Mapping Mountains – Significant Height Revisions – 200m Twmpau

Crasty Frain (SO 109 983)

There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill initially confirmed by LIDAR analysis instigated by Joe Nuttall who produced a summit analysis programme, with the LIDAR analysis initially conducted by Jim Bloomer and subsequently by Myrddyn Phillips, with the summit later surveyed with the Trimble GeoXH 6000 and which was conducted by Myrddyn Phillips.

Crasty Frain (SO 109 983)

The criteria for the list this height revision affects are:

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m in height with 20m or more and below 30m of drop.

The 200m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Crasty Frain and this was derived from the Tithe map, and it is adjoined to the Carnedd Wen group of hills which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned with a minor road to its north and the B4389 road to its south-west, and has the village of Tregynon towards the west north-west.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was included in the accompanying Hills to be surveyed sub list, and listed with a 253m summit height, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map and which is positioned at SO 107 981.

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

When the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed with an estimated c 27m of drop, based on the 253m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 226m bwlch height, with the latter based on interpolation of 10m contouring between 220m – 230m.

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.

LIDAR summit image of Crasty Frain

The height revision of this hill was initiated by Joe Nuttall who produced a summit analysis programme that used LIDAR with analternative height map (DEM) allowing identification of summits and cols and thereby drops.  The resulting spreadsheet that Joe produced contains over 29,600 hills.

This spreadsheet is being evaluated by a number of people, and for this particular hill it was Jim Bloomer who initially assessed this hill’s data against that produced via LIDAR.  Myrddyn Phillips then evaluated the details for this hill via LIDAR analysis and confirmed its summit height and position and hence its height revision and reclassification to 200m Twmpau status.  The summit of this hill has now been surveyed with the Trimble GeoXH 6000 and it is this evaluation on the hill and the subsequent survey that is being prioritised.

The Trimble GeoXH 6000 gathering data at the summit of Crasty Frain

The summit height and position produced by the Trimble GeoXH 6000 survey is 256.3m at SO 10995 98318, and this 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 summit 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 256.3m and this was derived from a Trimble GeoXH 6000 survey, this is positioned at SO 10995 98318 and is 3.3m higher than its previously listed height of 253m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map.

The full details for the hill are:

Group:  Carnedd Wen

Name:  Crasty Frain

OS 1:50,000 map:  136

Summit Height (new height):  256.3m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SO 10995 98318 (Trimble GeoXH 6000)

Bwlch Height:  226.1m (LIDAR)

Bwlch Grid Reference:  SO 10787 98754 (LIDAR)

Drop:  30.2m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips (October 2020)