QwtRasterData assumes that you have a regular matrix of data values, one for every cell in the matrix. It then maps this (x,y,z) data to a spectrogram using bilinear interpolation to create colors for the pixels between raster points.
This is what QwtMatrixRasterData does. QwtRasterData is simply an abstract API to bind the data ( however it looks like ) to a spectrogram.
So, you need to parse the rda into a QwtRasterData structure, ...
All what needs to be done is to implement the abstract methods usually fetching the values from any applications specific data structures.
And you probably want to use QwtMatrixRasterData; parse your rda file into a QVector< double > array of size (nRows * nCols) ...
When there is no other reason to have the values in memory this should be the easiest way to go.

Uwe