Computer Methods and Experimental Measurements · Britain's coastline and one such scheme is the Shoreham to Lancing Sea Defences [1], which are situated between Brighton and Worthing

Beach monitoring with GPS

William F Price

Department of Civil Engineering, University of Brighton,BRIGHTON, BN1 4GJ, UK

Abstract

In common with many other countries that have an extended coastline, seadefence work in the UK involves much time and expense. For these projects,topographical information is usually required to represent beach and other areasfor modelling and design purposes and to record the response of a sea defenceonce implemented. This survey information often has to be collected in ahostile environment in a relatively short time and this can be difficult to dowith conventional surveying equipment such as levels and total stations.However, with the advent of the Global Positioning System (GPS), surveyingequipment is now available that offers the ability to gather survey data at a fastrate that suits coastal and other inshore work. This paper gives details of someGPS surveying carried out by the University of Brighton for beach monitoringwork and describes the field procedures and data processing carried out. hiaddition, the results obtained are discussed and future recommendations given.

1 Introduction

In recent years, many flood defence schemes have been planned for protectingBritain's coastline and one such scheme is the Shoreham to Lancing SeaDefences [1], which are situated between Brighton and Worthing on the southcoast of England (Figure 1). The cost of these works is expected to be £14million over the next ten years.

Under the direction of the Environment Agency in the UK, Phase 1 of thesesea defences has involved the design and construction of three 50 m rockgroynes on the beach at Lancing, one of which is shown in Figure 2. In order tomonitor the effect on the beach area surrounding these groynes after

Transactions on Modelling and Simulation vol 16, © 1997 WIT Press, www.witpress.com, ISSN 1743-355X

Computer Methods and Experimental Measurements

Figure 1: Location of Lancing Beach between Shoreham and Worthing

Figure 2: Typical rock groyne


Computer Methods and Experimental Measurements 553

construction, monitoring surveys were carried out by the consultant responsiblefor the construction of Phase 1 during the period October-December 1995.From January 1996, these surveys were carried out by the Department of CivilEngineering, University of Brighton (UoB).

The specification for the UoB stated that for consistency, the same cross-sections that the consultant had been monitoring should continue to bemonitored and that all survey data was to be collected with a tolerance of ±100 mm both horizontally and vertically.

2 Data acquisition and processing

The original method used by the consultant for monitoring at Lancing involvedtaking measurements directly along a number of cross-sections specified for an800m segment of the beach as shown in Figure 3.

Figure 3: Positions of cross-sections monitored along Lancing Beach

This involved locating the start position of each section and then takingmeasurements from these, with conventional surveying equipment, along thedirection of each section. Although this is capable of producing satisfactoryresults it is a rather slow process and it limits the data available, at each survey,to the beach cross-sections only. Because of this, the method of data capturewas changed by the UoB and XYZ coordinates (plan and height information)are now obtained using GPS equipment such as to cover the entire beach atrandom but close intervals that do not follow any predetermined cross-sections.


554 Computer Methods and Experimental Measurements

This approach has the advantage of providing a complete record of the beachevery time it is surveyed from which it is possible to produce a Digital TerrainModel (DTM). The DTM is used to generate the required cross-sections andother spatial data can be extracted if required at any point and scale on thebeach. For all surveys, GPS rather than conventional surveying equipment isused as this enables a large number of points to be surveyed which in turn givesrise to an improved and more reliable DTM.

All the XYZ data for each survey has been acquired using a Wild GPS-System 200. This uses two single frequency, 6-channel receivers and has anominal baseline accuracy of 5 mm to 10 mm + 2 ppm for static surveys and10 mm to 30 mm + 2 ppm when used in kinematic mode. The surveysthemselves comprised two parts: the transfer of control from Ordnance Survey(OS) stations to establish reference points next to the beach at Lancing and thesurveys of the beach at regular time intervals.

In order to establish the first GPS station at Lancing, the System 200 wasused in static mode and GPS satellite data was collected with the referencereceiver located next to the beach as in Figure 4 and with the rover located atthree nearby OS stations in turn.

Figure 4: Wild GPS System-200 at reference station

In common with all other GPS equipment, the initial XYZ coordinatesobtained for any point surveyed with the System 200 are on the WGS84coordinate system and it is necessary to transform these to local coordinates in



the UK in the form of National Grid (NG) eastings and northings and heightsrelative to the Ordnance Datum (or mean sea level). At Lancing, this was madepossible by knowing the OS coordinates and heights of the three rover stationsused in the static survey as well as their observed WGS84 values. Some care isneeded when deriving transformation parameters for a NG or any other surveyand it was decided to use a Bursa-Wolf 4-parameter transformation to shift theorigin of the WGS84 system to the NG origin and to allow for a scale changebetween the two systems. Using these transformation parameters and therelevant GPS baseline data, the NG coordinates for the GPS reference station atLancing Beach were computed. In addition to this, a second GPS station had tobe established at Lancing, and this was done by taking further static readings,but between the two beach stations in this case. A check on the elevationsobtained for the GPS stations at Lancing was carried out by levelling to thesefrom two nearby OS bench marks. The results obtained by levelling agreedexactly with the GPS results.

Full surveys of the beach at Lancing have been carried out at regularintervals since January 1996. Because of the need to gather as much detail aspossible of the beach topography, the surveys are always completed within aperiod of about two hours before and after low tide. All satellite data isobserved using the System 200 in kinematic mode: this has been chosen as itenables surveys to be carried out at walking pace with the equipment back-packed as shown in Figure 5, this being the most efficient and comfortablemethod for manoeuvring on foot around the beach.

Figure 5: GPS equipment back-packed for kinematic survey of beach



As the longest baseline from the GPS stations to any other points to besurveyed on the beach is less than 1 km, the accuracy for the GPS coordinatesis expected to be 10 mm to 30 mm horizontally and up to 2-3 times this forheights, figures within the tolerance of 100 mm specified for these for thecontract. For each survey, the reference receiver is located throughout at one ofthe GPS stations and the other is used by the roving (back-packed) receiver as astart point to initialise each GPS kinematic chain. The sampling rate for thekinematic chains is set at 4 seconds and, at a steady walking pace, this seems toprovide the optimum rate for obtaining data on the more uniform parts of thebeach accepting that it is sometimes necessary to wait at changes of slope andat irregular points for each 4 second epoch to occur.

As the System 200 relies on post-processing of results, all the field data isprocessed immediately following each survey and a separate data file is createdwith lines comprising the NG Easting, Northing and height relative toOrdnance Datum of each point obtained by GPS and transformed fromWGS84. It is worth noting that each data file contains between 3000 and 5500points per survey. Occasionally, a kinematic chain does not compute and has tobe re-observed as soon as possible at the next low tide. The reason why thisoccurs has not always been established but it is usually caused by adeterioration in satellite coverage and geometry whilst surveying. In case ofproblems with the GPS software not being able to resolve ambiguities, nokinematic chains longer than about one hour are normally observed.

3 Surface modelling

The data files produced from the GPS readings are used to create a DTM foreach survey. This is done using the SURFER for Windows contouring and 3Dsurface mapping program. SURFER interpolates, from an irregular spread ofXYZ data, Z values on a regularly spaced XY set of coordinates in grid files.Irregular in this sense means that XY (and Z) values have been obtained alongrandom cross-sections and at other points where the beach surface wasirregular.

There are many methods available in SURFER for generating grid files andTriangulation with Linear Interpolation was chosen. The reasons for this arethat the triangulation method creates a patchwork of triangles by drawing linesbetween data points and it then interpolates heights at grid nodes within theseindividual triangles. Because the original data points are used to define thetriangles in this way, the data representation is good and break lines are alsopreserved, both of which were necessary as the beach data is gathered bysurveying as many changes of slope as possible. When creating grid files fromGPS data files with SURFER, a 1 m grid density has been chosen in order tobest represent the beach surface for producing cross-sections and it took thecomputer less than a minute to create a grid file with this density. This wasanother reason for choosing triangulation as with Kriging, another of the



recommended gridding methods, the estimated time for generating a grid filewould have been several hours with the size of data file in use.

4 Cross-sections

For the project, the NG coordinates of 19 cross-sections to be plotted alongLancing Beach were specified by the Environment Agency. To produce therequired cross-sectional data for these with SURFER, a blanking file is firstcreated for each section specifying the start and finish coordinates, and the slicecommand generates the cross-sections which are normally stored in separatedata files. Following this, the cross-sections are plotted from the data filesusing Microsoft EXCEL and some sample plots are shown in Figure 6.

5 Discussion

The survey method used for the monitoring of Lancing Beach has beensuccessful and cross-sections have been produced.

During the initial stages of setting up the measurement method a number ofminor operating difficulties were experienced but were soon resolved and afterthis each survey has progressed with few problems except when the GPSsatellite coverage tends to be poor at the same time as low tide.

Regarding accuracy, it is estimated that after being processed by SURFER,the data presented on the cross-sections is probably known to about ± 100 mm,the limit specified in the contract. This accuracy depends on many factors,including the accuracy of the original GPS data, the number of survey pointstaken, the grid spacing in the DTM, and so on. The original GPS results willhave a better accuracy than this, and are estimated to be known to about ± 50-75 mm. All of these estimates refer to the accuracy of horizontal points on theNG and the accuracy of levels relative to the Ordnance Datum. It is emphasisedthat the measurement method at Lancing was proposed and implementedbearing in mind the tolerance specified of ± 100 mm. With a different accuracyrequirement, the method could be modified to accommodate this.

In conclusion, it is suggested that the GPS/DTM approach to beachmonitoring is a valuable tool available to the Environment Agency and others.It is capable of producing a large amount of three-dimensional data in the sametime it would take to survey a limited number of cross-sections along a beachusing conventional methods and is easier to use in adverse weather as it doesnot involve the use of optical instruments. Furthermore, the operator(s) arealways moving. In these respects for beach surveys, GPS surveying is certainlymore 'user-friendly' as far as the surveyors on site are concerned!

Further work is being undertaken by the Department of Civil Engineering,University of Brighton, to find the optimum way in which to carry out beachmonitoring surveys with GPS and it is hoped that these results will bepublished at some time in the near future.


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Acknowledgement

The author wishes to thank the Environment Agency for giving permission forthis paper to be published.

Reference

1. Shoreham and Lancing Sea Defences, Environment Agency, Sussex AreaOffice, Worthing, BN11 1TF, UK, 1996.


Documents

Computer Methods and Experimental Measurements · Britain's coastline and one such scheme is the Shoreham to Lancing Sea Defences [1], which are situated between Brighton and Worthing