Mapping Mountains – Significant Height Revisions – Y Trichant – The 300m Hills of Wales

Dinas (SH 699 738)

There has been a Significant Height Revision to a hill that is now listed in the Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR summit image of Dinas (SH 699 738)

The criteria for the listing that this significant height revision applies to are:

Y Trichant – The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13^th May 2017.

The name the hill is listed by is Dinas and it is adjoined to the Carneddau group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A55 road to its north-west, and has the town of Penmaen-mawr towards the north-east and the town of Llanfairfechan towards the north-west. 

When the original 300m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was not included in the Hills to be surveyed sub list that accompanied the main P30 list, as it did not meet the criteria then used for this sub category. 

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated, but as the contemporary Ordnance Survey 1:25,000 Explorer map did not have a summit spot height and only an uppermost 320m continuous contour and bwlch contouring between 300m – 310m, an accurate interpolated drop value was hard to determine.

Extract from the Ordnance Survey 1:25,000 Explorer map

The details for this hill were re-examined when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online.  This mapping has many spot heights not available on any other publicly available Ordnance Survey map.

The details for this hill were also re-assessed when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals which are proving consistently more accurate compared to the 5m contours that sometimes appear on Ordnance Survey 1:25,000 Explorer maps and the online Vector Map Local.  These re-assessments resulted in the hill being listed with an estimated c 24m of drop, based on an estimated summit height of c 328m and an estimated bwlch height of c 304m.

Extract from the OS Maps website

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.

The summit height produced by LIDAR analysis in comparison to the previously listed estimated summit height comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis. 

Therefore, this hill’s new listed summit height is 325.9m and this was produced by LIDAR analysis.  This is not a dramatic revision in height compared to some revised heights and is 2.1m lower than the previous estimated height of c 328m which was based on interpolation of the uppermost 320m ring contour on contemporary Ordnance Survey maps.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Carneddau

Name:  Dinas

OS 1:50,000 map:  115

Summit Height (New Height):  325.9m (LIDAR)

Summit Grid Reference:  SH 69978 73828 (LIDAR)

Bwlch Height:  301.5m (LIDAR)

Bwlch Grid Reference:  SH 70198 73941 (LIDAR)

Drop:  24.4m (LIDAR)

Myrddyn Phillips (November 2019)