# USGS Mercury Relief

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## Usage

You access the global Messenger relief grid by specifying the special name

> @mercury_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-mercury_relief)=

| Code | Dimensions    | Reg | Size   | Description                                          |
| ---- | ------------- | --- | ------ | ---------------------------------------------------- |
| 01d  | 360 x 180     | g,p | 114 KB | 1 arc degree Mercury relief (Messenger @ 315 km)     |
| 30m  | 720 x 360     | g,p | 401 KB | 30 arc minute Mercury relief (Messenger @ 157 km)    |
| 20m  | 1080 x 540    | g,p | 839 KB | 20 arc minute Mercury relief (Messenger @ 105 km)    |
| 15m  | 1440 x 720    | g,p | 1.4 MB | 15 arc minute Mercury relief (Messenger @ 79 km)     |
| 10m  | 2160 x 1080   | g,p | 3.0 MB | 10 arc minute Mercury relief (Messenger @ 52 km)     |
| 06m  | 3600 x 1800   | g,p | 7.9 MB | 6 arc minute Mercury relief (Messenger @ 32 km)      |
| 05m  | 4320 x 2160   | g,p | 11 MB  | 5 arc minute Mercury relief (Messenger @ 26 km)      |
| 04m  | 5400 x 2700   | g,p | 17 MB  | 4 arc minute Mercury relief (Messenger @ 21 km)      |
| 03m  | 7200 x 3600   | g,p | 29 MB  | 3 arc minute Mercury relief (Messenger @ 16 km)      |
| 02m  | 10800 x 5400  | g,p | 63 MB  | 2 arc minute Mercury relief (Messenger @ 11 km)      |
| 01m  | 21600 x 10800 | g,p | 224 MB | 1 arc minute Mercury relief (Messenger @ 1.9 km)     |
| 56s  | 23040 x 11520 | p   | 262 MB | 56.25 arc second Mercury relief (Messenger @ 1.9 km) |

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 mercury_relief files being placed in an `mercury/mercury_relief` sub-directory. If you
do not specify a CPT then the default CPT for this dataset (the GMT master *@mercury_relief.cpt* CPT) will be used.

## Examples

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

```bash
gmt grdimage @mercury_relief_01d -Baf -pdf mercury_relief
```

## Technical Information

As you see, the 01m and lower resolutions are all derivatives of USGS' 56.25 arc-second Mercury_Messenger_USGS_DEM_Global_665m_v2.tif
(Becker_et_al.,_2016). We have downsampled it via Cartesian Gaussian filtering to prevent
aliasing while preserving the latitude-dependent resolution in the original 56.25 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 56.25 arc-seconds resolution (here called @mercury_relief_56s).

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 mercury_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. Messenger: [<https://www.hou.usra.edu/meetings/lpsc2016/pdf/2959.pdf>].