So the Australian river database doesn’t have Strahler numbers for streams, which I need to classify streams in order of importance. I got a very nice reply from the AHGF team asking about Strahler number with some ideas on what to do.
One option is to classify on a different measure. AHGFNetworkStream has a couple of features that are useful; UpstrGeoLn and UpstrDArea. They are “Combined geodesic length of contributing upstream AHGFNetworkStream features (incl. segment).” and “Combined albers area (m²) of contributing upstream AHGFCatchment features (incl. segment).”, respectively. Ie: how long the line is that feeds into this line, or how big the area that feeds into this line. It’s worth a look. Of course what I really want is actual water flow, but that’s not readily available.
The other option is to calculate the Strahler number myself. It seems like a simple enough algorithm. As long as the data is clean, that is, a strict tree. The AHGF folks warned me their data has loops and splits that might cause problems.
ESRI tools apparently calculate Strahler, but that’s not useful to me.
GRASS also apparently has some Strahler calculations. (Or Horton, which is similar. “Stream order” is the generic term.) More details here. I wonder which is easier; using GRASS or writing my own code? :-P