I need a DEM file for Scotland, UK. Do you know where I can get it from for free? I need to create a watershed for an area of study.
You should look at the geomatics LIDAR group site. registration is required but it is freely available in many part of UK as long as it is for non commercial use.
Otherwise, there are of course the SRTM and the ASTER DEM, but the resolution is not as good, respectively 90 and 30 m.
OS Terrain 50 is available through the Ordnance Survey OpenData portal. You can find details about OS Terrain 50 at this link: http://www.ordnancesurvey.co.uk/business-and-government/products/terrain-50.html
As well as Mark Bryant's answer.
Also available from the Ordnance Survey Open Data portal is the "Ordnance Survey Meridian2 Map set"
This is a vector (Shape file set) with many different layers, however one of the vector layers provided is a 1 meter gridded height point map that can be use to create a tin layer among other things.
I've used it previously to create dem like bitmaps (In fact I previously posted a question on GIS-SE trying to solve a problem with some banding which outlines how to use the data to produce a dem in QGis)
You can find out more about the map here:
my previous question (Showing how I converted the data to a dem) is here
(Oh and by the way, I still didn't solve the banding problem, well not in QGis anyway, Arcmap converted it perfectly)