National Oceanic and
Atmospheric Administration
United States Department of Commerce

Reversed Coordinates

Reversed Coordinates

Question:

Are there any disadvantages to using reversed coordinates? Example:

> I want to write my netcdf files using monotonically *decreasing*
> latitude coordinates, to make the file more directly compatible
> with the preferences of other software; for instance, right now
> if I use something like
>
> SET REGION/ *an XY region at one time step*
> LIST/FORMAT="STREAM"/FILE="uff.bil" uffvariable
>
> the raw binary image will come out "upside down", because that's how the
> data is written in the netcdf file.

Explanation:

The Ferret manual says the following about reversed-coordinate axes:

NetCDF axes may contain monotonically decreasing axis coordinates instead of monotonically increasing coordinates. Ferret will hide this aspect of the file data ordering.

There are only very minor performance issues from using a reverse-ordered axis in Ferret. When reading a large chunk of data, the amount of memory used will be momentarily doubled -- an issue in a few cases.

Ferret will hide the reversal of the axis in almost every way. The SHOW GRID command, for example, will show the coordinates in increasing order. When you write out a NetCDF file using Ferret (using the SAVE command) you will find that Ferret has reordered the data and coordinates in the file to increasing order. 

 


Last modified: Jul 17, 1997