LIDAR and Trimble GeoXH 6000 analysis for Fuches (SN 866 836)

Fuches Summit

LIDAR DSM shows elevation gradually increasing along the firebreak in a SE direction, with the highest points being on the SE end of the firebreak as follows:

·         LIDAR Position S1: 449.9m at SN 86623 83660.

·         LIDAR Position S2: 449.5m at SN 86631 83666.

·         LIDAR Position S3: 449.5m at SN 86639 83676.

However, these points all have a bridging nature when contours are analysed.  This suggests that individual trees, which are out of alignment with the line of the firebreak edges, are distorting the depth of the firebreak via scattering.  This is confirmed when the aerial image is compared to the LIDAR, where darker patches or bulges in the canopy line can be seen at these points (Figure 2).

The green circle in Figure 1 shows a gap between the S1 and S2 distorted high points, giving a glimpse of the real depth of the firebreak at the summit.  From this, an estimated summit elevation is 449.3 – 449.4m, i.e. between the orange and red contours.

Trimble: 449.297m at SN 86623 83656 (T1), which is comparable to the S1 LIDAR position.

Figure 1: LIDAR image of the summit firebreak.  Black = 449.0m; Green = 449.1m; Yellow = 449.2m; Orange = 449.3m; Red = 449.4m; Blue = 450.0m.

Figure 2: Aerial images of summit.  Top: S1, T1, S2 and S3 positions in relation to the OS map contours.  Bottom: Close up of S1, S2 and S3 where bulges in the canopy line are seen to extend into the firebreak, which probably distorts LIDAR data giving higher readings of elevation.

Ground Observations:

The high point of the forest track is relatively level for about 12 metres; two high points were noted (shown in Figure 3 and 4).  Ground to the immediate east of the track was judged to be the summit of the hill as the track is now down to the bedrock, whilst the ground at the base of the conifer trees is peat / soil and relatively stable when given a good thrust downward with a boot!  The Trimble set-up position was at SN 86623 83658 and this was 2 metres from where the summit was judged to be positioned.

Figure 3: Picture shows Trimble set-up position with alternate high point of track in background with summit of the hill on the right.

Figure 4: Picture shows Trimble set-up position with alternate track high point in foreground with the summit of the hill on left at base of / just in trees. 

Other measurements:

·         Fuches Bwlch Trimble: 419.134m at SN 86997 83837.

·         Fuches Bwlch LIDAR: 419.279m at SN 86997 83838.

·         Allt y Derw Summit Trimble: 449.958m summit at SN 87314 83429.

·         Allt y Derw Summit LIDAR: 449.922m at SN 87313 83431.

Figure 5: Summit of Allt y Derw; notable difference in ground conditions.

Conclusion:

·         Trimble height for Fuches is 449.297m at SN 86623 83656.

·         LIDAR DSM height for Fuches is 449.3 – 449.4m.

·         Trimble and LIDAR are in reasonable agreement with one another.

·       The pictures suggest that if the summit is slightly in the trees, where the ground is elevated by tree roots and moss vegetation.  It seems unlikely that Fuches would naturally make 449.958m.

Recommendation:

·         List Fuches as:

Summit = 449.3m at SN 86623 83656

Bwlch = 419.1m at SN 86997 83837

Drop = 30.2m (30.163m)

Aled Williams and Myrddyn Phillips (September 2018)