Equipment Rave For All You Boffins Out There

BACKGROUND

Surveying is driven by technology. Historically the Swiss were the renowned instrument-makers particularly when optical theodolites were the order of the day. These instruments were precision-engineered scientific instruments that relied on special manufacturing techniques.

With the advent of modern electronics, sophisticated optics & electro-magnetic technologies, surveying instruments have developed incredibly over the past 40 years!

The modern total station (an electronic version of the optical theodolite) combines all the above technologies in one instrument capable of measuring distance & angles in both horizontal & vertical planes to give an accurate position in 3-dimensional space relative to the instrument position of +/-2-3mm.

HOW MAN HAS USED THE PROPERTIES OF LIGHT TO REFINE ACCURACY

The mechanical chronometer used the effect of thousands of fast mechanical movements per unit time, to determine accurately small intervals of time, the total station electro-magnetic measuring system uses light of a narrow bandwidth pulsed & counted using mechanical technology and transmitted to a reflector & back to a receiver. Because the speed of light (even through our atmosphere) is well known, and the frequency of the light is known accurately by calibration at the factory, it is possible, using clever wave counting techniques to determine the distance accurately to around +/-2mm.

This same measurement principle is used in the modern GPS systems to determine ranges from satellites operated by both the USA military and latterly Russia (GLONASS) & Europe (Galileo Project). GPS made its way into our world in the late 1980’s and has revolutionised the way we survey our planet.

Both the above systems are used today often in conjunction with each other. Not all jobs are suited to the use of GPS, so the total station plays its part still.

Here are some strengths and weaknesses of the two systems. Bear in mind a typical GPS system costs around NZ$47k whereas a total station costs NZ$12k:

As you can see it’s a case of ‘horses for courses’. We carry both systems as an integral part of our field equipment so that every base is covered. Increasingly though we have to agree the GPS is slowly taking over many every-day surveying tasks.

These days the Japanese have joined the Swiss as major players in the surveying instrumentation game. We purchased our Sokkia RTK GPS system late in 2007 and this utilises technology mostly from North America. The core GPS modules are all manufactured by Nortel Technologies Inc. and the peripheral ‘black box’ technology is developed independently by Point Inc in Canada who also do the module case design.

A key feature of the latest Real-Time-Kinematic GPS system is that it is cable-less which means we are no longer restricted by cables! Ours utilises Bluetooth® technology which gives a range-capability of 10m between GPS antenna and the data controller. This means the pole man can be positioned in an awkward spot and the recorder can be in a safer position taking the shot!

So you understand why our systems are capable of 5mm accuracy as opposed to your GPS set you use for fishing, here are a few key differences:

• Higher measurement resolution – typically a higher-end hand-held system outputs a position to a resolution of +/- 2m with an accuracy of around +/-8m.
• RTK systems utilise a local base station that is essentially the same as the GPS rover but stays fixed on a known position. This being the case additional accuracy is obtained by utilising the known XYZ position in space of the base station, back-calculating the range corrections to the respective satellites (that are common to both base & rover) and transmitting these to the rover by UHF radio to improve the accuracy of fix.
• A special separate controller which talks to the unit and does some magic calculations to enable different mapping systems to be used at the flick of a button.
• A whole lot more expensive but worth it for your survey!!

In the future we are looking at utilising multiple GPS systems operating off the one base to improve efficiencies and affordability!

You will notice we even utilise the GPS rover in mobile applications such as hydrographic surveys and on top of our trusty Toyota Landcruiser!

WHERE WILL TECHNOLOGY TAKE US NEXT?

Probably the next phase in surveying technology will take us to hybrid or multi-type systems that rely on both the above technologies and perhaps inertial measurement too.

Battery technology continues to amaze us and this is something too that will assist new systems becoming a reality.

There is currently much research being done to develop positioning systems that are terrestrial-based that determine ranges from known positions (pseudolites). This is exciting as the traditional process of optical observation by the human eye is becoming redundant.

Whilst optical robotic total stations are popular, they are expensive and suitable for specialised uses e.g. accurate measurements in built-up areas. These instruments have developed their tracking and motor-drive technologies greatly over the past 20 years. The Trimble S6 uses magnetic levitation for turning its mechanical parts, rather than the traditional stepper-motor which is more jerky and vulnerable to breakdown. Magnetic levitation is where two magnets face each other N-pole to N-pole so they are repellent. To move one relative to the other, a variation of the magnetic field is induced by electric current, and very precise & smooth movements result.

Key to implementation of such technologies is making sure they work well, are compact & robust, and affordable. Presently when GPS systems won’t work (e.g. under trees) it’s operationally difficult and time-consuming to deploy a total station. With an integral inertial system one could continue surveying and the missed positions could be back-calculated in time by an onboard data-logger / computer.

HOW DOES AN INERTIAL SYSTEM WORK?

As the name rightly suggests it’s about utilising Newton’s laws of physics to determine movement. Historically this could be done using gyro-compasses but these are heavy, expensive, and require complex DC power supplies. These systems have been onboard aircraft for years but have been replaced in more recent times as back-up navigation using laser technologies which are lighter, becoming progressively more affordable and easier to power.

More recently the same can be achieved using lasers, prisms and sensor electronics. When the survey sensor moves the prisms are displaced so that the reference laser beam is altered. Thus displacement can be derived and logged & later used to back-calculate “missed points”. What is in fact measured is acceleration and utilising a mathematical tool known as numeric integration we can derive the first and second integrals being velocity & displacement respectively.

WE HOPE WE HAVE NOT BLINDED YOU TOO MUCH WITH SCIENCE!!