Address topic http:https://gmt.soest.hawaii.edu/boards/1/topics/7807

[PaulWessel]

While the longitudes where correctly inferred to be pixel registered, disaster struck for latitudes...
Should be a bit more foolproof now.

[joa-quim]

I closed this too soon. It's not wrong but not totally right either. It's transforming the grid into a pixel registered one but the original data is grid registered. Doing it via GDAL gives what I think is the correct result.

C:\v>grdconvert result_ice_repository.grd?Solid_earth_deformation[0] -Glixo.grd

C:\v>grdconvert result_ice_repository.grd=gd+b0 -Glixo2.grd
Warning 1: No UNIDATA NC_GLOBAL:Conventions attribute
Warning 1: No UNIDATA NC_GLOBAL:Conventions attribute
Warning 1: No UNIDATA NC_GLOBAL:Conventions attribute

C:\v>grdinfo lixo.grd
lixo.grd: Title: Produced by grdconvert
lixo.grd: Command: grdconvert result_ice_repository.grd?Solid_earth_deformation[0] -Glixo.grd
        (old cmd)
lixo.grd: Remark:
lixo.grd: Pixel node registration used [Cartesian grid]
lixo.grd: Grid file format: nf = GMT netCDF format (32-bit float), CF-1.7
lixo.grd: x_min: 0 x_max: 360 x_inc: 0.5 name: lon n_columns: 720
lixo.grd: y_min: -90 y_max: 90 y_inc: 0.5 name: lat n_rows: 360
lixo.grd: z_min: -0.000851899792906 z_max: 0.00316517567262 name: Solid_earth_deformation [m]
lixo.grd: scale_factor: 1 add_offset: 0
lixo.grd: format: netCDF-4 chunk_size: 144,180 shuffle: on deflation_level: 3

C:\v>grdinfo lixo2.grd
lixo2.grd: Title: Produced by grdconvert
lixo2.grd: Command: grdconvert result_ice_repository.grd=gd+b0 -Glixo2.grd
lixo2.grd: Remark:
lixo2.grd: Gridline node registration used [Cartesian grid]
lixo2.grd: Grid file format: nf = GMT netCDF format (32-bit float), CF-1.7
lixo2.grd: x_min: 0.25 x_max: 359.75 x_inc: 0.5 name:  n_columns: 720
lixo2.grd: y_min: -89.75 y_max: 89.75 y_inc: 0.5 name:  n_rows: 360
lixo2.grd: z_min: -0.000851899792906 z_max: 0.00316517567262 name: z
lixo2.grd: scale_factor: 1 add_offset: 0
lixo2.grd: format: netCDF-4 chunk_size: 144,180 shuffle: on deflation_level: 3

C:\v>

[joa-quim]

The problem is that we rely on the existence of "node_offset" or "node_offset" or "valid_range" or "valid_min"/"valid_max" to guess if data is grid or pixel but if none exists, we assume data is pixel reg.

[PaulWessel]

Given the original grid actually says the x-values are 0.25, .75, ..., 359.75 it is admitting it is pixel registered, which matches the even number of nodes. Even more so that min = -89.25 so I think only pixel registration makes sense.

[joa-quim]

Yes, it has an even number of nodes but that doesn't necessarily make it more pixelated

length(0.25:0.5:359.75)
720

For this data, why a pixel reg -R[0 360] makes more sense then a grid reg of -R[0.25 359.75]?

C:\v>ncdump -h result_ice_repository.grd
netcdf result_ice_repository {
dimensions:
        lon = 720 ;
        lat = 360 ;
        time = 113 ;

[PaulWessel]

Since we know it is a global grid it should be 360 by 180 degrees. I would say 0/360/-90/90 as pixel reg make much more sense than 0.25/359.75/-89.75/89.75 as gridline reg. Same file.

[joa-quim]

OK, I buy the global argument but the gmt_nc code is not basing its choice on a test for global regions.

[PaulWessel]

No, it is basing it on xmin/dx giving a remainder of 0.5 *dx which is normally a good test to guess pixel registration. It cannot know for sure of course, so a grid form 0.25 to 9.75 with dx 0.5 could be 0/10 pixel or 0.25/9.75 gridline. Hard to know what the user meant without more info in the header, but it is still a reasonable guess. However, when the adjusted min/max gives a 360 range then I think we can be very sure. I don't think we would treat these two cases differently but we certainly would be more confident in the latter case and could print a long-verbose message to that effect.

[PaulWessel]

Note that the original grid fails to use COARDS convention for longitude attribute: It has "degrees east" instead of "degrees_east". So it is also seen as a Cartesian grid.
Of course, it should not stop GMT from perhaps still figuring this out that it is geographic.