Overlay grid on top of another grid - clipping

Q&A
#1

I want to create a simple map with earth_relief (https://docs.generic-mapping-tools.org/latest/datasets/remote-data.html#global-earth-relief-grids). On top of this, I would like to plot the earth_age (https://docs.generic-mapping-tools.org/latest/datasets/remote-data.html#global-earth-seafloor-crustal-age-grids).

What is the best way to do this? Making a custom clip path? Generate a mask with grdmask?

In simple words, what I want to do:
Take this map, remove all the gray and superimpose it on this (the longitude is shifted, but please ignore that).

#2

If you only worry about the figure, then you should create a clip path.

#3

I just dont know what the best way to create the clippath is. I want to get some ideas before i start manually tracing the earth_age grid.

#4

I think that the gray area are NAN. So with grdimage -Q you could make those nodes transparent. So you just plot first the relief grid and then the age grid with -Q.

#5

That’s a bingo, Esteban82! Thank you, exactly what I was looking for.

Just after I made this clip script… maybe it will be of help to me or others some other time:

#make clip path in other software (e.g. QGIS) and export as shapefile

#convert shapefile to gmt
ogr2ogr -F GMT no_age_clip.gmt no_age_clip.shp &> /dev/null

ps=clip.ps
#basemap
gmt grdimage -R-5/53/90/81r @earth_relief_05m -JU+34/20c -t50 -K > $ps

#make clip path. everything that is situated inside the polygon defined in no_age_clip.gmt is plotted
gmt psclip no_age_clip.gmt -J -R -K -O >> $ps

#fill the plot slot
gmt grdimage -R -J @earth_age_05m.nc -K -O >> $ps

#end clipping
gmt psclip -C -O >> $ps

#convert ps to png
gmt psconvert -Tg -A -Z $ps
#6

I just make this map with relief and the geological ages based on the age grid.

I use the following batch script (I change the extension from .bat to .txt to uploaded it).
Earth_Relief_Age_CPT-City.txt (1,9 KB)

#7

That’s a fantastic map! Thank you for sharing. Hope it’s OK that I borrow some of your ideas :slight_smile:

1 Like
#8

Yes it is ok. The original idea is yours.