Survey of Idlicote Hill
04 January 2018
The Team: John Barnard, Graham Jackson and Richard Mclellan
1) Introduction
Idlicote Hill (Hill Number 17404, Section 39, OS 1:50000 Map 151, OS 1:25000 Map 205, Grid
Ref. SP288433) is listed as a Tump (Thirty & Upward Metre Prominence) in the Database of
British and Irish Hills (DoBIH). The summit area is quite level making identification of the highest
point difficult. Several visitors have recorded ten-figure grid references, all within 30m of one
another and located in a narrow strip near a hedge. One of the team, Richard Mclellan, used an
Abney level to determine the summit position and found this to lie about 10m North of the hedge
near the remnants of a maize crop. More recently, another team member, Dave Marshall studied
LIDAR (Light Detection and Ranging) data for Idlicote Hill and found a summit position lying
about 20m SE of Richard’s position.
The purpose of the survey was to determine the highest point of this hill and then to measure
accurately its height and position. This would also allow a comparison between survey data and
LIDAR data and forms part of a wider project to determine the usefulness of the latter for the
Database of British and Irish Hills.
2) Equipment used and Conditions for Survey
A Leica NA730 Professional Automatic level (X30 telescopic system)/tripod system and a “1m” Estaff extendable to 5m were used to determine the relative heights of candidate positions for the
summit.
The absolute height of the summit position was measured using a Leica Viva GS15 receiver. This
receiver is a dual-frequency, multi-channel instrument, which means it is capable of locking on to a
maximum of 12 GPS and 8 GLONASS satellites, as availability dictates, and receiving two signals
(at different frequencies) from each of these satellites. The latter feature reduces inaccuracies that
result from atmospheric degradation of the satellite signals. As a stand-alone instrument, it is
capable of giving position and height to an accuracy of about two metres and five metres
respectively. Despite the on-board features of the GS15 receiver, there are still sources that create
residual errors. To obtain accurate positions and heights, corrections were made to the GNSS
(Global Navigation Satellite System) data via imported RINEX data from Ordnance Survey which
were post-processed using Leica GeoOffice 8.3. Repeated measurements with the Leica Viva GS15
instrument made on the same point yield a height precision of +/-0.06m.
Note that small hand-held GPS receivers used for general navigation can only receive up to 12 GPS
satellites and each at a single frequency and therefore these instruments have a poorer positional
accuracy of +/-8 metres and a height accuracy of no better than +/-15 metres. Some recently
produced hand-held GPS Garmin receivers can also receive signals from GLONASS satellites
which greatly improve the speed at which these instruments can achieve a satellite “fix”.
Conditions for the survey, which took place between 11.30hr and 14.40hr GMT, were fair. The
weather was cool. Although the temperature was about 10 degrees Celsius, a fresh westerly wind
averaging about 15mph but gusting up to 35mph provided extra chill. Visibility was good even
though the sky was mostly overcast.
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3) Character of Hill
Idlicote Hill lies about 13km due South East of Stratford-Upon-Avon and is about 1km South East
of the small village of Idlicote. This whole area consists of rolling fields which form a patchwork of
crops and grassland. Consequently, many farms lie in the vicinity of Idlicote Hill. The terrain is
broken up with areas of mature trees and diverse hedgerows. The Long Distance Path, the
Centenary Way, passes in a South West to North East direction close to the hill’s summit. This path
was established to celebrate 100 years of Warwick County Council and runs for 99 miles from its
start at Kingsbury Water Park in Warwickshire (SP204959) to its finish at Upper Quinton in
Warwickshire (SP177464).
The summit of Idlicote Hill is on private land which we believe may be owned by the adjacent
Idlicote Hill Farm. Access is most easily gained from the village of Idlicote either using the
footpath from Idlicote Hill Lodge or the parallel one about 200m South West. However, we chose
to use the Centenary Way as access and parked in the small village of Whatcote, North East of the
hill, where there is limited roadside parking. The Centenary Way is followed for about 1.5km to
Pumpground Coppice where one turns off the track to follow another footpath in a South Easterly
direction for approximately 200m. Here, a hedge is followed in a Westerly direction on the edge of
a field, which was planted with crops at the time of the survey, to the summit area of the hill.
An extract of the Ordnance Survey 1:25000 scale (Crown Copyright Ordnance Survey) is shown
below. The “summit” is marked with a spot height of 134m just on the North side of the hedge that
runs in a West to East direction. As shown by the large 130m ring contour the whole area is quite
flat.
The summit area itself comprises a wide grass strip with a hedge on its northern boundary and
ploughed fields to the North and South. At the time of our visit a crop was just beginning to grow
in both of these fields. The ground covered by the grass strip dips slightly from the hedge before
rising to a narrow and shallow ridge on its southern edge. The hedge itself has a ridge of raised
earth along some of its length although this is only a few centimetres high and may have been
caused by root growth of the hedge. At one point along the hedge the ground has been significantly
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engineered, by either badgers or foxes, with a pile of bare earth resulting from extensive tunnelling
by the animals concerned. To the unaided eye the summit position could lie anywhere in the two
fields, along the hedge or along the grass ridge.
4) LIDAR Analysis
Analysis of the Environment Agency’s LIDAR (Light Detection and Ranging) data had previously
been conducted by Dave Marshall. Data at a resolution of 1m 2 were available and these confirmed
the flat nature of the summit area. A section of the Excel worksheet used to analyse the LIDAR data
is shown below, covering an area 35m x 20m. Grid references submitted to the Database are shown
in purple squares and the LIDAR summit is shown in a red square. These five positions can be
correlated with those shown on the satellite image.
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The grid references (OSGB) and LIDAR heights for the five positions, together with the names of
the submitters, are as follows:
RichardM
133.4m
SP 28876 43302
Rob Woodall
133.3m
SP 28864 43288
Aye Jimmy
133.4m
SP 28869 43287
Jonathan Russell
133.7m
SP 28888 43293
LIDAR
133.8m
SP 28895 43293
LIDAR DSM (Digital Surface Model) data were also available and these were compared with the
DTM data. Differences between the two models were up to a metre along a narrow line consistent
with the satellite image (the line which four of the five points sit close to) and this was assumed in
the analysis to be vegetation consistent with a low hedge.
The Environment Agency states that the height error for LIDAR data is ±15cm and positional error
is ±5cm.
4.1) Survey of the Summit
The first task was to locate the positions of the various candidates for the summit that previous
visitors to the hill had found. This was done using a Garmin Oregon 450. The “Go To” feature on
the receiver was used to locate the grid references for each of the various candidates. Each position
was determined twice by approaching from different directions and the grid reference recorded by
the Oregon 450 compared with the reported position. Grid references taken separately and at a later
time during the survey agreed with the submitted values to within two metres. We note that the
reproducibility of a hand-held receiver has been measured as eight metres (3 standard deviations),
so the reproducibility may actually be poorer than two metres reported above. These positions were
marked with flags and are shown in the diagram below and the photograph in the appendix.
Next the Leica NA730 level was set up on a tripod a few metres East of the gap in the hedge and
staff readings were taken on the ground parallel to the hedge and South of it. This enabled the
highest point South of the hedge to be found which was marked with a flag. Ground in the “Short
crop” field further to the South of the hedge was also checked with the level and this was found to
be lower.
The Leica NA730 was repositioned to a position a few metres North of the hedge so that the highest
point in that area could be found from systematic staff readings. This was found to be the point
marked “Richard Mclellan & Summit” on the diagram. Some staff readings were also taken from
ground within the old sweetcorn stems but it was clear this area was lower, as was all ground to the
North and East. However, a higher point was found in the base of the hedge further to the East; this
point was the ground excavated by animals, as described above, and therefore this position was
rejected. Nevertheless, staff readings were taken on this point and the highest point North of the
hedge for the purpose of this report. Note this position is within 3 metres of the flagged LIDAR
position and is therefore almost certainly the position shown above on the LIDAR diagram.
Finally, optical work was completed by setting up the Leica level on its tripod near the highest point
South of the hedge which allowed a view through the gap in the hedge to the field to the North of it.
Staff readings for all the identified points were then taken so that a comparison of heights could be
made. The staff readings for each point are shown on the Summit diagram. However, the key ones
were:-
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Staff reading at highest point North of the hedge = 0.743m
Staff reading at highest point South of the hedge = 0.804m, which is 0.06m lower than the summit.
Staff Reading at Badger’s sett North of the hedge = 0.644m, which is 0.1m higher than the summit.
Staff Reading at LIDAR summit = 0.980m, which is 0.24m lower than the summit.
To obtain an absolute measure of height the Leica Viva GS15 receiver was mounted on a 2.000
metre pole supported by a Leica “Quickset” tripod (see photograph in Appendix 1) over the summit
position. GNSS data were collected at the point for 1 hour and 40 minutes with an epoch time of 15
seconds.
The data for the Leica Viva GS15 were processed in Leica GeoOffice 8.3 using the ten nearest base
stations. The results are given in the table below: -
System
Easting
error(1SD)
GS15
428876.510 0.002
Northing
error(1SD)
Height(m)
error(1SD)
243301.617
0.001
133.526
0.012
The height of the GS15 set-up position = 133.53m (this result takes into account the 2m pole)
The ten figure grid references recorded by hand-held Garmin GNSS receivers for the summit were:Garmin Oregon 450
SP 28879 43309
Accuracy: averaged
Height = 138m
Garmin Montana 600
SP 28879 43310
Accuracy: averaged
Height = 136m
Garmin Etrex 20
SP 28878 43310
Accuracy: averaged
Height = 134m
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5) Summary of Operating Conditions
The latest geoid model was employed and 100 minutes of GNSS data were collected at the summit.
The cut-off angle of 15 degrees prevents data from satellites close to the horizon being employed in
the processing; at these low angles errors due to atmospheric effects compromise the accuracy of
the data.
Parameter
GS15
Data Collection BT summit (min)
100
Number of Base Stations used in
Processing for all points
10
Epoch Time (sec)
15
Tropospheric Model
Computed
Geoid Model
OSGM15
Cut off Angle (degs)
15
6) Coordinate Recovery Analysis
In order to verify the precision and consistency of a GNSS dataset, Ordnance Survey recommends a
procedure called Coordinate Recovery Analysis. Instead of processing the data with reference to all
the nearest OS Base Stations under approximately100km distance, as used in this report, the data is
first processed with reference to only the nearest Base Station (in this case Church Lawford as the
actual nearest, Gaydon, was not operating). The data is then reprocessed with the survey point taken
as a Reference Point and all the remaining Base stations taken as survey points. These measured
values for the OS Base Stations can then be compared directly with the actual OS values for
position and height. (This has been carried out via an Excel spreadsheet supplied to us by OS).
Although the spreadsheet calculates a number of different parameters, two important ones are
presented in the tables below. “Height Difference U metres” is the vertical height difference
between the height of the Base Station as measured in this survey compared with the actual OS
value. “Separation Dij metres” is the distance in 3-d space between the measured and actual OS
values for each Base Station. The results for the summit are presented below.
Base Station
Code
Distance to
Survey Point
km.
Height Difference
U metres
Separation
Dij metres
Church Lawford
CLAW
35
Reference
Reference
Oxford
OXFR
36
-0.0013
0.0040
Buckingham
BUCI
41
-0.0712
0.0716
6
Droitwich
DROW
45
0.0209
0.0260
Stroud
STRU
62
-0.0237
0.0261
Prince
Risborough
PRIS
67
-0.0640
0.0654
Wellingborough
WELL
71
0.0046
0.0082
Lichfield
LICF
71
0.0399
0.0399
Hungerford
HUNG
76
-0.0576
0.0585
Amersham
AMER
84
-0.0023
0.0079
Shobdon
SHOD
90
0.0107
0.0113
The results show a consistent dataset, as all measured OS Base Stations are within 0.07m distance
and height of the OS actual values, even for Base Stations up to 100km distance.
7) Discussion of Results
For the Leica Viva GS15, a 100-minute data collection time gives results with a measurement
uncertainty of +/-0.05m. The summit position was on agricultural ground and we estimate the
height uncertainty for this to be +/-0.05m. Therefore, the overall measurement uncertainty for both
summit determinations from the Leica Viva GS15 is +/-0.07m [square root (0.052 +0.052)].
The LIDAR height for the summit position is 133.4m which compares with the measured height of
133.5m. Given that the measurement uncertainty of the GNSS measured height is +/0.07m and that
of the LIDAR height is +/-0.15m, there is good between the agreement between the two
measurements. The highest LIDAR position of 133.8m corresponds with the animal burrows. This
is 0.1m higher than the summit position, as measured with level and staff, but 0.35m higher
according to LIDAR (which is outside the expected measurement uncertainty). It is possible that
the earth thrown up by burrowing has reduced in height since the LIDAR measurements were made,
although some of the earth appeared to be relatively fresh and free from vegetation. There are also
several positions along the line of the hedge where the LIDAR heights (133.6m -133.7m) are
incorrectly higher relative to the true summit height of 133.5m. Level and staff measurements
showed the ground in and around the hedge to be lower than the summit position. One explanation
is that the DTM heights do not reflect complete removal of the vegetation, in this case the hedge.
The LIDAR and level and staff measurements are in good agreement regarding the shallow ridge
about 10m South of the hedge with both finding this to be 133.4m. The LIDAR data and the on-site
survey with level and staff also agree on the topography of the wider landscape in the summit area.
In both cases the land drops to the North and East and to the South, although this is not immediately
obvious to the unaided eye.
8) Summary and Conclusions
The summit of Idlicote Hill is at grid reference * SP 28877 43307 and is on agricultural ground a
few metres North of the hedge, running West to East, that marks the field boundary. Its height is
133.5m+/-0.07m. The height of the corresponding position as measured with LIDAR is 133.4m the
difference of 0.1m being within the measurement uncertainty of the two determinations. The
highest point measured by LIDAR corresponds to an extensive animal burrow and therefore does
not qualify for consideration as the summit. The highest point along the shallow ridge about 10m
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South of the hedge, as measured by LIDAR is 133.4m. In this survey it was also measured to be
133.4m.
Given the level nature of the extensive summit area it is testament to all the contributors of tenfigure grid references for this hill that they were so close in locating the true summit position.
*NB: Grid references for Garmin receivers are quoted in the summary.
John Barnard, Graham Jackson & Dave Marshall, 18 Jan 2018
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Appendix 1
Looking West showing candidate summits marked with flags and GS15 setup to right of hedge
Leica Viva GS15 set up on summit
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Photograph showing the summit positions recorded by others: true summit is to the left out of the picture
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