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What is Nodata?

R
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I am working with rasters in many formats with various GIS-software.

In particularly I am having problems displaying rasters which have backgrounds which need to be made transparent. For one project I need to deliver plan in ECW format. Some plans are black and white, some coloured. If the plan is black and white and the background is also white, by defining the color 255,255,255 als Nodata will remove the white cells inside the plan. In ESRI ArcGIS I can clip a raster and define all areas outside the polygon as nodata. But what is nodata? Is this just a signal to the software to treat a certain pixel value is a different way? If a pixel has a n unsigned 8bit depth, we can only assign a value from 0 - 255. Is there a standard Nodata definition or is it totally software specific and nothing to do with the source data at all?

Best answer by david_r

As you say, a raster is composed of one or several bands with a value for each cell (or pixel, if you prefer). These values are in a certain range depending on the data type of the raster band, e.g. UINT8 can contain values in the range 0..255.

For each band, you can designate a specific cell value to be the NODATA value, this must be some value inside the data type range, for example 0. Depending on the output format, this is also saved in the resulting file, where the metadata will say that for this specific band, the pixel value 0 (or whatever you define) shouldn't be interpreted literally, but as a NODATA value. It's up to the creator of the raster file to define which value corresponds to NODATA, but common values are either 0 or the maximum or minimum value allowed by the raster data type. But in principle the NODATA value can be whatever you like that doesn't interfer with your "real" data.

But what is nodata? Is this just a signal to the software to treat a certain pixel value is a different way?

Yes, exactly. When visualizing the raster in e.g. ArcMap or QGIS, it's up to the client software to read the NODATA metadata setting in the raster file and then interpret it accordingly, e.g. by not plotting the pixels containing those values. If the software used to visualize the raster doesn't know how to treat NODATA values, those pixel value will most probably be plotted as if they contained a regular value, for example 0.

There's also a more detailed description of NODATA concepts here: http://desktop.arcgis.com/en/arcmap/10.3/manage-data/raster-and-images/nodata-in-raster-datasets.htm

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7 replies

david_r
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  • david_rEvangelist
  • Best Answer
  • January 17, 2018

As you say, a raster is composed of one or several bands with a value for each cell (or pixel, if you prefer). These values are in a certain range depending on the data type of the raster band, e.g. UINT8 can contain values in the range 0..255.

For each band, you can designate a specific cell value to be the NODATA value, this must be some value inside the data type range, for example 0. Depending on the output format, this is also saved in the resulting file, where the metadata will say that for this specific band, the pixel value 0 (or whatever you define) shouldn't be interpreted literally, but as a NODATA value. It's up to the creator of the raster file to define which value corresponds to NODATA, but common values are either 0 or the maximum or minimum value allowed by the raster data type. But in principle the NODATA value can be whatever you like that doesn't interfer with your "real" data.

But what is nodata? Is this just a signal to the software to treat a certain pixel value is a different way?

Yes, exactly. When visualizing the raster in e.g. ArcMap or QGIS, it's up to the client software to read the NODATA metadata setting in the raster file and then interpret it accordingly, e.g. by not plotting the pixels containing those values. If the software used to visualize the raster doesn't know how to treat NODATA values, those pixel value will most probably be plotted as if they contained a regular value, for example 0.

There's also a more detailed description of NODATA concepts here: http://desktop.arcgis.com/en/arcmap/10.3/manage-data/raster-and-images/nodata-in-raster-datasets.htm

takashi
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  • takashiContributor
  • Contributor
  • January 17, 2018
david_r wrote:

As you say, a raster is composed of one or several bands with a value for each cell (or pixel, if you prefer). These values are in a certain range depending on the data type of the raster band, e.g. UINT8 can contain values in the range 0..255.

For each band, you can designate a specific cell value to be the NODATA value, this must be some value inside the data type range, for example 0. Depending on the output format, this is also saved in the resulting file, where the metadata will say that for this specific band, the pixel value 0 (or whatever you define) shouldn't be interpreted literally, but as a NODATA value. It's up to the creator of the raster file to define which value corresponds to NODATA, but common values are either 0 or the maximum or minimum value allowed by the raster data type. But in principle the NODATA value can be whatever you like that doesn't interfer with your "real" data.

But what is nodata? Is this just a signal to the software to treat a certain pixel value is a different way?

Yes, exactly. When visualizing the raster in e.g. ArcMap or QGIS, it's up to the client software to read the NODATA metadata setting in the raster file and then interpret it accordingly, e.g. by not plotting the pixels containing those values. If the software used to visualize the raster doesn't know how to treat NODATA values, those pixel value will most probably be plotted as if they contained a regular value, for example 0.

There's also a more detailed description of NODATA concepts here: http://desktop.arcgis.com/en/arcmap/10.3/manage-data/raster-and-images/nodata-in-raster-datasets.htm

In addition to @david_r's excellent explanation, just be aware that the Nodata mechanism may not be supported depending on the raster file format. The ECW is one of those formats not to support Nodata. cf. ER Mapper ECW Quick Facts

 

Therefore, if your goal is to create a raster dataset with ECW format, you might have to think of other approach to represent invalid cells.

 

david_r
Evangelist
  • david_rEvangelist
  • January 17, 2018
takashi wrote:
In addition to @david_r's excellent explanation, just be aware that the Nodata mechanism may not be supported depending on the raster file format. The ECW is one of those formats not to support Nodata. cf. ER Mapper ECW Quick Facts

 

Therefore, if your goal is to create a raster dataset with ECW format, you might have to think of other approach to represent invalid cells.

 

Yes, exactly, that's important to know re the ECW format.
R
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Thankyou for your answers. So what would in fact be a good way to create ECW rasters transformed from TIFs which did in fact have Nodata values?

david_r
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  • david_rEvangelist
  • January 17, 2018
robertdbuckley wrote:

Thankyou for your answers. So what would in fact be a good way to create ECW rasters transformed from TIFs which did in fact have Nodata values?

I would just leave the rasters as-is, but make sure to tell the end users which value corresponds to NODATA. That way they can manually configure the NODATA value in their client (e.g. ArcMap, QGIS, etc).
I
david_r wrote:
I would just leave the rasters as-is, but make sure to tell the end users which value corresponds to NODATA. That way they can manually configure the NODATA value in their client (e.g. ArcMap, QGIS, etc).
In addition to @david_r suggestion, you might want to consider increasing the image depth in case the values you will assign to the NODATA pixels already exist. If for example the 255,255,255 is already a value in other pixels, 256,256,256 should be idealy your NODATA.
david_r
Evangelist
  • david_rEvangelist
  • January 18, 2018
ioannakostara wrote:
In addition to @david_r suggestion, you might want to consider increasing the image depth in case the values you will assign to the NODATA pixels already exist. If for example the 255,255,255 is already a value in other pixels, 256,256,256 should be idealy your NODATA.
Just be aware that going from e.g. UINT8 to UINT16 will probably have an adverse effect on the file size.

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