Drawing ice-sheet flowlines
When I wanted to draw flowlines for a modelled ice-sheet DEM I was surprised to find limited options in my usual GIS program ArcGIS 9.2.
There is a tool in the 3D Analyst toolbar which gives a ‘steepest path’ on a raster, but it can’t be batch processed from multiple points for example.
After hours trying to find a workaround trying various hydrological toolboxes, I found a solution using the opensource GIS program GRASS. The necessary tool is r.flow, with options available to set the spacing of the calculated flowlines and maximum number of segments per flowline. The added bonus is that files can be exported as polylines that are readable in ArcGIS. Below is an example from the Welsh Ice Cap made from an ice-surface elevation raster, with 50 m contours (note the flowlines are perpendicular).