GM Surveys Ltd

Control Surveys

Whatever type of survey you need for your project you will need accurate horizontal and vertical control on which to base the measured data. Horizontally this can be a simple baseline, an arbitrary grid system or a fully co-ordinated network of control stations related to Ordnance Survey National Grid. Levels are also an essential factor for any development and accuracy is of paramount importance, particularly when the proposed construction is adjacent to a flood plain.

Most of us are familiar with the old OS benchmark system where cut marks in buildings were related to a mean height above sea level. However these benchmarks are becoming harder to find as buildings are demolished or replaced and ground movement in some parts of the country have had the result of making them unreliable. New technology allows us to use Networked Real Time Kinematic systems as described in the RICS virtually level guide to establish accurate level data without the need for the old benchmark system.

At GM Surveys we recommend that all control points are related to National Grid using GPS equipment. Through experience we have often arrived on site ready to set out a new development only to find that all of the control points from the original land survey have been destroyed. This can potentially cause a major delay right at the start of the works. But if the same control points are related to National Grid they can be swiftly re-established using our Trimble VRS system.

However care must be taken when the design of a site is based on GPS co-ordinates or National Grid. Traditionally most small surveys would be based on a ‘flat earth’ grid system. However on sites over one kilometre in length it becomes necessary to apply a scale factor to take into account the curvature of the earth. The RICS map projection scale factor guide gives a brief guide on the effects of scale factor and when it should be used.

Even on smaller sites GPS equipment can still be used by placing a control station approximately in the middle of the proposed development then traversing around the perimeter of the site using a conventional total station. The site is then said to be ‘centred’ on the original control point and by performing a site calibration using the software onboard the field computer all subsequent GPS data is transformed to local grid co-ordinates.

The recent development of networked GPS correction services means that co-ordinates can now be achieved in real time thereby saving on the lengthy and costly computations required with previous equipment. Instead of spending several hours collecting data then returning to the office to post process the information we can now obtain accurate co-ordinates within minutes of setting up on site. A good insight on the benefits and pitfalls of networked GPS is explained in the RICS virtually right –networked GPS guide. Further Information can also be obtained from the TSA best practice guide to networked RTK.

Using our state of the art total stations, precise traversing equipment and GPS equipment we can ensure that all the control on your site is accurate and easily updated should the machine drivers get a little over zealous.