid cells not entirely square-shaped?
fdetsch
# GIMMS NDVI3g grid cells not entirely square-shaped?

(Asked by: username_0 on 2016-01-27)

The [official data description](https://nex.nasa.gov/nex/projects/1349/wiki/general_data_description_and_access/) 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?

**Tags:** remote-sensing, data-formats, cartography, geospatial, vegetation


---

## 1 answers

### Answer by username_1 on 2016-01-27. Score: 4 | Accepted answer

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.

> **Comments:**
> **username_0**: Great, thanks! I wasn't entirely sure what the annotation "coordinates located UL corner of pixel" in the README was supposed to mean.  

---

