Survey of Stanky Hill
16th February 2012
The Team:
John Barnard, Graham Jackson, Myrddyn Phillips and Aled Williams
1) Introduction
Stanky Hill (Section 31B, OS 1:50000, Maps 136 & 148, Grid Ref. SO163764) is on Michael Dewey’s list of 500m hills and is recorded with a drop of just 31m. It was included in the list following a survey in May 2000 by Myrddyn Phillips who used a staff and spirit level to determine drop. The accuracy of this technique is of the order of +/-1m (one standard deviation). The 1:50k map shows a spot height of 506m on the summit and 477m on the bwlch thus indicating a drop of only 29m although the 477m spot height appears not to be on the lowest point of the bwlch. The 1:25k map also shows a spot height of 506m on the summit, but none for the bwlch. The contours show that the bwlch lies between 470m and 480m.
Consequently, an accurate survey is required to resolve these issues and determine an accurate value for the drop.
2) Equipment used and Conditions for Survey
Ground surveys to determine the positions of the bwlch and summit were carried out using a Leica NA730 Professional Automatic level (X30 telescopic system)/tripod and a “1m” E-staff extendable to 5m.
Absolute heights were measured using a Leica Geosystems 530 GPS receiver. It is a dual-frequency, 24-channel instrument, which means it can lock on to a maximum of 12 satellites and receive two signals (at different frequencies) from each of these satellites. The latter feature reduces inaccuracies that result from atmospheric degradation of the satellite signal. As a stand alone instrument it is capable of giving position and height to an accuracy of about one and five metres respectively. Note that a hand-held GPS receiver can only receive up to 12 satellites and each at a single frequency and therefore it has a poorer positional accuracy of +/-5m and a height accuracy of no better than 10 metres. Despite the on-board features of the 530 GPS receiver, there are still sources that create residual errors. To obtain accurate positions and heights, corrections were made to the GPS data via imported RINEX data from the Ordnance Survey which was post-processed using Leica Geo Office 7 software.
Conditions for the survey, which took place between 12.00hr and 16.00hr GMT, were fair. The weather was cool, 5 degrees Celsius and the wind was light. Although rain had been forecast for later in the day, it remained dry but overcast throughout the survey.
3) The Survey
3.1) Character of Hill
Stanky Hill is situated about 15km SSE of Newtown and 12km WNW of Knighton in a large area of upland which exceeds 500m in several places. It is best accessed from the North via the A483, the B4355 and then a minor road from Felindre. The hill is situated about 5km South of Felindre and there is limited parking for a couple of vehicles at the highest point of the minor road at SO153765. From here a good track passes to the North of Stanky Hill and to the bwlch between it and Beacon Hill (Appendix 2). The slopes of Stanky Hill are nowhere steep and are covered with heather and rough grass. Nearby Beacon Hill is a Marilyn and Dewey and 2km SE of Stanky Hill lies Pool Hill, which is also a Dewey. The three hills make a pleasant half-day round with easy going under-foot.
3.2) Summary of Survey Method
The first task was to determine the position of the very broad bwlch, which lies between Stanky Hill and its higher neighbour Beacon Hill. Closer inspection of the terrain revealed a slight undulation in the ground, thus giving rise to two possible locations. Consequently, a preliminary survey was carried out using one line of flags set in a hill to hill direction and a line set in a valley to valley direction at each of the two possible bwlch positions. The level and staff were then used to determine the approximate bwlch position of each of the two candidates and thereby determine which was the lower and therefore the critical bwlch. Having determined which of the two candidates was the correct bwlch to study, we next set out a grid of flags at 5m intervals and determined the exact position of that bwlch by the method described in previous reports. While this process was being carried out, the Leica 530 GPS was set up at the approximate position of the critical bwlch and data collected. This practice of collecting GPS data while the exact position of bwlch is being determined saves much time. Once the exact bwlch position had been identified, data collected and the GPS removed, the height difference between the bwlch position and the GPS set-up position was determined using the level and staff. This correction was then applied to the GPS result.
Next the Leica 530 GPS was taken to the summit of Stanky Hill, where it was once again set up and data collected. The summit position was first determined by Abney level and then later confirmed by the Leica NA730 level and staff.
While this was being done, two members of the team began a line survey from bwlch to summit to determine drop (Appendix 1). Again the exact method has been described in detail in previous reports. The line survey was repeated in the summit to bwlch direction to confirm the drop measurement and also to obtain a closing error for the two line surveys.
Thus, once complete, the survey would produce an accurate height for Stanky Hill and an accurate position and height for the bwlch from which a value for drop could be determined. In addition the line survey would also produce a value for drop and thereby allow a comparison of the two very different methods.
As described above, the absolute heights of the summit and the bwlch were measured with the Leica 530GPS. The instrument was set up with tripod support to hold it firmly over the point to be measured. The AT502 antenna was mounted on a 2.000 metre pole for all measurements. Data at each point were collected for 60 minutes with an epoch time of 30 seconds.
3.3) The Bwlch
As described in Section 3.2 two candidate bwlch positions were identified from our preliminary survey. The Leica NA730 was set up at a convenient position between the two candidates and staff readings were then taken of each.
Staff reading of easterly candidate = 0.98m
Staff reading of westerly candidate = 1.17m
Therefore the westerly bwlch is 0.19m lower than easterly bwlch and is therefore the critical one for this survey.
The ten-figure Grid References for the easterly higher bwlch were:-
Garmin Map60CSx SO 16988 76439 Accuracy 5m Height = 477m
Garmin Etrex SO 16987 76441 Accuracy 5m Height = 476m
Having identified that the westerly bwlch was indeed the critical bwlch we next surveyed it more systematically with level and staff as described in Section 3.2 and we are confident this was achieved to well within +/-0.1m of height. Indeed our staff readings showed a variation of only 0.05m over a distance of 20m in the hill to hill direction.
The ten-figure Grid References for the GPS set-up position were:-
Garmin Map60CSx SO 16873 76421 Accuracy 4m Height = 476m
Garmin Venture SO 16872 76419 Accuracy 3m Height = 482m
Garmin Etrex SO 16872 76419 Accuracy 5m Height = 477m
Magellan Explorist 100 SO 16874 76417 Accuracy 7m Height = 485m
The ten-figure Grid References for the bwlch were:-
Garmin Map60CSx SO 16860 76415 Accuracy 3m Height = 478m
Garmin Venture SO 16863 76416 Accuracy 3m Height = 477m
Garmin Etrex SO 16861 76413 Accuracy 5m Height = 479m
Magellan Explorist 100 SO 16859 76418 Accuracy 5m Height = 475m
The position and height data for the bwlch that were recorded by the Leica 530 and post-processed with Leica GeoOffice 7 using imported OS RINEX data from the eight nearest OS base stations and the Hopfield tropospheric model were:-
System / Easting / error(1SD) / Northing / error(1SD) / Height(m) / error(1SD)SR 530 / 316869.761 / 0.003 / 276415.379 / 0.002 / 476.352 / 0.006
Staff reading at GPS set-up position = 0.887m
Staff reading at bwlch = 0.905m
Therefore the GPS was set up at a point that was (0.905 – 0.887) = 0.018m higher than the true bwlch.
Height of bwlch = 476.352 - 0.018 = 476.33m
Note that the true position of the bwlch is just 0.02m lower than the GPS set-up position and 12m from it in distance.
3.4) Summit of Stanky Hill
The exact position of the summit was established using the level and staff and is a small area of loose flat stones near a tiny pool of water.
The ten-figure Grid References for the summit are:-
Garmin Map60CSx SO 16395 76426 Accuracy 2m Height = 510m
Garmin Venture SO 16393 76425 Accuracy 2m Height = 514m
Magellan Explorist 100 SO 16392 76427 Accuracy 4m Height = 508m
The position and height data for the summit recorded by the Leica 530 and post-processed with Leica GeoOffice 7 using imported OS RINEX data from the nearest eight OS base stations and the Hopfield tropospheric model were:-
System / Easting / error(1SD) / Northing / error(1SD) / Height(m) / error(1SD)SR 530 / 316390.865 / 0.002 / 276419.939 / 0.001 / 506.646 / 0.019
From the GPS data the drop for Stanky Hill is measured to be 506.65 – 476.33 = 30.32m
3.5) The Line Survey
The line survey was carried out from the bwlch to summit and back to the bwlch with a closing error of 0.004m. The value of drop was calculated to be 30.31m. The details are shown in Appendix 1.
4) Discussion of Results
The largest measurement uncertainty associated with both the GPS and line survey measurements was the correct location of the bwlch which is +/-0.10m in its height. We also estimate that both the summit height and bwlch height were measured to better than +/-0.08m by the Leica 530. Therefore the overall error in drop measurement for the hill is no more than +/-0.15m by GPS and +/-0.1m by line survey.
The agreement of just 0.01m between the line survey measurement and the GPS measurement of drop is excellent.
The drop measurement is 30.32+/-0.12m and therefore Stanky Hill does achieve the 30m required to give it Dewey status.
5) Summary and Conclusions
The summit of Stanky Hill is at grid reference * SO 16393 76426 and is an area of loose stones near a tiny pool. Its height is 506.6+/-0.1m.
The bwlch separating Stanky Hill from Beacon Hill is at grid reference * SO 16861 76416. Its height is 476.3+/-0.1m.
The re-ascent from the bwlch to the summit is 30.3m and therefore Stanky Hill retains its Dewey status
* NB average hand-held Garmin/Magellan GPS grids are quoted in the summary.
John Barnard and Graham Jackson, 21st February 2012
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Appendix 1
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Appendix 2
1:25k map of Stanky Hill and the Bwlch
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