Basic understanding grdimage & -JL projection

Q&A
1

Plotting gridded data in the Hawaii area I wonder if the plot is actually correct or if I’m missing something:

mauna_loa_so2
mauna_loa_so22633×1535 113 KB

Take a closer look at the N20° parallel. The Lambert Conic Conformal projection makes the parallel bend southwards. When you look at the gridded data, it appears to be oriented parallel to the plot borders and not curved as I expected it:

Bildschirm­foto 2022-12-01 um 10.32.22
Bildschirm­foto 2022-12-01 um 10.32.222116×508 109 KB

Green: orientation of gridded data
Blue: I expected the data to bend like the parallel

I’m not sure if I just don’t understand how grdimage works or if I’m missing some option in my grdimage call? Thank you for your help!

Simplified example code:

gmt begin so2_data png
  gmt basemap -R-165/5/-105/35+r -JL-135/22.5/15/25/20c -B+f
  gmt grdimage so2.nc -Ccmocean/matter
  gmt coast -W0.2p,black -Bg10 -A200
gmt end show

Download so2.nc (~1.2 MB)

Attribution: Generated using Copernicus Atmosphere Monitoring Service information 2022

2

grdimage calls the PostScript image operator which expects a rectangular raster. Hence, the map projection converts lon/lat grids into projected rectangular grids which are then imaged as you see. So this is expected. You can try grdview -T for “raw” tiles.

1 Like
3

Thank you for your explanation @pwessel!

I should have used the grdview -T command.