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POC: Add a separate documentation for supported plot and distance units #4359

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1 change: 1 addition & 0 deletions doc/techref/index.md
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Expand Up @@ -10,6 +10,7 @@ visit the {gmt-docs}`GMT Technical Reference <reference.html>`.

common_parameters.md
projections.md
units.md
fonts.md
text_formatting.md
encodings.md
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34 changes: 34 additions & 0 deletions doc/techref/units.md
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@@ -0,0 +1,34 @@
# Units

GMT uses two unit groups: **dimension units** (plot units) and **distance units** (Earth distances).

(plot-units)=
## Dimension Units (Plot Units)

Use these for map widths, offsets, symbol sizes, pen widths, etc.

| Code | Unit | Notes |
|------|------|-------|
| `c` | Centimeter | Default for most dimensions |
| `i` | Inch | 1 inch = 2.54 cm |
| `p` | Point | 1 point = 1/72 inch |

**Defaults:** If no unit is given, GMT uses {gmt-term}`PROJ_LENGTH_UNIT` (default is `c`).
Fonts and pen thicknesses default to `p`.

(distance-units)=
## Distance Units

GMT supports various distance units for geographic calculations.

| Code | Unit | Notes |
|------|------|-------|
| `d` | Degree | Arc degree |
| `m` | Minute of arc | 1/60 degree |
| `s` | Second of arc | 1/3600 degree |
| `e` | Meter | Default distance unit |
| `k` | Kilometer | 1000 meters |
| `f` | Foot | 0.3048 meters |
| `u` | US Survey Foot | 1200/3937 meters |
| `M` | Statute Mile | 1.60934 km |
| `n` | Nautical Mile | 1.852 km |
15 changes: 7 additions & 8 deletions pygmt/src/grdproject.py
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Expand Up @@ -81,16 +81,15 @@ def grdproject( # noqa: PLR0913
When set to ``True``, do the inverse transformation, from rectangular to
geographical [Default is ``False``].
unit
Set the projected measure unit. Valid values are ``"c"`` (centimeters),
``"i"`` (inches), and ``"p"`` (points) [Default is set by
:gmt-term:`PROJ_LENGTH_UNIT`]. Cannot be used with ``scaling``.
Set the projected measure unit. See :doc:`/techref/units#plot-units` for
supported units [Default is set by :gmt-term:`PROJ_LENGTH_UNIT`]. Cannot be used
with ``scaling``.
scaling
Force 1:1 scaling, i.e., output (or input, see ``inverse``) data are in actual
projected meters. To specify other units, set it to ``"f"`` (feet),
``"k"`` (kilometers), ``"M"`` (statute miles), ``"n"`` (nautical miles),
``"u"`` (US survey feet), ``"i"`` (inches), ``"c"`` (centimeters), or
``"p"`` (points). Without ``scaling``, the output (or input, see ``inverse``) is
in the units specified by :gmt-term:`PROJ_LENGTH_UNIT` (but see ``unit``).
projected meters. To specify other units, set it to
a supported distance unit or plot unit (see :doc:`/techref/units`). Without
``scaling``, the output (or input, see ``inverse``) is in the units specified
by :gmt-term:`PROJ_LENGTH_UNIT` (but see ``unit``).
$projection
$region
$verbose
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