Resampling of map

Q&A
1

Hello everyone,

Before continuing with my message, I would like to mention that I am in no way an expert in using maps and related technologies, so please forgive any errors in using incorrectly any terms. It just happened that I have to create a map and… here I am. I have already put a lot of time to understand how GMT tools work, but I have an issue I cannot solve.

I downloaded elevation maps (?) in GeoTIIF (?) format from https://www.eorc.jaxa.jp/ALOS/en/aw3d30 for the area I am interested in, kept only the tiles I need and put these tiles into a VRT file using the command:

gdalbuildvrt all.vrt *.tif

Then, I created my map:

gmt grdimage all.vrt -R34/45/36/42 -JM26 -B2WESN -Xc -Yc -Cmby.cpt -V -K > map.ps

and also added some cities, lakes, rivers and other information to the map. This works great, but my issue is that the output of the above grdimage command is huge (about 650MB) and even transforming it into a PDF leaves a very big file (about 22MB) which cannot be incorporated into any paper.

The resolution of the maps I downloaded is about 30m, which is too much for my purposes. Is there any way to resample the map and reduce its size?

Thank you in advance for any help!

2

@Ioannis GMT has programs to resample grids, grdsample and grdfilter but what you want has already been done for you by GMT6. See the datasets page

3

Thanks for the pointers. I was not aware of the datasets, but in any case it is currently a bit difficult to install GMT 6 to use them (I am working with GMT 5). Nonetheless, grdsample did the trick.

Thank you so much for the help!