Using pygmt.project to invert UTM projection

Q&A PyGMT Q&A
1

Hello, I’m trying to get a grid in UTM coordinates into Geographic coordinates (particularly the grids mentioned in this paper which can be found in this link) but I’m having no luck transforming the coordinates, and I don’t understand the documentation on the function and projection well enough. For now I’ve tried a couple options, all along the lines of:

masked_bed='path/to/grid.nc'
utm_zone=-18
lat_rad=-50*np.pi/180
scale=(1/111)#*np.cos(lat_rad)
# scale=1/4.370079e6#*np.cos(lat_rad)
masked_bed=pygmt.grdproject(
    grid=masked_bed,
    projection=f"U{utm_zone}/{scale}",
    scaling='k',
    inverse=True)

I have tried a lot of different ways to set the scale (either km/deg, m/deg, cm/deg, all of them at the mean latitude of the region and at the equator, in/deg, and all their corresponding inverses), and tried with and without the scaling option, but none of them gave me the correct result, I’m getting longitudes of -160 when I should be getting -70, and latitudes of -0.7 when I should be getting -50. I also tried to set the utm zone with the GMT convention for regions (this one would be 18F) but it made no difference.

I’m sure I’m missing something, maybe some parameter I should set, or a different way to set the details of the projection. Any help is greatly appreciated!

2 
grid="PIF_fuerst_2023_v10_npi_masked_bed_consens.nc"
outgrid="outgrid.grd"
pygmt.grdproject(grid,outgrid,projection="EPSG:32718",inverse=True)

gmt grdinfo outgrid.grd 
outgrid.grd: Title: Produced by grdproject
outgrid.grd: Command: gmt grdproject PIF_fuerst_2023_v10_npi_masked_bed_consens.nc -Goutgrid.grd -I -J+EPSG:32718
outgrid.grd: Remark: 
outgrid.grd: Gridline node registration used [Geographic grid]
outgrid.grd: Grid file format: nf = GMT netCDF format (32-bit float), CF-1.7
outgrid.grd: x_min: -74.101230585 x_max: -73.1143548636 x_inc: 0.00135373898683 name: longitude n_columns: 730
outgrid.grd: y_min: -47.5489070766 y_max: -46.4505966809 y_inc: 0.000909197347368 name: latitude n_rows: 1209
outgrid.grd: v_min: -574.163024902 v_max: 3823.58081055 name: bedrock topography [m a.s.l. (above sea level)]
outgrid.grd: scale_factor: 1 add_offset: 0
outgrid.grd: format: netCDF-4 chunk_size: 146,135 shuffle: on deflation_level: 3
outgrid.grd: Default CPT: 
GEOGCS["unknown",
    DATUM["Unknown based on WGS 84 ellipsoid",
        SPHEROID["WGS 84",6378137,298.257223563,
            AUTHORITY["EPSG","7030"]]],
    PRIMEM["Greenwich",0,
        AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.0174532925199433,
        AUTHORITY["EPSG","9122"]],
    AXIS["Longitude",EAST],
    AXIS["Latitude",NORTH]]

EPSG:32718 was indicated in the .nc metadata:

gdalinfo PIF_fuerst_2023_v10_npi_masked_bed_consens.nc 
Driver: netCDF/Network Common Data Format
Files: PIF_fuerst_2023_v10_npi_masked_bed_consens.nc
Size is 730, 1209
Origin = (568962.171469999942929,4854277.314640000462532)
Pixel Size = (100.000000000000000,-100.000000000000000)
Metadata:
  masked_bed#comment=Primary target is the basal topography beneath the icefield. The map is limited to the glacier outline from RGI v6.0. The basal topography was created as the difference between the c-SRTM digital elevation model (v2.1) and the thickness field. For SRTM please refer to: Farr, T. et al. The Shuttle Radar Topography Mission. Reviews of Geophysics 45 (2007), http://dx.doi.org/10.1029/2005RG000183.
  masked_bed#long_name=bedrock topography
  masked_bed#units=m a.s.l. (above sea level)
  masked_bed#_FillValue=-9999
  NC_GLOBAL#AUTHOR=Johannes J. Fuerst
  NC_GLOBAL#CRS=UTM18S (EPSG:32718)
  NC_GLOBAL#DATE=November 21, 2023
  NC_GLOBAL#REFERENCE=Fürst et al. (2022) The Foundation of the Patagonian Icefields, Communications Earth & Environment
  NC_GLOBAL#RESOLUTION=100 m
  NC_GLOBAL#TITLE=Mapping of the Northern Patagonian Icefield
  NC_GLOBAL#VERSION=1.0.0
  x#long_name=x coordinate of projection
  x#standard_name=projection_x_coordinate
  x#units=m
  y#long_name=y coordinate of projection
  y#standard_name=projection_y_coordinate
  y#units=m
Corner Coordinates:
Upper Left  (  568962.171, 4854277.315) 
Lower Left  (  568962.171, 4733377.315) 
Upper Right (  641962.171, 4854277.315) 
Lower Right (  641962.171, 4733377.315) 
Center      (  605462.171, 4793827.315) 
Band 1 Block=730x1 Type=Float32, ColorInterp=Undefined
  NoData Value=-9999
  Unit Type: m a.s.l. (above sea level)
  Metadata:
    comment=Primary target is the basal topography beneath the icefield. The map is limited to the glacier outline from RGI v6.0. The basal topography was created as the difference between the c-SRTM digital elevation model (v2.1) and the thickness field. For SRTM please refer to: Farr, T. et al. The Shuttle Radar Topography Mission. Reviews of Geophysics 45 (2007), http://dx.doi.org/10.1029/2005RG000183.
    long_name=bedrock topography
    NETCDF_VARNAME=masked_bed
    units=m a.s.l. (above sea level)
    _FillValue=-9999
3

Very much my bad, thank you!