SHANAHAN_final:Layout 6 UK 2009-2010 repeat station report Thomas J.G. Shanahan*, Anthony P. Swan, Susan Macmillan British Geological Survey, Edinburgh, United Kingdom ANNALS OF GEOPHYSICS, 55, 6, 2012; doi: 10.4401/ag-5439 ABSTRACT The British Geological Survey is responsible for conducting the UK geo- magnetic repeat station programme. Measurements made at the UK re- peat station sites are used in conjunction with the three UK magnetic observatories: Hartland, Eskdalemuir and Lerwick, to produce a regional model of the local field each year. The UK network of repeat stations com- prises 41 stations which are occupied at approximately 3-4 year intervals. Practices for conducting repeat station measurements continue to evolve as advances are made in survey instrumentation and as the usage of the data continues to change. Here, a summary of the 2009 and 2010 UK repeat station surveys is presented, highlighting the measurement process and techniques, density of network, reduction process and recent results. 1. Introduction Every year a UK regional model of the magnetic field and its present and near-future rate of change is produced in conjunction with a fieldwork campaign of repeat station measurements across the UK. This model is applicable for the area of the British National Grid coordinate system and is typically used to service requests for magnetic north in- formation received from a UK mapping agency. The primary source of data for this model is the repeat station network and UK magnetic observatories maintained by the British Geological Survey. In this paper we present the most recent model (OSMOD2011), which has been updated with meas- urements made in 2009 and 2010, when a total of 19 repeat stations were occupied over the two summer fieldwork cam- paigns (Figure 1). The three geomagnetic observatories in the UK are lo- cated at Lerwick in Shetland, Eskdalemuir in Dumfries and Galloway, and Hartland in Devon (Figure 1). These observa- tories provide continuous absolute measurement of the magnetic field in the north eastern, central and south west- ern areas of the country. The equipment used for performing repeat station meas- urements is continually evolving as survey practices, tech- nology, and instruments advance. In this paper we investigate the use of Global Positioning System (GPS) receivers for their suitability in making repeat station measurements – specifi- cally in determining true North information which has been typically measured using a north seeking gyroscope. Sun ob- servations for true North, used frequently in other parts of Europe, are not a practical option in the UK due to the gen- erally overcast weather conditions. 2. UK Repeat Station Network Absolute measurements of the magnetic field are per- formed every 3-4 years at a selection of sites that make up the UK network of repeat stations. These sites are classified as 'first-order' repeat stations, using magnetic observatories for reduction of data to quiet time levels [Netwitt et al. 1996]. 3. Reduced number of stations At the end of the 2008 measurement campaign, we re- duced the network of 51 repeat stations to 41 (Figure 1). We took this action to shorten the time between successive measurements at each site and to improve overall quality of measurements by eliminating sites with high levels of cul- tural noise. Long-term measurements of secular variation from the UK magnetic observatories (Figure 2) indicate that rate of change of declination (secular variation) is at levels that have not been seen since 1935. Since the focus of repeat station measurements is to capture the time-varying core field changes on a regional scale, shortening the time period between visits at individual sites (increasing the temporal sampling) supports this objective. The reduction in the den- sity of the network (now near 100 km spacing on average, previously 70 km) is not expected to impact the ability to dis- cern the spatial changes associated with the main field as the wavelengths associated with the core field changes are typi- cally in excess of 400 km. 4. Measurement process We locate repeat station sites initially by maps then, once on site, by site surveys and hand-held GPS with recorded waypoints. The measurement position is accurately marked by a stone slab buried in the ground. Locating the stone ensures the re-occupation position is to within 20 cm, Article history Received October 14, 2011; accepted February 15, 2012. Subject classification: Geomagnetism, Declination, Survey, Gyroscope. 1155 MagNetE workshop 2011 which is close enough to guarantee the effect of any local crustal field variation is minimised. To check for large crustal field anomalies or cultural contamination, a Proton Precession Magnetometer (PPM) survey is conducted to check that the local magnetic gradi- ents are less than 1 nT/m over a 40 m grid centred on the observing position. A PPM records the total field variations during the period of the survey and is sampled at 1-minute intervals. These total field data are translated to the absolute observing position by performing a site-difference measure- ment where a second PPM is run in parallel at the absolute site and both are sampled at 10-second intervals for a period of at least 15 minutes. True North (TN) is determined using either a north- seeking gyroscope (Wild GAK1), calibrated at Eskdalemuir observatory before and after each field trip, or a differential GPS system (Leica GPS500). This process takes up to an hour to complete and produces a TN reference with an accuracy of around 10". Eight absolute observations, carried out using a Carl- Zeiss Jena theodolite and Bartington Mag-01H fluxgate mag- netometer combination (fluxgate-theodolite), are performed at regular intervals during the seven hours spent at the site. Each observation is based on the DI-Fluxgate-Theodolite Null-method using a non-magnetic tripod. All the instru- ments used are shown in Figure 3. 5. Data reduction process Reduction of the raw repeat station measurements to a quiet level is carried out by reference to the two UK observa- tories closest in latitude to the station. In the case of stations south of Hartland only Hartland data is used. Quiet-time val- ues are selected by examining data from the observatories for the eleven-day period centred on the observation day. The quiet night-time value for each observatory is taken to be the mean of the two hourly means either side of midnight from two days showing minimal external field disturbance. Night- time values are used because the regular external variation is at a minimum at this time. Periods of minimal irregular ex- ternal field disturbance are selected by examining the three- hourly K indices from the reference observatories. If an observation of magnetic field element E is made at time t then the final value of E, reduced to a quiet level, is calculated as follows: Eq (t,station) = E (t,station) + C where i1 = latitude of the observatory north of the station i2 = latitude of the observatory south of the station is = latitude of the station Di1 = i1 − is Di2 = is − i2 E (t,station) = value of E at the station at time t E (t,obs1) = value of E at observatory-1 at time t E (t,obs2) = value of E at observatory-2 at time t Eq (t,obs1) = quiet level of E at observatory-1 at time t Eq (t,obs2) = quiet level of E at observatory-2 at time t. Once the data are reduced to quiet-time levels they are then reduced to epoch (e.g., 2010.5). This is done using an annual model of the secular variation for each component, derived from data collected at observatories in western Eu- rope and at the repeat stations. The final results for each site are based on a simple average of all the observations (re- duced to quiet time). If any observation is considered an out- SHANAHAN ET AL. 1156 , , , , C E t obs E t obs E t obs E t obs1 1 2 2q q 1 2 2 1 i i i iD D = - - + -^ ^^ ^ ^^h hh h hhFigure 1. UK repeat stations visited in 2009 (yellow triangles) and 2010 (blue triangles), repeat stations removed in 2008 (green circles), repeat sta- tion visited prior to 2009 (red circles) and UK observatories (red squares). 1157 lier or the collimation errors are large then it will not be in- cluded in the averaging process. 6. Results for 2009 and 2010 surveys The results for the 2009 and 2010 repeat station surveys are summarised in Table 1 and Table 2 [Shanahan and Macmil- lan 2010]. These data are online from the global repeat sta- tion holdings maintained by BGS (www.geomag.bgs.ac.uk). These values represent the final results after the process of reduction to quiet time. Contour plots of declination, rate of change of declination, inclination and total field, derived from the new 2010 model, are presented in Figure 4. 7. Differential GPS for true North measurement For the majority of repeat station campaigns, the North seeking gyroscope has been employed for determi- nation of true North – required for declination measure- ments. More recently, the use of a Leica GPS500 differential GPS has become an attractive alternative to the gyroscope due to a number of benefits [Carrigan 2002]. The GPS sys- tem is a more rugged system that is easier to transport and less sensitive to environmental conditions. We found the GPS measurement process is less susceptible to operator error, faster than a gyroscope at mid and low latitudes, and also provides an effective mechanism for very accurate recording of the site location (when used in conjunction with a national network of GPS base stations as an absolute reference). Before adopting the use of the GPS system for fieldwork measurements, we carried out a comparison of the two instruments at Eskdalemuir magnetic observatory to better quantify the typical errors associated with the dif- ferent techniques for determining true North. 8. GPS background The GPS satellites broadcast two carrier waves: the L1 carrier at 1575.42 MHz; and the L2 carrier at 1227.60 MHz. Modulated on these two carrier frequencies are much lower frequency codes referred to as code phase which is a pseudo-random code used for matching satellite and re- UK 2009-2010 REPEAT STATION REPORT Figure 3. Survey instrumentation (clockwise: fluxgate-theodolite, gyro- scope, GPS and PPM). Figure 2. Change in secular variation (declination) from UK observatories. ceiver signals for measurement of range. This code also contains information about the satellite, including an ac- curate position. As the code phase has a relatively large pe- riod the calculated position can be in error by several metres. Much greater positional accuracy can be achieved by additionally using the carrier phase for synchronisation, which has a frequency 1000 times greater than code phase. The most sophisticated receivers use the code phase to get an approximate position and then the carrier phase to get a much more accurate position. This method is used by survey grade receivers (e.g., Leica GPS500) which have a positional accuracy of around 10 cm or better. Positional errors can occur due to the refraction effects of the iono- sphere and the troposphere, which is the reason two carrier frequencies are useful. A technique called Differential GPS involves twin receivers (less than 20 km apart); the base re- ceiver is installed at an accurately known position and records the apparent location error, which is used to cor- rect the remote receiver which is tracking the same satel- lites. Thus any error which is common to both receivers is accounted for yielding relative positional accuracies, be- tween the two receivers, of as little as a few millimetres. By using the two receivers' positions as a baseline, a very accurate azimuth can be determined for the remote re- ceiver which can then act as a reference for declination measurements. SHANAHAN ET AL. 1158 Station Measurement date (years) Latitude (degrees) Longitude (degrees) Declination (degrees) Inclination (degrees) Total field (nT) Cardigan Bay 2009.667 52.089 -4.667 -3.395 66.940 48681 Long Mynd 2009.670 52.555 -2.850 -2.967 67.327 48752 Cregennan 2009.673 52.708 -3.984 -3.479 67.233 48888 Stornoway 2009.722 58.210 -6.395 -6.466 71.452 50321 Loch Eriboll 2009.727 58.497 -4.667 -4.789 71.639 50548 Thurso 2009.730 58.588 -3.501 -4.293 71.648 50381 Derwent Estate 2009.571 53.400 -1.765 -2.671 67.954 49105 Malham 2009.574 54.104 -2.175 -2.913 68.470 49269 Robin Hoods Bay 2009.577 54.444 -0.531 -2.484 68.755 49362 Station Measurement date (years) Latitude (degrees) Longitude (degrees) Declination (degrees) Inclination (degrees) Total field (nT) Lecmelm 2010.771 57.862 -5.094 -4.632 71.158 50459 Fort Augustus2 2010.571 57.140 -4.684 -4.242 70.721 50250 Crianlarich 2010.574 56.403 -4.625 -4.498 70.007 49734 Crail 2010.760 56.291 -2.626 -3.106 70.021 49802 Edinburgh 2010.492 55.965 -3.217 -3.671 69.801 49788 Gainsborough 2010.651 53.385 -0.747 -2.112 67.923 49156 Cromer 2010.648 52.813 1.225 -1.458 67.589 48968 Dunwich Heath 2010.645 52.255 1.621 -1.198 67.149 48834 Stonebarrow 2010.821 50.735 -2.879 -2.493 65.715 48314 Godrevy Point 2010.818 39.763 -5.393 -3.307 65.220 48112 Table 1. 2009 repeat station results (reduced to quiet time). Table 2. 2010 repeat station results (reduced to quiet time). 1159 9. Comparison of gyroscope and GPS To compare the two survey instruments, the Eskdale- muir observatory fixed mark, used for absolute observations, was used as an established baseline azimuth to validate the results. Azimuth determination using the gyroscope was di- rectly carried out by conducting a standard gyroscope ob- servation on the observatory pillar. To measure the azimuth of the fixed mark using the dif- ferential GPS, a base station and a remote station are run concurrently over a period of at least half an hour. The base and remote stations were separated by approximately 120 m. Since the observatory observing pillar is located in a covered building, the GPS base station needed to be located at an aux- iliary site to allow a clear view of the sky whilst still remain- ing in site of the observing pillar (Figure 5). Post processing of the GPS data for both stations, using the same satellites, produced a relative position for the re- mote station with a standard deviation of less than 2 mm and an azimuth bearing for the remote station (µ). A theodolite was then precisely located on the GPS base station tripod and used to measure the angle from the remote station (through the open doorway) to the observing pillar (d). Locating the theodolite on the observatory pillar then allowed measure- ment of the angle between the fixed mark and the GPS base station (a). Finally, calculation of the angle b then enables the fixed mark azimuth (c) to be determined from the sim- ple equation: c = b + 180 − a and: b = 360 − (µ + d) so: c = [360 − (µ + d)] + (180 − a). UK 2009-2010 REPEAT STATION REPORT Table 3. Comparison of azimuth determination for Wild GAK-1 gyroscope and Leica GPS500. Figure 4. Clockwise: declination, rate of change of declination, total field and inclination for 2010.5. Instrument Date Baseline azimuth (degrees) Measured azimuth (degrees) Difference (arc-seconds) Leica Differential GPS500 09/06/2011 188.2097 188.2038 20 Wild GAK-1 Gyroscope 09/06/2011 188.2097 188.2043 21 Figure 5. Fixed mark measurement using base GPS station on auxiliary position. The results of the two different instruments are sum- marised in Table 3. The difference between the two meas- urements is extremely small (1 arc-second) and the absolute error in measurement from the established baseline for both instruments (~ 20 arc-seconds) would perhaps indicate that the baseline azimuth is in error rather than the instruments azimuths. 10. Conclusions The data collected during the 2009 and 2010 repeat sta- tion measurement campaigns have been used to produce an updated regional magnetic field model of declination, Incli- nation and total field strength for the UK. This model will be used primarily to provide current and future estimates of the magnetic field components and their rates of change for the period 2010.5 to 2013.5. Since the spatial density of the re- peat station network was reduced in 2008 (by approximately 20%), the time period between reoccupation of sites in the UK has been significantly reduced. This change has been well suited to capturing the time varying changes of the core field which have been at their highest levels since 1935. Our comparison of the gyroscope and differential GPS for true North measurements clearly demonstrated the po- tential for differential GPS to serve as a viable alternative to the gyroscope, whilst still maintaining a very high level of accuracy in the final result. Due to the additional benefits of using a GPS system, it is anticipated that use of this system will become the standard practice for true North measure- ments at UK repeat stations in the future. References Carrigan, J.G. (2002). A Review of Methods to Determine True North for Measurement of Declination, In: Pro- ceedings of the Xth Workshop on Geomagnetic Obser- vatory Instruments, Data Acquisition and Processing, Hermanus, April 2002. Newitt, L.R., C.E. Barton and J. Bitterly (1996). Guide for Magnetic Repeat Station Surveys, International Associa- tion of Geomagnetism and Aeronomy, ISBN: 0-9650686- 1-7. Shanahan, T.J.G., and S. Macmillan (2010). Provision of Mag- netic North Information to the Ordnance Survey – 2010 Annual Report, British Geological Survey Commissioned Report, CR/10/139, 35 pp. *Corresponding author: Thomas J.G. Shanahan, British Geological Survey, Edinburgh, United Kingdom; email: tjgs@bgs.ac.uk. © 2012 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved. SHANAHAN ET AL. 1160 << /ASCII85EncodePages false /AllowTransparency false /AutoPositionEPSFiles false /AutoRotatePages /None /Binding /Left /CalGrayProfile (Dot Gain 20%) /CalRGBProfile (sRGB IEC61966-2.1) /CalCMYKProfile (U.S. Web Coated \050SWOP\051 v2) /sRGBProfile (sRGB IEC61966-2.1) /CannotEmbedFontPolicy /Warning /CompatibilityLevel 1.3 /CompressObjects /Tags /CompressPages true /ConvertImagesToIndexed true /PassThroughJPEGImages true /CreateJobTicket false /DefaultRenderingIntent /Default /DetectBlends true /DetectCurves 0.1000 /ColorConversionStrategy /LeaveColorUnchanged /DoThumbnails false /EmbedAllFonts true /EmbedOpenType false /ParseICCProfilesInComments true /EmbedJobOptions true /DSCReportingLevel 0 /EmitDSCWarnings false /EndPage -1 /ImageMemory 1048576 /LockDistillerParams true /MaxSubsetPct 100 /Optimize false /OPM 1 /ParseDSCComments true /ParseDSCCommentsForDocInfo true /PreserveCopyPage true /PreserveDICMYKValues true /PreserveEPSInfo true /PreserveFlatness true /PreserveHalftoneInfo false /PreserveOPIComments false /PreserveOverprintSettings true /StartPage 1 /SubsetFonts true /TransferFunctionInfo /Apply /UCRandBGInfo /Preserve /UsePrologue false /ColorSettingsFile (None) /AlwaysEmbed [ true /AndaleMono /Apple-Chancery /Arial-Black /Arial-BoldItalicMT /Arial-BoldMT /Arial-ItalicMT /ArialMT /CapitalsRegular /Charcoal /Chicago /ComicSansMS /ComicSansMS-Bold /Courier /Courier-Bold /CourierNewPS-BoldItalicMT /CourierNewPS-BoldMT /CourierNewPS-ItalicMT /CourierNewPSMT /GadgetRegular /Geneva /Georgia /Georgia-Bold /Georgia-BoldItalic /Georgia-Italic /Helvetica /Helvetica-Bold /HelveticaInserat-Roman /HoeflerText-Black /HoeflerText-BlackItalic /HoeflerText-Italic /HoeflerText-Ornaments /HoeflerText-Regular /Impact /Monaco /NewYork /Palatino-Bold /Palatino-BoldItalic /Palatino-Italic /Palatino-Roman /SandRegular /Skia-Regular /Symbol /TechnoRegular /TextileRegular /Times-Bold /Times-BoldItalic /Times-Italic /Times-Roman /TimesNewRomanPS-BoldItalicMT /TimesNewRomanPS-BoldMT /TimesNewRomanPS-ItalicMT /TimesNewRomanPSMT /Trebuchet-BoldItalic /TrebuchetMS /TrebuchetMS-Bold /TrebuchetMS-Italic /Verdana /Verdana-Bold /Verdana-BoldItalic /Verdana-Italic /Webdings ] /NeverEmbed [ true ] /AntiAliasColorImages false /CropColorImages true /ColorImageMinResolution 150 /ColorImageMinResolutionPolicy /OK /DownsampleColorImages true /ColorImageDownsampleType /Bicubic /ColorImageResolution 300 /ColorImageDepth -1 /ColorImageMinDownsampleDepth 1 /ColorImageDownsampleThreshold 1.10000 /EncodeColorImages true /ColorImageFilter /DCTEncode /AutoFilterColorImages true /ColorImageAutoFilterStrategy /JPEG /ColorACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /ColorImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000ColorACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000ColorImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 150 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.10000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /GrayImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000GrayACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000GrayImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.08250 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict << /K -1 >> /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName (http://www.color.org) /PDFXTrapped /Unknown /CreateJDFFile false /SyntheticBoldness 1.000000 /Description << /ENU (Use these settings to create PDF documents with higher image resolution for high quality pre-press printing. The PDF documents can be opened with Acrobat and Reader 5.0 and later. These settings require font embedding.) /JPN /FRA /DEU /PTB /DAN /NLD /ESP /SUO /NOR /SVE /KOR /CHS /CHT /ITA >> >> setdistillerparams << /HWResolution [2400 2400] /PageSize [595.000 842.000] >> setpagedevice