Ensuring the accuracy, usefulness and orientation of your building and site survey data is essential for a solid foundation
Something my uncle always drilled into me from an early age was: 'If you get it right first time, you'll get it right sooner.' Over time I have come to realise that getting it right in one area can also reap benefits across other areas that depend upon the integrity of the first. In a practical, construction sense there is no better example than the foundations to a structure; get them wrong and the whole building may suffer as a result.
The foundation of any design (the part upon which all decisions are based) is the building and site survey.
Indeed, in the spirit of the 'train hard, fight easy' approach to lowering barriers, I am a great advocate of the motto: 'Survey accurately, design with certainty.' But how can you be sure that your survey is accurate and, moreover, how do you want to receive the survey data so that you can make the best use of it?
From experience on a recent project, I outline below some 'must-have' requirements for any survey tender document. Many may seem obvious, but often it is the obvious things that are overlooked in the search for complex answers to simple problems.
What needs surveying?
The first thing you must decide is what you want surveyed. Naturally, the site and any existing buildings need to be picked up, but the detail required in measuring the existing buildings will depend on what you intend to do with them. If you just require some typical plans and elevations with a couple of cross-sections, a standard two-dimensional survey will be the quickest and cheapest solution. However, if you need greater detail and accuracy from your survey, with detailed sections at close and regular intervals, you may wish to consider a three-dimensional laser survey.
The 3D laser survey will cost substantially more than its regular 2D sibling, due in part to the time taken to collate and interpret the data, and in part to the fact that there are fewer survey companies that have the technology. The benefits of the 3D survey are clear: when it is set up correctly, the entire building can be measured to an accuracy of less than 10mm.
The system works by firing lasers at the fabric of the building and plotting spheres about the size of a large marble in 3D space. Accuracy is determined by the number and spacing of the lasers, and the number and placement of spheres at the point of impact.
Once the survey is complete the 3D model (looking like the pingpong ball equivalent of a matchstick model) can be 'sliced' at any point to give an accurate representation of a plan or section.Unfortunately, that is not the end of it.
From the slice taken, somebody then has to perform the CAD version of dot-to-dot to link the centres of each sphere with a series of lines to define the section profile. This is not only enormously time consuming but the profile path is also open to interpretation by the dot-joiner.
Receiving the data
Having decided on the survey method, the next step is to inform your surveyor of the desired presentation format and orientation. Most survey companies will present the survey data on an OS grid but, if requested, will re-orient the data to fit a site or building grid. You should also give thought to the elevations and sections being drawn in their 'true' location so that snapping to any point will reveal its X, Y and Z coordinates, giving an accurate floorlevel indicator which can make life much easier later on when you want to add notation to a drawing.
It is also prudent to request that the survey information be returned to you in a number of formats. Naturally AutoCAD DWG is the most widely used CAD-file format and, as such, you will automatically be in line to receive your survey data in that format. But most surveyors will happily provide you with the data in other file formats to suit your needs.
So if you are using ArchiCAD or MicroStation, add the proprietary file format of choice to your tender specification and make sure that you request the correct units of measurement too, otherwise you are likely to receive the data in metres as a default.
You should also consider the structural makeup of the CAD data.
If you have a layer standard, pass it on to the surveyor so that they can draw to your standard. Furthermore, if the area to be surveyed includes many buildings or a single large building with obvious zones, it would be wise to subdivide the CAD data into different model files, each representing a floor level, elevation view or section cut for each different zone. This will make it easier to develop your design for each part without having the constant overhead of large quantities of data on the screen.
Finally, be sure to select a surveyor who has a history of producing accurate and reliable data. If the survey is poor and your design relies upon tight tolerances for prefabricated elements inside the existing building, a surveyor that will go the extra yard to provide more reliable data will give you greater certainty of fit with the design.
Money spent on the foundation of the design is money invested in the design.
Joe Croser can be contacted via email at joe@croser. net