Inverted Mercator

Is there an easy way to generate a Mercator projection with south up?

yes. Look at common options “-p”
Using -p180 on a mercator projection puts South “up”
You will have to play around more carefully with positioning and rotation of text

Yes, could use -p0/90 (azimuth 0, view from the north, and elevation 90º).

gmt grdimage @earth_relief_30m -png mapa -p0/90 -Baf -JM15c -RAfrica

Thanks biggle and Esteban. Of the two, I found the -p0/90 the better option as it has the lat/long annotations orientated the correct way up. There’s probably a -B setting to do that with -p180, but I haven’t looked hard enough. To get the text the correct way up I’ve added +a180 to the gmt text -F option and reversed the offset. All good except for one thing: The symbols are still upside down. How do I rotate them 180°? My code and output as it currently stands is below.

gmt coast -R-7/2/50/59 -JM10c -Glightyellow -Slightblue -B -p0/90

:: Symbols.txt has structure Long,Lat,Symbol(t)
gmt plot Symbols.txt -S0.3c -Wred -Gred -p

:: Names.txt has structure Long,Lat,Name
gmt text Names.txt -F+f0.5c,black+jLB+a180 -D-0.25c -p

gmt plot Names.txt -W0.02c,red -p

Use -Si... instead of -St... (reversed triangle)

Yes, I found that for the triangle. Thanks. I also use symbols that don’t have an inbuilt inverted version (e.g. star and volcano). I’m wondering if there is simple command line option/switch to invert those symbols? It’s not a problem if there isn’t, as I have designed my own inverted custom symbols. I’m just looking for an easier option.

Might be simplest to make a few custom symbols and just start with a 180 rotation before plotting one of the build in ones, e.g.,

cat << EOF > rstar.def
# Rotate 180 first
180	O
# Plot the reversed star
0 0 1 a

echo 30 30 > t.txt

This has the top of the N-star pointing south

gmt psxy -Rg -JH5i -Baf -Skrstar.def/1c -Gred t.txt -P >

while this is the normal star pointing north

gmt psxy -Rg -JH5i -Baf -Sa1c -Gred t.txt -P >

1 Like

Thanks Paul. That makes creating my custom inverted symbols a whole lot easier. Cheers.