# 5. GMT Coordinate Transformations¶

GMT programs read real-world coordinates and convert them to positions on a plot. This is achieved by selecting one of several coordinate transformations or projections. We distinguish between three sets of such conversions:

- Cartesian coordinate transformations
- Polar coordinate transformations
- Map coordinate transformations

The next Chapter will be dedicated to GMT map projections in its
entirety. Meanwhile, the present Chapter will summarize the properties
of the Cartesian and Polar coordinate transformations available in
GMT, list which parameters define them, and demonstrate how they are
used to create simple plot axes. We will mostly be using
basemap (and occasionally plot) to demonstrate the various
transformations. Our illustrations may differ from those you reproduce
with the same commands because of different settings in our `gmt.conf`

file.)
Finally, note that while we will specify dimensions in inches (by
appending **i**), you may want to use cm (**c**), or points (**p**) as
unit instead (see the gmt.conf man page).

## 5.1. Cartesian transformations¶

GMT Cartesian coordinate transformations come in three flavors:

- Linear coordinate transformation
- Log\(_{10}\) coordinate transformation
- Power (exponential) coordinate transformation

These transformations convert input coordinates *(x,y)* to
locations *(x’, y’)* on a plot. There is no coupling between
*x* and *y* (i.e., *x’ = f(x)* and *y’ = f(y)*);
it is a **one-dimensional** projection. Hence, we may use separate
transformations for the *x*- and *y*-axes (and
*z*-axes for 3-D plots). Below, we will use the expression
*u’ = f(u)*, where *u* is either *x* or *y* (or
*z* for 3-D plots). The coefficients in *f(u)* depend on the
desired plot size (or scale), the chosen *(x,y)* domain, and the
nature of *f* itself.

Two subsets of linear will be discussed separately; these are a polar
(cylindrical) projection and a linear projection applied to geographic
coordinates (with a 360º periodicity in the *x*-coordinate). We
will show examples of all of these projections using dummy data sets
created with gmtmath, a “Reverse
Polish Notation” (RPN) calculator that operates on or creates table data:

gmt math -T0/100/1 T SQRT = sqrt.txt gmt math -T0/100/10 T SQRT = sqrt10.txt

### 5.1.1. Cartesian linear transformation (**-Jx** **-JX**)¶

There are in fact three different uses of the Cartesian linear transformation, each associated with specific command line options. The different manifestations result from specific properties of three kinds of data:

Regular floating point coordinates

Geographic coordinates

Calendar time coordinates

Examples of Cartesian (left), circular (middle), and geo-vectors (right) for different attribute specifications. Note that both full and half arrow-heads can be specified, as well as no head at all.

#### 5.1.1.1. Regular floating point coordinates¶

Selection of the Cartesian linear transformation with regular floating
point coordinates will result in a simple linear scaling
*u’ = au + b* of the input coordinates. The projection is defined
by stating scale in inches/unit (**-Jx**) or axis length in inches (**-JX**).
If the *y*-scale or *y*-axis length is different from that of the
*x*-axis (which is most often the case), separate the two scales (or
lengths) by a slash, e.g., **-Jx**0.1i/0.5i or **-JX**8i/5i. Thus,
our \(y = \sqrt{x}\) data sets will plot as shown in
Figure Linear transformation of Cartesian coordinates.

The complete commands given to produce this plot were

```
gmt begin GMT_linear
gmt plot -R0/100/0/10 -JX3i/1.5i -Bag -BWSne+gsnow -Wthick,blue,- sqrt.txt
gmt plot -St0.1i -N -Gred -Wfaint sqrt10.txt
gmt end show
```

Normally, the user’s *x*-values will increase to the right and the
*y*-values will increase upwards. It should be noted that in many
situations it is desirable to have the direction of positive coordinates
be reversed. For example, when plotting depth on the *y*-axis it makes
more sense to have the positive direction downwards. All that is
required to reverse the sense of positive direction is to supply a
negative scale (or axis length). Finally, sometimes it is convenient to
specify the width (or height) of a map and let the other dimension be
computed based on the implied scale and the range of the other axis. To
do this, simply specify the length to be recomputed as 0.

#### 5.1.1.2. Geographic coordinates¶

While the Cartesian linear projection is primarily designed for regular
floating point *x*,*y* data, it is sometimes necessary to plot
geographical data in a linear projection. This poses a problem since
longitudes have a 360º periodicity. GMT therefore needs to be informed
that it has been given geographical coordinates even though a linear
transformation has been chosen. We do so by adding a **g** (for
geographical) or **d** (for degrees) directly after **-R** or by
appending a **g** or **d** to the end of the **-Jx** (or **-JX**)
option. As an example, we want to plot a crude world map centered on
125ºE. Our command will be

```
gmt begin GMT_linear_d
gmt set MAP_GRID_CROSS_SIZE_PRIMARY 0.1i MAP_FRAME_TYPE FANCY FORMAT_GEO_MAP ddd:mm:ssF
gmt coast -Rg-55/305/-90/90 -Jx0.014i -Bagf -BWSen -Dc -A1000 -Glightbrown -Wthinnest -Slightblue
gmt end show
```

with the result reproduced in Figure Linear transformation of map coordinates.

#### 5.1.1.3. Calendar time coordinates¶

Several particular issues arise when we seek to make linear plots using
calendar date/time as the input coordinates. As far as setting up the
coordinate transformation we must indicate whether our input data have
absolute time coordinates or relative time coordinates. For the former
we append **T** after the axis scale (or width), while for the latter we
append **t** at the end of the **-Jx** (or **-JX**) option. However,
other command line arguments (like the **-R** option) may already
specify whether the time coordinate is absolute or relative. An absolute
time entry must be given as [*date*]**T**[*clock*] (with *date*
given as *yyyy*[-*mm*[-*dd*]], *yyyy*[-*jjj*], or
*yyyy*[-**W***ww*[-*d*]], and *clock* using the
*hh*[:*mm*[:*ss*[*.xxx*]]] 24-hour clock format) whereas the
relative time is simply given as the units of time since the epoch
followed by **t** (see TIME_UNIT and TIME_EPOCH for
information on specifying the time unit and the epoch). As a simple
example, we will make a plot of a school week calendar
(Figure Linear transformation of calendar coordinates).

When the coordinate ranges provided by the **-R** option and the
projection type given by **-JX** (including the optional **d**, **g**,
**t** or **T**) conflict, GMT will warn the users about it. In
general, the options provided with **-JX** will prevail.

```
gmt begin GMT_linear_cal
gmt set FORMAT_DATE_MAP o TIME_WEEK_START Sunday FORMAT_CLOCK_MAP=-hham FORMAT_TIME_PRIMARY_MAP full
gmt basemap -R2001-9-24T/2001-9-29T/T07:0/T15:0 -JX4i/-2i -Bxa1Kf1kg1d -Bya1Hg1h -BWsNe+glightyellow
gmt end show
```

### 5.1.2. Cartesian logarithmic projection¶

The \(\log_{10}\) transformation is simply
\(u' = a \log_{10}(u) + b\) and is selected by appending an **l**
(lower case L) immediately following the scale (or axis length) value.
Hence, to produce a plot in which the *x*-axis is logarithmic (the
*y*-axis remains linear, i.e., a semi-log plot), try (Figure Logarithmic
transformation)

```
gmt begin GMT_log
gmt plot -R1/100/0/10 -Jx1.5il/0.15i -Bx2g3 -Bya2f1g2 -BWSne+gbisque -Wthick,blue,- -h sqrt.txt
gmt plot -Ss0.1i -N -Gred -W -h sqrt10.txt
gmt end show
```

Note that if *x*- and *y*-scaling are different and a
\(\log_{10}-\log_{10}\) plot is desired, the **l** must be
appended twice: Once after the *x*-scale (before the /) and once after
the *y*-scale.

### 5.1.3. Cartesian power projection …¶

This projection uses \(u' = a u^b + c\) and allows us to explore
exponential relationships like \(x^p\) versus \(y^q\).
While *p* and *q* can be any values, we will select *p
= 0.5* and *q = 1* which means we will plot *x* versus
\(\sqrt{x}\). We indicate this scaling by appending a **p** (lower
case P) followed by the desired exponent, in our case 0.5. Since
*q = 1* we do not need to specify **p**1 since it is identical
to the linear transformation. Thus our command becomes (Figure Power
transformation)

```
gmt begin GMT_pow
gmt plot -R0/100/0/10 -Jx0.3ip0.5/0.15i -Bxa1p -Bya2f1 -BWSne+givory -Wthick sqrt.txt
gmt plot -Sc0.075i -Ggreen -W sqrt10.txt
gmt end show
```

## 5.2. Linear projection with polar coordinates (**-Jp** **-JP**) …¶

This transformation converts polar coordinates (angle \(\theta\) and
radius *r*) to positions on a plot. Now \(x' = f(\theta,r)\)
and \(y' = g(\theta,r)\), hence it is similar to a regular map
projection because *x* and *y* are coupled and *x*
(i.e., \(\theta\)) has a 360º periodicity. With input and output
points both in the plane it is a **two-dimensional** projection. The
transformation comes in two flavors:

- Normally, \(\theta\) is understood to be directions counter-clockwise from the horizontal axis, but we may choose to specify an angular offset [whose default value is zero]. We will call this offset \(\theta_0\). Then, \(x' = f(\theta, r) = ar \cos (\theta-\theta_0) + b\) and \(y' = g(\theta, r) = ar \sin (\theta-\theta_0) + c\).
- Alternatively, \(\theta\) can be interpreted to be azimuths clockwise from the vertical axis, yet we may again choose to specify the angular offset [whose default value is zero]. Then, \(x' = f(\theta, r) = ar \cos (90 - (\theta-\theta_0)) + b\) and \(y' = g(\theta, r) = ar \sin (90 - (\theta-\theta_0)) + c\).

Consequently, the polar transformation is defined by providing

- scale in inches/unit (
**-Jp**) or full width of plot in inches (**-JP**) - Optionally, insert
**a**after**p| P**to indicate CW azimuths rather than CCW directions - Optionally, append /
*origin*in degrees to indicate an angular offset [0] - Optionally, append
**r**to reverse the radial direction (here,*south*and*north*must be elevations in 0–90 range). - Optionally, append
**z**to annotate depths rather than radius.

As an example of this projection we will create a gridded data set in polar coordinates \(z(\theta, r) = r^2 \cdot \cos{4\theta}\) using grdmath, a RPN calculator that operates on or creates grid files.

```
gmt begin GMT_polar
gmt grdmath -R0/360/2/4 -I6/0.1 X 4 MUL PI MUL 180 DIV COS Y 2 POW MUL = tt.nc
gmt grdcontour tt.nc -JP3i -B30 -BNs+ghoneydew -C2 -S4 --FORMAT_GEO_MAP=+ddd
gmt end show
```

We used grdcontour to make a contour map of this data. Because the data file only contains values with \(2 \leq r \leq 4\), a donut shaped plot appears in Figure Polar transformation.