[visit-users] DEM display

Cyrus D. Harrison cyrush at llnl.gov
Mon Mar 1 18:41:03 EST 2010

Hi Dianne,
Do you have zone centered or node centered data?
I am wondering if  problem may be that threshold is interpolating the 
value of the cell.

If you send an example I would be happy to look at it for you,


Dianne Brien wrote:
> Brad,
> I have had some similar problems displaying .asc files.  Adding a 
> threshold operator helps, but the nodata values seem to be assigned a 
> value close to the minimum of the dataset.  If I set the minimum 
> threshold to the minimum value in the legend, I am able to get rid of 
> the nodata values, but I also lose some of the values from my .asc 
> file that have a value equal to the minimum. Can I send you and example?
> Dianne
> Brad Whitlock_ whitlock2 at llnl.gov_
> Tue Feb 16 13:24:23 EST 2010
> VisIt's GDAL plugin is not currently set up to remove portions of the
> data using "nodata" values. This probably is not too difficult to
> correct since it looks like the "nodata" value can be queried from
> Since the nodata values are not currently removed, adding a Threshold
> operator is a good workaround. The Pseudocolor plot will need its
> limits set relative to the "current plot" instead of the "original
> data" for the legend to exclude the nodata values.
> As for the Z-axis values, which mesh are you visualizing? The GDAL
> plugin provides both flat and elevated versions of the mesh. As
> written, the "elevated" version of the mesh provided by the GDAL
> reader uses the Z values from the file. If your height units are not
> the same as the x/y units then a Transform operator may be needed to
> bring the plot into a 1:1 aspect ratio with height vs width.
> If you're using the flat mesh with the Elevate operator, you'll get a
> plot where the vertical values appear to be normalized into [0,1] so
> the data can be plotted in roughly a cube shape. The Z-axis would
> then be labeled with values in [0,1]. Is this the 0 that you're
> talking about?
> Brad
> On Mon, 2010-02-15 at 21:38 -0600, Brian Clark wrote:
> > Hi,
> > I have a DEM formatted file with nodata values (-9999) in portions of
> > the area. When viewing in VisIt, the nodata values show, even if a
> > threshold is set (though normally, the nodata areas seem to inherit the
> > same value and the lowest value in the DEM). What am I doing wrong? Is
> > there a nodata option to specify for these DEM's related to the GDAL 
> plugin?
> >
> > Also the DEM contains negative values that show up in the color legend,
> > but the Z axis appears to cut everything off below zero? Is it possible
> > to show the full scale of DEM values (those below zero)?
> >
> > thanks,
> > brc
> > --
> -- 
> Dianne L. Brien, Hydrologist
> U.S. Geological Survey, Landslide Hazards
> 345 Middlefield Road, MS 910
> Menlo Park, CA 94025
> dbrien at usgs.gov
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