Real Time Surveying GPS and HYDRO Software for Tide and Swell Compensation

Transcription

1 Real Time Surveying GPS and HYDRO Software for Tide and Swell Compensation by Mr Michael Walker, B.Surv. (Otago) HYDRO Division Trimble Navigation New Zealand Ltd. Abstract This paper focuses on the use of HYDRO and Real Time Surveying (RTS) for the real time corrections of ocean tides and swells. With the HYDRO and RTS combination there is no need to search for or establish tide gauges in the area which you are working. This is specifically useful in locations where there is a large amount of low frequency swells, caused by water streaming through gulfs, which cannot be accurately accounted for by tide gauges. In this paper two examples are given illustrating the use of HYDRO and RTS for positioning. The first is the reef area which was surveyed by Australia s Gold Coast City Council and the second is an exposed coastal strip of New Zealand s Pegasus Bay. The cost effectiveness of the package, in both situations can be seen, highlighting the benefits of such a system. Introduction Over the years Hydrographic surveyors have always dealt with the issue of tidal reduction when carrying out both inshore and offshore surveys. Up till now, the only way to correct survey data for tidal effects was by: applying depth corrections, after the survey, collected from a tide gauge. having the corrections transmitted by the tide gauge in real time over a radio link to the data acquisition package on the boat. by vertical angle measurements from shore based Range/Azimuth positioning device. In addition, there was always the uncertainty of swell that could not be monitored by tide gauges. To remove the effect of swell surveys were historically either carried out in calm conditions or a heave compensator was used. Conventional Determination of Tide Data A constraint for conventional hydrographic surveying is the neccessity to have tide gauges close to the local area where survey operations are being carried out. In the case where tidal effects are complex (eg. large coastal river mouths), then more than one tide gauge needs to be established and monitored. This requires considerable time and expense. If tide gauges are not established prior to surveying then they would have to be surveyed into the area where the work is taking place. This can be a major cost, especially if the location for the area was miles away from the nearest survey control. Trimble Navigation 55

2 Also, if the coastline is rugged such that there are no wharfs or structures to mount tide gauges, then to establish a seabed mounted gauge requires another days work. Conventional Determination of Swell Swell is another major factor which can attribute to the errors in depth measurement. Swell can be a result of factors such as wind, wakes from other vessels and wave reflections in confined harbours. Swell can be divided into two categories: High Frequency Swell Low Frequency Swell High frequency swell is experienced when there is a rapid vertical movement of the sea surface. This movement can be measured by a heave compensator on a boat. However, the frequency range in which most heave compensators operate is limited to swells that range from 2 to 20 seconds. An example of a heave compensator is a TSS 333 Heave compensator, as pictured at right: The standard heave (swell) compensator contains a built in accelerometer which measures the vertical movement from a zero datum. Heave values can be applied to the depths in real time, but are only really required for the final chart. The negative aspect about a heave compensator is the pricing. Most survey companies are unable to stretch the budget to spend $ US on such a product. Figure 1. TSS Heave Compensator Low frequency swell, with a period in the order of a few minutes, is experienced in areas such as gulfs where there is a vertical movement in water level which cannot be sensed by an instrument such as the heave compensator. Determination of Tide and Swell with HYDRO and RTS The Equipment Needed An alternative method for compensation of errors due to tide and swell is to use the latest technology from Trimble s stable : The 4000 Series GPS receivers - the 4000SSE with RTS Hydrographic Survey Software - HYDRO This Real Time Surveying GPS is a land survey technology that has been commercialised by Trimble, and comprises of a 4000SSE and radio transmitter set up at the base station with another 4000SSE and radio receiver set up onboard the survey vessel. This technology means precise carrier phase positioning is now usable in the hydrographic environment. Real Time Surveying GPS generates positions with accuracy s of better than 5cm in all dimensions. 56 Users Conference Proceedings 1995

3 The land base station is situated on a point whose three dimensional coordinates are known on the WGS84 Spheroid. This can be established by conventional survey techniques. From here carrier phase corrections are transmitted out to the survey vessel via the radio link. The accuracy of the system is 1 cm + 1 ppm. Best accuracy is achieved if the vessel is operating within 10 kilometers (6 miles) from the base station. This centimeter precise position of the GPS antenna on the survey vessel is output into the HYDRO computer, and latitude and longitude is transformed into the local coordinate system. The GPS height is used by HYDRO to calculate the height above or below a zero datum, that is, the tide and swell. HYDRO combines the depth sounder values with the GPS data to produce precise profiles of the seabed. The Geoidal Separation In order for the above values to be calculated, one important element must be known - the Geoidal Separation. This is the vertical separation between the WGS84 Spheroid and Mean Sea Level (MSL). The geoid is often known as MSL. This separation is also established by conventional survey techniques. Typically a GPS receiver can be set up over a survey mark near the survey area, whose three dimensional coordinates are known. The known mark will typically have a height above mean sea level and the GPS receiver will record a height above the WGS84 Spheroid. The difference between the two values is the Geoidal Separation. However, the geoidal separation does differ from place to place due to factors such as varying gravitational pull and proximity to dense objects such as mountain ranges. The change in the geoidal separation over the range of the RTS system (10 kilometers) is usually no more than a few centimeters. Reduced Level Profiles and Tide Calculation With HYDRO you are able to input the geoidal separation value for the area that you are working in, as well as being able to change it when the separation changes. Tide and reduced level values are calculated as follows: Tidal Correction Tide Correction = GPS height - Antenna height - Raw depth - Datum separation Tide Corrected Depth = Raw depth - Tide correction Reduced Level Profile Reduced Level of Water Surface = GPS height - Antenna height - Datum separation Reduced Level of Sounding = Reduced Level of Water surface - Raw depth Trimble Navigation 57

4 Application examples of HYDRO and RTS As introduced, a good solution to the above two issues (of tide and swell) can be found by the use of the combination of the HYDRO software package and the 4000SSE GPS receivers. This is illustrated by the two surveys which were carried out over the past year. Australia s Gold Coast City Council swell corrected soundings using Trimble s HYDRO and RTS. New Zealand s Pegasus Bay tidal corrections by Trimble s HYDRO and RTS. HYDRO and RTS used for Swell Correction on the Gold Coast Issues The Gold Coast City Council (GCCC) is vitally interested in coastal processes as the sandy beaches are a major tourist feature. The GCCC has the task of surveying the area from the shoreline to the inner reef, which is approximately 1.5 kilometers from the beach. Naturally this area is tidal. However the coral reef tends to make the tide values, along the beach, very irregular, hence this is a situation where a tide gauge would not be a great deal of use; especially when precise depths were required. Tide aside, the main issue to confront in the survey was the amount of swell experienced as one approached the shore. The higher frequency swell could be compensated for by the use of a heave compensator, such as a TSS model as aforementioned. However a heave compensator is deemed to be an extra expense. In addition to that, they do not have the ability to compensate for the low frequency swell which is typically attributed to areas within a coral reef. Solution Figure 2. Relationship between datum, GPS datum and measured depth. The solution - carry out the survey with the Trimble RTS GPS receivers, and use HYDRO for real time data capture, reduction and processing. Horizontal information, captured by HYDRO, was reduced to a local datum while the raw GPS height was reduced to a profile above a known datum. In post processing HYDRO matched the actual GPS position update with the raw depth sounding, then added the values together (as explained previously) to output a depth corrected for tide and swell. Results With reference to the longitudinal plot (Figure 4), there are two profiles displayed above a known datum. The left hand side of the plot is the beach end and the right hand side is the reef end, which if course, is deeper. 58 Users Conference Proceedings 1995

5 The top profile is the raw depth, uncorrected for tide or swell. The sinusoidal pattern is a result of the ocean swell affecting the depths from the echosounder. It can truly be seen that this does not give a true indication of the profile progressing from the shore out to the start of the inner reef. As the geoidal separation, raw depth, GPS height and Antenna height are known, then the true reduced level for each sounding can be deduced. HYDRO matches position with soundings to ensure that the raw depths are corrected for tide and swell. The resultant reduced level profile can be seen from the same plot. This is represented by the lower line which shows the reduced level for the profile from the beach to the start of the inner reef. The savings are in having no need to purchase a heave compensator or to monitor and record data from a tide gauge. HYDRO and RTS used for Tidal correction in Pegasus Bay The Task The Canterbury Regional Council required a number of benchmark profiles to be surveyed as part of its coastal studies. A local hydrographic contractor, Eliot Sinclair and Partners, were awarded the contract. These profiles were to extend from the surf zone out 3 kms. The study area was 40 kms long and 5 profiles were to be surveyed. The intention is to resurvey them every 12 months or after a significant storm to determine the coastal accretion and erosion process. These profiles are to be integrated with the beach profiles and so the vertical datums have to coincide. The Council required decimetre accuracy in depth for the profiles. Horizontal positioning to 3 meters was sufficient. Due to the nature of the coastline (open surf beaches) tide gauges could not be installed close to the survey area. The closest tide gauge was at the southern end of the survey area 5 kms from the nearest profile and it was felt this may introduce an error due to it operating in a different tidal regime. Figure 3. Aeiral view of Surfer's Paradise on Australia's sunny Gold Coast. Trimble Navigation 59

6 The Solution Trimble s Real Time Surveying (RTS) GPS system was used as it: Gave accuracy s of better than 5cm in 3 dimensions Removed the vertical datum error due to distant tide gauge data Quick to mobilise the reference station Automatically initialised the GPS while on the boat (termed On the Fly) The maximum range from the reference station GPS to the boat is 10 kilometers so as the profiles were completed the reference station was moved down the coast to other control points. A 2 watt UHF telemetry link operating at 4800 baud was used to transmit corrections from the reference station to the boat. Even though the survey was carried out on a calm day there was 1.6m swell so a TSS Heave Compensator was utilised. The echosounder was an ODOM Echotrac DF3200. Unlike the Gold Coast survey procedure the RTS was used here to determine long period effects such as tide rather than short period swell. During the survey, Trimble s HYDRO navigation software was used to integrate all sensors, record data and determine the tidal correction. The HYDRO software took the GPS antenna position and transformed it into the local datum and map projection. Prior to the hydrographic survey, a GPS land survey established control points at the shore end of the five profile lines. The difference between WGS84 GPS height and local height datum was determined at these five sites and entered into HYDRO for the surveys. Figure 6. The survey boat on location. The height of the GPS antenna above the echosounder transducer was measured prior to survey. Tidal corrections were computed and stored, and tide corrected depths displayed. Precise guidance was available on a helmsman s monitor and HYDRO stored 10 raw depths and heave readings each second. Results and Conclusions When the boat received the corrections from the reference station the RTS GPS system initialised automatically within 40 seconds. The system remained initialised in all cases until the reference station was turned off. The tidal corrections as computed by HYDRO and RTS GPS were compared with those from the tide gauge. No significant difference between the gauge readings and the tide by RTS GPS was noted. However the GPS system provided significant benefit in revealing the existence of a long period (2-3 minute) surge in the survey area. The heave compensator corrected for short swell periods (approx. 6 seconds) 60 Users Conference Proceedings 1995

7 but could not compensate for the larger period surge. An effect which must be determined and compensated for when using RTS GPS is Geoidal Separation. The Geoidal shape (ie. the shape of the surface of the water if absolutely calm) is not necessarily parallel to the WGS84 spheroid. In the test area there was a variation of 10 centimeters over 3 kilometers. The variation was determined during the GPS land survey of the coastal reference points. Raw sounding data was edited to remove incorrect data, tide, heave and surge corrections applied and profiles generated using HYDRO. These were submitted to the client in plotted and digital form relative to the local height datum. Summary of the Technology Used - Real Time Surveying with On the Fly Initialisation The benefits of HYDRO software and Real Time Surveying GPS are: No need for tide gauge, draft or squat correction to be individually measured and calibrated The effect of long period waves outside the range of a heave compensator can be measured and compensated for No need for co-tidal computations as the vertical datum is based on GPS height not sea level 3D position solution with accuracy s better than 5cm High precision in a dynamic environment for coastal engineering works (e.g. pile placements) Figure 7. Interior of the survey vessel showing GPS, radio telemetry, Odom sounder, PC running HYDRO and Helmsmans display. Trimble Navigation 61

8 No need to set up or maintain tidal gauges Not only can GPS be used for positioning, but also to supply tide and swell corrections without needing to purchase tide gauges or heave compensators. All the advantages of GPS (no line of sight required, works in all weather conditions, automatic system initialisation, no user charges for satellite signals, etc) Conclusion The evidence can be clearly seen as to the advantages in using the technology, from GPS for positioning in all three dimensions. From tidal to swell - these can all be corrected for by the use of the Trimble 4000 GPS receiver. The example from Australia a Gold Coast illustrated that swell was as issue, and the effective use of the one GPS system to provide the entire solution. The beach profile survey in New Zealand s Pegasus Bay illustrated the issue that it is not neccessary to install tide gauges in remote areas along a rugged coast. Also, the RTS system increased accuracy of the profiles as it detected a long period swell. The tide corrections, supplied by the HYDRO - RTS combination proved to be an invaluable cost and time saver. About the Author Mr Michael Walker was born in New Zealand in He graduated from the School of Surveying at Otago University in Since then he has been involved in a wide range of hydrographic survey operations in New Zealand, South East Asia, South America, Western Europe and the United States. His specialty is Trimble 4000 series RTS and Differential GPS receivers and the hydrographic survey package, HYDRO. 62 Users Conference Proceedings 1995