# NASA Venus Relief

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## Usage

You access the global Magellan Venus relief grid by specifying the special name

> @venus_relief[\_*rru*[\_*reg*]]

This grid contains observed relief measured via altimetry.
The following codes for the optional *rru* and *reg* are supported (dimensions are listed
for pixel-registered grids; gridline-registered grids increment dimensions by one):

(tbl-venus_relief)=

| Code | Dimensions    | Reg | Size   | Description                                    |
| ---- | ------------- | --- | ------ | ---------------------------------------------- |
| 01d  | 360 x 180     | g,p | 107 KB | 1 arc degree Venus relief (Magellan @ 299 km)  |
| 30m  | 720 x 360     | g,p | 374 KB | 30 arc minute Venus relief (Magellan @ 149 km) |
| 20m  | 1080 x 540    | g,p | 783 KB | 20 arc minute Venus relief (Magellan @ 100 km) |
| 15m  | 1440 x 720    | g,p | 1.3 MB | 15 arc minute Venus relief (Magellan @ 75 km)  |
| 10m  | 2160 x 1080   | g,p | 2.8 MB | 10 arc minute Venus relief (Magellan @ 50 km)  |
| 06m  | 3600 x 1800   | g,p | 7.4 MB | 6 arc minute Venus relief (Magellan @ 30 km)   |
| 05m  | 4320 x 2160   | g,p | 10 MB  | 5 arc minute Venus relief (Magellan @ 25 km)   |
| 04m  | 5400 x 2700   | g,p | 16 MB  | 4 arc minute Venus relief (Magellan @ 20 km)   |
| 03m  | 7200 x 3600   | g,p | 27 MB  | 3 arc minute Venus relief (Magellan @ 15 km)   |
| 02m  | 10800 x 5400  | g,p | 57 MB  | 2 arc minute Venus relief (Magellan @ 10 km)   |
| 01m  | 21600 x 10800 | g,p | 208 MB | 1 arc minute Venus relief (original)           |

Note that resolutions 05m and higher are tiled.
See {gmt-docs}`Remote Datasets <datasets/remote-data.html>` for more information.

See {gmt-docs}`GMT remote dataset usage <datasets/remote-data.html#usage>` for when resolution codes are optional or required.
All of these data will, when downloaded, be placed in your ~/.gmt/server directory, with
the venus_relief files being placed in an `venus/venus_relief` sub-directory. If you
do not specify a CPT then the default CPT for this dataset (the GMT master *@venus_relief.cpt* CPT) will be used.

## Examples

To create a PDF map of the Venus Magellan relief at 1 arc degree resolution, try:

```bash
gmt grdimage @venus_relief_01d -Baf -pdf venus_relief
```

## Technical Information

As you see, the 02m and lower resolutions are all derivatives of USGS' 1 arc-minute Sandwell_Final_Topo_Venus_2012_meter.grd
(Sandwell, 2012). We have downsampled it via Cartesian Gaussian filtering to prevent
aliasing while preserving the latitude-dependent resolution in the original 12.1468873601 arc sec grid.
The full (6 sigma) filter-widths are indicated in parenthesis. Almost all grids
are available in both gridline- and pixel-registered formats except the original pixel-registered
grid at 12.1468873601 arc-seconds resolution (here called @venus_relief_12s).

We also scale and reformat the original data to take up very little space so that downloads
from the servers are as fast as possible. For the venus_relief grids this means we chose 0.5
meter as the grid z step, which is well below the uncertainties in the data. Elevations
are scaled and shifted to fit in a short integer grid that is highly compressed by netCDF
lossless compression and chunking.

**Note**: Because of this quantization, these grids should be used with care in morphometric
studies (e.g., slope, curvature or roughness analyses), since even sub-meter rounding can bias
slope and aspect calculations [Carter, 1992]. We suggest using the original dataset instead for
such applications.

## Data References

1. Carter, J.R., 1992: [<https://doi.org/10.3138/aj35-34h3-524k-0685>].
1. Magellan: [<https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940013181.pdf>].