Hi,
I've got some problems with actual splitter and a combined input of osm and srtm-data. I merged them with osmconvert, so I think they are not mixed (--mixed gave the same result as without --mixed).

I use splitter with an polygon-file of New Zealand [1] and max-nodes=1000000, which is no problem if I use only osm-data as input. Splitter generates the following density-map [2] and then ends with the message:

Failed to calculate areas.
Sorry. Cannot split the file without creating huge, almost empty, tiles.
Please specify a bounding polygon with --polygon-file parameter.

Tiles should get smaller with srtm data, so my guess would be that the tiles, which covers parts of the sea could be problematic. If you need further information, let me know.

Henning

[1] http://www.aighes.de/data/NewZealand.poly
[2] http://www.aighes.de/data/densities-out.zip