ASCII grid disregards nodatavalue when choosing data type

I agree that the nodata value should be considered in the data range for ascii files.

If I understand this report correctly, there are actually two issues:

1. Use nodata value to determine data type of a grid according to following algorithm:

   if nodata is float-point number
   {
     set grid data type to GDT_Float32
   }
   else // nodata is integral number
   {
     for pixels in sample chunk)
     {
       if float-point numbers found
         set data type to GDT_Float32
       else
         set data type to GDT_Integer16 (default type)
     }
   }
   

2. When calculating statistics, nodata value should be also compared, so no data is not out of range of min/max value:

   min = MIN( calculated_min, nodata_value )
   max = MAX( calculated_max, nodata_value )
   

The 1st issue can be solved directly in AAIGrid driver. The 2nd issue seems to be solvable in GDALComputeRasterMinMax function only, so the fix would apply to all GDAL drivers. I think , this is generally correct that nodata value is in range of all values:

min <= nodata < max
or
min < nodata <= max

Could you confirm if I've caught the problem correctly?

Mateusz,

I believe point 2 is not right. nodata values should not be included in min/max statistics if possible. I think you can just focus on point 1.

Frank,

I'm an idiot. I've no idea why I wanted to use nodata in statistics computation. This is completely stupid idea that nodata value is in range of all values. Sorry for that.

Fixed in trunk (r13404)

Backported to branches/1.5 (r13407)

Actually that is not what I meant. My problem was overflow. It doesn't matter if the nodatavalue is float if there are no nodatavalues in the data or if the nodatavalues in data are represented as ints.

I'm nut sure it is a good idea to make data type float just because the nodatavalue is float. Some drivers (I think GDAL included) always write the nodata as a float regardless of the "real" data type.

What actually meant was something like:

if nodata < -32,768 or nodata > 32,767
{
  set grid data type to GDT_Float32
}
else
{
  for pixels in sample chunk)
  {
    if float-point numbers found
      set data type to GDT_Float32
    else
      set data type to GDT_Integer16 (default type)
  }
}

I'm sorry, I din't come back to this before. You two are just too fast for me :-)

Thanks for the clarification. Right, this is slightly different than I understood previously.

Re-fixed in trunk (r13432, r13433) and branches/1.5 (r13434)