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I'm looking at the transparency window of the layer properties of my "Heat map". How does the No Data Value effect a raster layer. I'm assuming an initial setting of -9999 sets it out of the way, so when should I consider setting it.

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The idea behind no data is that this is a way to identify potentially invalid pixels in a regular grid of data. Unlike a vector representation, raster formats can not support empty spaces.

Support for No Data Values varies based on format. Some formats support more than one NDV, some support only one. Usage can vary - from identifying those invalid pixels, to masking certain values.

Unless you have a specific reason for manually setting the NDV, you are likely safe to let the program use the default value.

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What tool should I use to identify NDV, or does QGIS automatically bring it to my attention? Thank you for your help –  Fred Feb 20 '13 at 16:29
    
If you have GDAL on your system you can use gdalinfo. In QGIS, the properties pane, checkout the metadata tab. It will show you the NDV. –  Jay Laura Feb 20 '13 at 16:34
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