# Plotting only in projected cartesian coordinates

**URL:** <https://forum.generic-mapping-tools.org/t/plotting-only-in-projected-cartesian-coordinates/5472>\
**Category:** Q&A\
**Created:** [October 19, 2024, 7:28am UTC](https://forum.generic-mapping-tools.org/t/plotting-only-in-projected-cartesian-coordinates/5472 "2024-10-19T07:28:39Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![gabrahamastro](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/gabrahamastro/32/4552_2.png) [@gabrahamastro](https://forum.generic-mapping-tools.org/u/gabrahamastro)\
**Post date:** [October 19, 2024, 7:28am UTC](https://forum.generic-mapping-tools.org/t/plotting-only-in-projected-cartesian-coordinates/5472/1 "2024-10-19T07:28:40Z")

</div>

I have data in cartesian coordinates (easting northing from Ordnance Survey projection) and would like to plot only using this system. However, I’m struggling to understand what projection command to provide in order that I can show coastline data.  
Here’s my code so far:

```auto
fig = pygmt.Figure()
fig.grdimage(
    "@earth_relief_01s",
    region=region_deg,
    projection=projection_gmt,
    shading="+a45+nt0.7",
    cmap="gray",
)
fig.coast(shorelines=True, region=region_deg)

maxabs = vd.maxabs(Bouguer_Anomaly)

pygmt.makecpt(cmap="polar", series=[-maxabs, maxabs])
fig.plot(x=long, y=lat, fill=Bouguer_Anomaly, cmap=True, style="c2p", projection=projection_gmt)
fig.basemap(projection=projection_gmt, frame=True)
fig.colorbar(cmap=True, frame=["a50f25", "x+lBouguer Anomaly", "y+lmGal"])
fig.plot(data=np.array([[ed,sd,wd,nd]]), style='r+s', pen="3p,yellow")
fig.show()

```

where `ed,sd,wd,nd` are the east, south, west and north values for an area, `Bouguer_Anomaly` is the data I’m looking to plot and `region_deg` is the region I’m plotting.

Note that I’m currently plotting using `projection_gmt = "T-2/49/15c"` since this is the projection of OSGB36 coordinates. However, I’d instead like to use easting northing data to plot, and therefore would like to define the conversion (i.e where the origin of the coordinates is, in order that `pygmt` knows what coast data to plot underneath my `Bouguer_Anomaly` data.

The main purpose behind this is that I want to reduce error accrued from constantly converting between cartesian and geographic coordinates (with the analysis processes I’m using requiring cartesian data).

I’m assuming the way forward would be to use `pygmt.grdproject`, but I’m still confused as to how to define the projection later on in the figure.

The system I’m using has true origin easting 40000, northing -10000; true lat long origin 49ºN,2ºW, scale factor on the central meridian of 0.9996012717, and uses the Transverse Mercator projection with the Airy 1830 ellipsoid.

Lastly, I should note that I’m using `Verde`, a gridding python library, meaning that `vd.maxabs` simply chooses the maximum value from the `Bouguer_Anomaly` data, for colorbar scaling purposes.

---

<div class="post-metadata">

**Author:** ![mkononets](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/mkononets/32/4029_2.png) [@mkononets](https://forum.generic-mapping-tools.org/u/mkononets)\
**Post date:** [October 19, 2024, 11:10am UTC](https://forum.generic-mapping-tools.org/t/plotting-only-in-projected-cartesian-coordinates/5472/2 "2024-10-19T11:10:48Z")

</div>

> [@gabrahamastro](#):
>
> I have data in cartesian coordinates (easting northing from Ordnance Survey projection) and would like to plot only using this system.

as your data are already projected, you can plot your data using Cartesian projection: `projection = X6c` or `projection=x6c` changing size or scale as needed.

> [@gabrahamastro](#):
>
> However, I’m struggling to understand what projection command to provide in order that I can show coastline data.

To overlay GMT coastlines you need to correctly specify your projection to `pscoast`:

> [@gabrahamastro](#):
>
> The system I’m using has true origin easting 40000, northing -10000; true lat long origin 49ºN,2ºW, scale factor on the central meridian of 0.9996012717, and uses the Transverse Mercator projection with the Airy 1830 ellipsoid.

This may or may not work, see more below.

You would need to indicate your projection ellipsoid as shown here [**PROJ\_ELLIPSOID**](https://docs.generic-mapping-tools.org/latest/gmt.conf.html#term-PROJ_ELLIPSOID)  
This is done using `pygmt.config`:

```auto
pygmt.config(PROJ_ELLIPSOID="6378137")

```

Check GMT manual for the Transverse Mercator projection projection, indicated by letter T or t: [6.3.2. Transverse Mercator projection ( **-Jt**  **-JT** )](https://docs.generic-mapping-tools.org/latest/reference/map-projections.html#transverse-mercator-projection-jt-jt). However I can see no way of specifying origin easting and northing, these `true origin easting 40000, northing -10000` using (py)gmt `-R` syntax.

There is another possibility: to save coastlines in the region of interest from `coast` to files and reproject these to your coordinate system using `mapproject`. For reprojection you can use more or less any source and destination coordinate system using e.g. `PROJ` specifications, while the subset that actually works for plotting and drawing map frames is limited which is the problem for directly using `pygmt.coast`. The reprojected coastlines are free from this issue and can be potted using `pygmt.plot()` and the same Cartesian projection using `projection=X6c`.

Or, as you guessed, you could reproject your Cartesian grid to geographic coordinates and do all your plotting in geographical coordinates. Specifying the source ellipsoid and projection would also be necessary.