I need the most precise and highest resolution free digital elevation model for the UK. Would OS terrain 50 or ASTER GDEM be more suitable for view shed analysis?
As the name suggests, OS Terrain 50 is a product with a 50 metre grid resolution (http://www.ordnancesurvey.co.uk/business-and-government/products/terrain-50.html).
OS Terrain 50 has been compared with GPS points in a range of sample areas to provide a Route Mean Square Error (RMSE) value for the height points in each geographic area; urban and major communication routes, rural and mountain and moorland. OS Terrain 50 grid has been verified to be 4 m RMSE.
ASTER GDEM is about 30metres (http://asterweb.jpl.nasa.gov/gdem.asp).
There's also SRTM, which while it's currently only available at 90 metre accuracy for the UK, will be increasing to 30m (what it is for the USA) within a year (http://www.jpl.nasa.gov/news/news.php?release=2014-321).
For a comparison of the accuracy of these satellite derived products (plus NEXTMap), see this paper:
The accuracy varies considerably depending on the surface, the averages being:
SRTM (15.27 m RMSE), and ASTER (18.52 m RMSE) while the published RMSE's are 16m and 20m respectively.
Considerably lower than OS Terrain 50.
Resolution vs Accuracy
Because OS Terrain 50 is I believe a scaled-down version of flown data, probably in combination with ground survey data, the accuracy is much better than the space-acquired data. On the other hand you have a reduced resolution.
I leave it as an exercise for someone else to determine whether the increased accuracy of OS Terrain 50 offsets the lower resolution.
In short - I don't believe there's a definitive answer, it depends which you value more. I'd guess accuracy is more important for a viewshed though.
If you choose ASTER GDEMv2 as your DEM option, you may transform the Geographic Coordinate System (GCS) from WGS-84 (GCS of Earth) to OSGB-36 (GCS of UK). In this case, you might have more accurate DEM data.
See the comparison between ASTER GDEMv2.0, EU-DEMv1.1, and Google Earth: https://github.com/HeZhang1994/digital-elevation-model-tool-tutorial