Scale grid coordinates

Q&A
1

Is there a way to scale the grid coordinates?

I have a grid with the following extent:

$ gmt grdinfo is.tif -I1
-R321761/876155/7016802/7403589

I would like to edit the grid, and divide the coordinates by 1000, so that the extent becomes:

$ gmt grdinfo is.tif -I1
-R321.761/876.155/7016.802/7403.589

The x_inc and y_inc also have to be changed, but maybe this is implicit?

I was looking at grdmath, but didn’t find anything very obvious.

The X and Y symbols may be interesting, but I dont’t understand how to use them; the list does not provide info about the number of arguments, as the other operators table does:

$ gmt grdmath is.tif X = temp.nc 
grdmath [WARNING]: 1 more operands left on the stack!
2

grdedit?

3

Yes, maybe that’ll work. Just input my ‘new’ region.

Thanks! Will try.

4

If these data were in NetCDF format, the NetCDF operators (ncap2 in particular) would get you out of jail. Hopefully grdedit works - if you want a NetCDF operator solution let me know here and I’ll find the code.

5

@timhume My grid is absolutely available as netCDF (it’s converted to tif by me) - I’m very interested to see some NetCDF magic!

6

Hi Andreas,

Here’s how you do this to your original NetCDF file. If you don’t already have them on your computer, get the NetCDF operators from here: https://nco.sourceforge.net/

It’s highly likely your operating system’s packages or ports come with these, so installation should be easy enough.

I’m going to demonstrate on a file called ecmwf_202506021200.nc - your file will look different, but it should be easy enough to translate what is done here to your needs.

First examine the structure of your file with ncdump:

/Users/tim/ecmwf> ncdump -c ecmwf_202506021200.nc
netcdf ecmwf_202506021200 {
dimensions:
	longitude = 1440 ;
	latitude = 721 ;
	time = 41 ;
variables:
	float longitude(longitude) ;
		longitude:units = "degrees_east" ;
		longitude:long_name = "longitude" ;
	float latitude(latitude) ;
		latitude:units = "degrees_north" ;
		latitude:long_name = "latitude" ;
	int time(time) ;
		time:units = "hours since 1900-01-01 00:00:00.0" ;
		time:long_name = "time" ;
		time:calendar = "gregorian" ;
	float tp(time, latitude, longitude) ;
.
.
.
 longitude = -180, -179.75, -179.5, -179.25, -179, -178.75, -178.5, -178.25,
    -178, -177.75, -177.5, -177.25, -177, -176.75, -176.5, -176.25, -176,
    -175.75, -175.5, -175.25, -175, -174.75, -174.5, -174.25, -174, -173.75,
    -173.5, -173.25, -173, -172.75, -172.5, -172.25, -172, -171.75, -171.5
.
.
.
 latitude = 90, 89.75, 89.5, 89.25, 89, 88.75, 88.5, 88.25, 88, 87.75, 87.5,
    87.25, 87, 86.75, 86.5, 86.25, 86, 85.75, 85.5, 85.25, 85, 84.75, 84.5,
    84.25, 84, 83.75, 83.5, 83.25, 83, 82.75, 82.5, 82.25, 82, 81.75, 81.5,
.
.
.

The dimensions I want to scale down by 1000 are longitude and latitude which currently range from -180.0 to 179.75 and 90.0 to -90.0 respectively.

This is done easily with ncap2 from the NetCDF operators:

ncap2 -O -s "longitude=longitude/1000; latitude=latitude/1000" ecmwf_202506021200.nc scaled.nc

Now if you run the previous ncdump command you’ll see this:

/Users/tim/ecmwf> ncdump -c scaled.nc
netcdf scaled {
dimensions:
	longitude = 1440 ;
	latitude = 721 ;
	time = 41 ;
variables:
	float longitude(longitude) ;
		longitude:long_name = "longitude" ;
		longitude:units = "degrees_east" ;
	float latitude(latitude) ;
		latitude:long_name = "latitude" ;
		latitude:units = "degrees_north" ;
.
.
.
 longitude = -0.18, -0.17975, -0.1795, -0.17925, -0.179, -0.17875, -0.1785,
    -0.17825, -0.178, -0.17775, -0.1775, -0.17725, -0.177, -0.17675, -0.1765,
    -0.17625, -0.176, -0.17575, -0.1755, -0.17525, -0.175, -0.17475, -0.1745,
.
.
.
 latitude = 0.09, 0.08975, 0.0895, 0.08925, 0.089, 0.08875, 0.0885, 0.08825,
    0.088, 0.08775, 0.0875, 0.08725, 0.087, 0.08675, 0.0865, 0.08625, 0.086,
    0.08575, 0.0855, 0.08525, 0.085, 0.08475, 0.0845, 0.08425, 0.084,
.
.
.

Which is probably all you need to do. But if you’re being picky, you’ll notice the units for the longitude and latitude dimensions are now incorrect (they’re still in units of degrees_east and degrees_north rather than in units of kdegrees_east and kdegrees_north). Another NetCDF operator, ncatted can solve this:

ncatted -O -a units,longitude,o,c,kdegrees_east \
    -a units,latitude,o,c,kdegrees_north scaled.nc scaled.nc

You can rerun ncdump to confirm the units attribute has indeed been changed.

7

Thank you so much @timhume!
I’m going to play around with this.

(Not sure if I have time for much before summer though)