= RFC 58: Removing Dataset Nodata Value =

Authors: Sean Gillies

Contact: sean at mapbox.com

Status: Draft

== Summary ==

This RFC adds a GDALDeleteNoDataValue() function to the GDAL C API. This function removes a dataset's nodata value. When it succeeds, the dataset will have no nodata value. When it fails, the nodata value will be unchanged.

== Rationale ==

In the C API the nodata value has accessors {{{GDALGetNoDataValue()}}} and {{{GDALSetNoDataValue()}}}. For GeoTIFFs, the value is stored in a TIFFTAG_GDAL_NODATA TIFF tag. Newly created GeoTIFF files can have no nodata value (no tag), but once a nodata value is set and stored it can only be given new values, it can not be removed. Nor can it be set to a value outside the range of the data type; for 8-bit data passing {{{nan}}}, {{{-inf}}}, or {{{256}}} to {{{GDALSetNoDataValue()}}} has the same effect as passing 0.

The problem with un-removable nodata values is this:

 * Nodata masks (see GDAL RFC 15) can cover up a nodata value but if the .msk file gets lost (and this is ever the problem with sidecar files), the nodata value you were hiding is exposed again.
 * Nodata masks are not available everywhere in GDAL, nodata values are the only definition of valid data in some parts of GDAL.

The current recommended practice for removing a nodata value is to copy the GeoTIFF using gdal_translate, specifying that the nodata tag not be copied over along with the data. By making the nodata value fully editable and removable we could avoid copying unnecessarily.

== Changes ==

{{{GDALDeleteNoDataValue()}}} will be added to gdal.h.

=== Updated drivers ===

The following drivers will be updated: GTiff, MEM, VRT.

The {{{GDALPamRasterBand}}} class will also be updated (for drivers that have no built-in mechanism nodata mechanism and rely on .aux.xml sidecars).

== SWIG bindings (Python / Java / C# / Perl) changes ==

No changes to the SWIG bindings are proposed. 

== Utilities ==

No changes to utilities are proposed.

== Documentation ==

No new documentation is proposed.

== Test Suite ==

TBD.

== Compatibility Issues ==

None.

== Implementation ==

Implementation will be done by Even Rouault and be sponsored by Mapbox.

== Voting history