The Golden Valley (Landranger Sheet 161 SO3141 to SO3927), lying in the eastern foothills of the Black Mountains west of Hereford, has been the scene of intense human activity since the late Stone Age. Several amateur history groups are engaged in exploring and recording the visible sites and alignments of landscape features for possible professional investigation at a later date. One significant area of interest is the indications of a Roman cavalry patrol route from Abergavenny to Hay-on-Wye, engineered with anti-ambush features and dating from AD 50-51, when the Romans were pushing westward along a line broadly similar to the modern A40 trunk road. Another group is investigating the remains of a very ambitious 18th-century land drainage and irrigation scheme, developed by a local landowner with utopian dreams for improving the prosperity of the area.

The survey requirement is therefore to record the positions of many fairly insignificant features in the hope of interpreting their locations into a broader picture and, hopefully, to provide a permanent record to guide future investigators. In some cases, where a site sketch survey is to be made, a secondary requirement is to lay a datum line across the site in a known orientation. In numerical terms the objective is to be able to determine spot location to an accuracy of 1m or better in terms of its British National Grid position. Such a requirement would be trivially easy to achieve using well-established GPS survey techniques, but the cost of the requisite survey-grade equipment is completely beyond the means of unsupported amateur groups.

Three developments during the past year (2000) have, however, completely transformed the prospects for low-cost GPS surveying:

• The removal, by order of the US president, Bill Clinton, in May 2000 of the deliberate degradation in positional accuracy, Selective Availability. The result of this development is not only to reduce the errors of raw GPS position information to the order of 5-10m, but also to make the error statistics much smoother and better conditioned in their effect.



• The change in policy by Ordnance Survey in making available via their Internet web site the raw GPS (Receiver INdependent EXchange format - RINEX) data recorded from the active stations of the UK National GPS Network. Public access was also granted to the catalogue of precise coordinates of the triangulation pillars incorporated in the network. The ready availability of these two classes of data brought the possibility of calculating the precise coordinates of user sites through post-processing.



• The discovery of undocumented features in the software of the inexpensive Garmin GPS12 family of consumer-grade receivers, by which raw GPS data could be output to a PC, converted to RINEX format and then post-processed against the GPS data from the active stations. The results of this detective work are available in the user-friendly and modestly-priced GRINGO package sold by the IESSG at Nottingham University. A companion program performs the post-processing functions. Full details of GRINGO and a limited function demonstration version can be obtained from the IESSG web site at: http://www.nottingham.ac.uk/iessg/gringo.

Thus, the user on even a quite limited budget now has the means to achieve much better levels of survey accuracy than have hitherto been achievable.

Over the past winter a quite extensive range of familiarisation and evaluation trials have been performed.

Firstly, to examine the observing and post-processing tasks a Garmin GPS12XL receiver attached to a laptop PC running the GRINGO software was positioned at the triangulation pillar on the summit ridge of Blakemere Hill. The processed position (using RINEX data from the Ordnance Survey Droitwich reference site, some 50km distant) was compared with the precise coordinates published on the Ordnance Survey web site. It was quickly established that positions could be measured to within about 50cm mean error, albeit the standard deviation of the data points collected during a one-hour observation period was of the order of 2.5m.

The second phase of the trials aimed to repeat the exercise at a reference station to be established at the home of the author of this report, where a GPS receiver could be connected to a desktop PC. Although the observing conditions, with the receiver propped in a south-facing window, were far from ideal, mean positions within a 70cm diameter circle and consistent with the coordinates read from a 1:500 scale local planning map could be obtained, provided that some care was taken to exclude data points taken at times where one or more satellites were in the process of rising or setting behind the nearby ridge of hill that obscures the sky view up to some 20 degrees elevation. While such an event is in progress the processed differential GPS position makes very large excursions (c 500m or so) along a line parallel to the direction of the horizon-obscuring ridge.

On the basis of these trials, the purchase of an additional GPS receiver can now be justified. The object will then be to use a differential GPS technique, operating over a distance of only a few kilometres from the local reference station.

The Royal Commission on the Ancient and Historical Monuments of Scotland (RCAHMS) undertakes an archaeological recording and mapping programme across the whole of Scotland, targeting areas on a strategic basis. The aim is to add to and improve the quality of the information held within the National Monuments Record of Scotland (NMRS), and to provide the Ordnance Survey with selected new or improved archaeological depiction for base-scale mapping. Survey data is checked and edited before being transferred to the publicly accessible NMRS geographical information system. This resource is used by archaeologists involved in planning applications, research students and the general public. The data may also provide the basis for RCAHMS publication maps or illustrations.

Prior to the implementation of GPS at RCAHMS in 1997 mapping was carried out using total stations, with local map detail surveyed to position our divorced surveys on the National Grid. GPS not only presented many obvious advantages in terms of field practice but also allowed our work to be fitted directly to the National Grid - removing the inaccuracies which occur when fitting to map detail.

In order to use GPS, however, an expensive, time consuming and resource intensive control network had to be undertaken in each survey area. Observations from a trig station network were taken to fix our reference station and to compute the transformation parameters to convert from WGS84 to OSGB36^®. This transformation could then be applied to all kinematic work. Unfortunately, any errors in trig coords would adversely affect the quality of results.

The Ordnance Survey National GPS Network will allow RCAHMS to make huge time and resource savings in controlling its GPS field survey. The active network will be used to establish the ETRS89 coordinates of a local reference station. Kinematic surveys will then be processed against this position, and the results transformed using the online coordinate converter to obtain OSGB36 coordinates accurately and consistently without actually occupying any Ordnance Survey control stations.

Dennis Sinnott is employed as a research member of staff at the University of Central Lancashire's Department of Environmental Management. He is part of a small team working on the conservation of wetlands. An important element of this work involves developing the use of GPS to help evaluate past damage to the self-sustaining hydrology of raised peat bogs.

Raised peat bog systems are considered a rare and valued habitat globally, having often been compromised by threats such as peat extraction for fuel or horticultural use and drainage for agriculture.

The work involves carrying out a topographical and hydrological survey of a site. Such topographic surveys using conventional techniques were difficult and time-consuming given the large and remote areas sometimes involved, the nature of the vegetation cover and the difficulty in establishing fixed stations in peaty terrain. To react to this an innovative approach that uses a differential GPS to create survey-quality data has been adopted.

The data was subject to post-processed differential correction (DGPS) for further accuracy. Positions were computed using carrier phase data at the local receiver and from the reference stations. The purpose of this DGPS survey was to evaluate the relationships between the patterns of topography, artificial drainage and the water table.

The data were used to compute representations of the profiles of the water table, at the time of the survey, and topography across the raised bogs and peripheral zones. Field notes of any key features, made at individual survey points, were subsequently plotted onto the profiles using their National Grid references.

Results enable valuable understanding of hydrological functions, which provides for highly informed evaluation of the quality of a site. These data also allow identification and assessment of past damage, and threat of damage, to the raised water table. This provides for highly informed recommendations to be made for the hydrological restoration and conservation of these wetland habitats.

Jared Ware, an engineer officer in the United States Army, is a student at the Royal School of Military Survey in Hermitage, England. He is studying for an MSc in Defence Geographic Information from Cranfield University. His research project was conducted for the Ordnance Survey of Great Britain.

In the autumn of 2000, the active network station at Saint Catherine's Point (SCP1) on the Isle of Wight began to gradually shift south towards the English Channel from its original coordinate position. However, no movement was evident in any other GPS active network station based upon the GPS data processed at Ordnance Survey. The active network consists of 30 continuously operating GPS reference stations located throughout Great Britain. SCP1 is one of a network of 9 stations owned and operated by the General Lighthouse Authority which contribute GPS data to the Ordnance Survey's active network. These stations are permanently installed, precisely coordinated active GPS stations. The extensive rainfall in the autumn and winter of 2000 was raised as a factor contributing to the perceived movement, potentially having affected the soil properties at the base of the lighthouse. Given this situation SCP1 was taken out of the active network until an investigation into its coordinate shift could be conducted. The lighthouse and the GPS antenna are shown below in Figure 1.

The investigation looked into the GPS, rainfall, and soil information available during the time interval in which the lighthouse shifted south. The aim of this project was to investigate the sources of data from St. Catherine's Point to determine the magnitude of movement of the lighthouse. An additional aim was to determine if the lighthouse had stopped moving so that SCP1 could be returned to operation in the active GPS network with its new coordinate position. The objective was to provide information that may help develop potential solutions to mitigate the effects of the lighthouse's movement during periods of extensive rainfall. The focus of the project was to determine the sources of error (if any) in the network, provide information that determines (1) the extent to which the lighthouse has shifted south and (2) if the lighthouse has ceased moving, and to establish new coordinates for the SCP1 station based upon coordinate corrections. The results of the investigation were based upon the GPS data provided by the Ordnance Survey and the data gathered in the field by the author.

It was determined that between September 2000 and March 2001 the lighthouse moved in a southern direction and produced GPS residuals outside of the accepted 95% confidence level. There is a strong correlation between the amount of rainfall experienced at Saint Catherine's Point and the rate of change of movement south of the lighthouse. Analysis indicated St. Catherine's Lighthouse moved approximately 10 centimetres south over a seven months period. It was also determined that SCP1 ceased moving in the second quarter of 2001. A future cumulative rainfall total equivalent to that of the autumn of 2000 coupled with a short duration of intense precipitation could potentially produce an additional southward shift of up to 2 centimetres.

The GPS data from the lighthouse revealed that a significant southern shift in the lighthouse began in October 2000 and gradually continued until late March 2001. This can be seen in Figure 2 below, where the movements are determined in centimetres. Note that no further movement has occurred since April 2001.

The comparison of epochs were essential to determine whether the shifts at St. Catherine's Point were due to network deformation, or whether the lighthouse actually shifted due to another factor. It was necessary to compare the stations in the network at times of normal precipitation, no precipitation, and during period of heavy rainfall to determine if rainfall had an overarching effect on the entire network;

The base epoch, 2000.335, was chosen because this was a period when the residuals in the network were small and there was little rainfall occurring during this period. Epoch 2000.779 was chosen because it was the day on which the greatest amount of rainfall occurred at St. Catherine's Point. Epochs 2001.089 and 2001.122 were chosen because this was a period during the first two weeks of February where the lighthouse experienced almost 1 centimetre of movement to the south. Epoch 20001.251 was chosen because the cumulative rainfall total decreased significantly, and the overall movement of the lighthouse appears to have subsided. Epochs on the first day of each month were chosen in an effort to match monthly rainfall totals with the movement of the lighthouse.

St. Catherine's Point experienced 940.8 millimetres of rainfall from 1 January 2000 to 31 December 2000. From 1 May 2000 to 1 April 2001 the area received 1067.7 millimetres of rainfall, which is over 200 millimetres more rainfall than experienced in a normal year. This indicates that St. Catherine's Point was affected by a significant amount of rainfall, which contributed to the shift in the lighthouse with respect to the soil shear at its base. The lighthouse began to shift southwards immediately after the largest daily rainfall total of the year, which was approximately 38 millimetres in October 2000. Similarly, the greater the daily rainfall totals for amounts over 10 millimetres, the steeper the rate of change of the shift of the lighthouse. Figure 3 shows the shift data (in centimetres to the left) superimposed with the rainfall data (in millimetres to the right).

The analysis of the overall network indicates that the residuals fall within the accepted range of 3s at a 99% confidence level. The entire network achieved millimetric precision throughout the entire data period analysed. In general, the eastings and northings for each station tend to be twice as precise as the up component. The only station in the network to experience a significant coordinate shift is SCP1, and this is mainly in the north component. Note that additional sources of error such as receiver, atmospheric, and multipath were investigated and determined not to have adversely impacted SCP1 or any other station in the national active network.

In comparing various epochs it is evident that SCP1 has moved due to soil shear and not because of deformations in the network. When Epoch 2001.335 is compared with Epoch 2001.497, the change between the adjusted coordinate values for SCP1 is approximately 2 centimetres in east, 9 centimetres in north, and 4 centimetres in up. When comparing both epochs for the active station at Ordnance Survey Headquarters in Southampton, there is no coordinate change at all. The average coordinate change for any one station in the network is 3 millimetres in east, 3.5 millimetres in north, and 1 centimetre in up. The active network was determined to have a strong geometry and does not suffer from horizontal or vertical deformation.

The station at St. Catherine's Point has experienced over 9 centimetres of shift southwards since October 2000. The station appears to have stopped moving, and will not move until another significant period of rainfall occurs. This equates to approximately 40mm of rainfall in a four-day period based upon a trend analysis model developed during the research of St. Catherine's Point. It was determined that short periods of intense rainfall are directly proportional to the rate of change of movement of the lighthouse. This finding was consistent with other groundwater level and horizontal deformation studies conducted in similar environments to that of St. Catherine's Point. It was determined that no structural damage occurred within the structure to cause movement, and no twisting or bending had occurred with the GPS antenna to cause movement. This was confirmed after a visual inspection of the antenna in September 2001, checking maintenance inspection records at Ordnance Survey, and analysing the statistical results of the GPS data.