Global Mapper v25.0

handling void value -32767

dmr
dmr Global Mapper UserTrusted User
edited June 2009 in Elevation Data
I exported some NED (.adf) files as 16 bit GeoTiff elevation files using the batch convert/reproject... command. A few of the output files have trouble (seem corrupt) in another program (Terra Vista). So I am trying to track it down. A common "void/null" value in elevation file formats that I am aware of is -32767. How does Global Mapper deal with this value when doing a 16bit TIF?
If I analyze the pixel values (with ERDAS), I see this -32767 value down the entire left edge of my output. But if I view the file I exported in Global Mapper, it shows me a value of zero there.
When I exported, I did use the option to "Interpolate to fill small gaps". But maybe an edge of a file is a special case that is not handled with this option?
The NED files I had trouble with were at the following locations 121W 35N and 121W 34N if you want examples. Both are mostly ocean with a little bit of coastline.

dmr

Comments

  • global_mapper
    global_mapper Administrator
    edited June 2009
    When writing out elevation TIFF files Global Mapper does use a void value of -32767 for places where no data is available. You would never see this in Global Mapper because running your cursor over a void area in a layer doesn't show you anything about that layer, but whatever is beneath it (likely nothing in your case).

    The "fill gaps" option is for filling holes where there is surrounding valid data, it would not fill to an edge as there would not be data on all sides to interpolate with.

    What you can do if you don't want the no data value is to replace any void areas in your original data with some other value, like 0, prior to export. To do this, simply open the Control Center, select the elevation layer(s), press Options, then go to the Adjust Elevations tab and specify to replace void/no data values with some elevation.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • dmr
    dmr Global Mapper User Trusted User
    edited June 2009
    If I do tell it to replace void using the Adjust Elevations tool as you suggest, does that override the "fill gaps" option? Or does it do it a little more smartly? I can forsee problems if it overrides it.

    I didn't quite get what you were saying in your first paragraph about layers with nothing berneath. By another layer, do you mean something to the effect of including a super low res .dem in the background so it will see those elevation values and plug them in place of void values.
  • global_mapper
    global_mapper Administrator
    edited June 2009
    If you use the Adjust Elevations too to replace voids then every void area in your layer will be set to whatever elevation you specify and will no longer be treated as void. Since the Fill Gaps option only fills in areas of missing data and those areas are no longer missing, the Fill Gaps option wouldn't have anything to do then.

    By another layer I mean you could have something, like a lower res DEM or anything else loaded in addition to your topmost DEM. That would show through in any void areas of the topmost data.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com