In reply to Alan Bowring:

I can't add anything on how OS maps are printed, but on looking at
my copies of Sheet 160 I see that near to `Nant y
Rhaeadr', `Waterfall' at SN 753436 changes from blue to black on
edition A3. While `Nant y Rhaeadr' reverts to being all blue type on
edition B (2000), `Waterfall' remains in black, and is still black
on edition B1/. As a water feature it should be blue, like the other
identified waterfalls nearby at SN 730417 and 754429.

This area around Mynydd Mallaen is on the overlap between sheets
146, 147 and 160. On 146 and 147, `Nant y Rhaeadr' is always in
blue, but `Waterfall' at SN 753436 changes to black on sheet 146
edition B (2000) and 147 edition C (2001). This is probably due to
the adoption of a single database from which to produce 1:50,000
Select Maps. Perhaps the detail on sheet 160 was preferred, thereby
inadvertently extending the use of erroneous black type for
this `Waterfall' to all 3 sheets.

Returning to the 2 colour `Nant y Rhaeadr', there is a similar
oddity on sheet 18 edition B1/ (2003). Here at NF 874648 is `Loch
Tarruinn an Eithir' (2 lines of text). `Loch Ta…' and `an Ei…' are
in blue, while `…rruinn' and `…thir' are in black.

Bill Henwood
27/07/2006