Posted 3 years ago
A Guide to Topographical Surveys
Our guide to topographical surveys is intended to give readers a broad introduction to the field and explain when, why and how a topographical survey might be undertaken. As far as is practicable, we have removed jargon to explain the process in simple terms. As such, this guide will prove useful to the layperson as well as developers, planners, property professionals, engineers, architects and surveyors etc.
As well as explaining what topographical surveys are, the guide will touch on the process for engaging an experienced land surveyor to conduct your survey, including what to look out for and essential information to include in your brief for the best possible result.
What is a topographical survey?
A topographical survey is a detailed examination of a specific area of land, mapped to include the features - man made and naturally occurring - of the area under scrutiny. Topographic surveys are generally commissioned for planning or posterity; they provide a record of what existed in a particular place at a particular moment in time.
Topographic surveys may also be referred to as topo surveys, topographic surveys and, quite often, land surveys. Whatever term is used, the process and end result is usually the same.
That said, however, there is a marked difference between individual topographical surveys. Every survey will be subtly different depending on the level of detail required by a client or the particular reason for commissioning the survey. For example, a 3D mapped survey will demand a higher level of detail than a survey to ascertain the boundary of a property. Topographical surveys are, therefore, unique to the project to which they relate and cannot necessarily be used as multipurpose documents.
Why is it needed?
The resulting drawing is presented digitally or on paper, depending upon a client’s requirements. These highly-detailed plans, previously hand-drawn but now created using sophisticated 3D CAD software, are used for many different purposes, some of the most common being mentioned here:
- To identify and locate specific features of an area
- To create a 3D model of the survey area
- To describe the existing layout of an area with its defining features
- Construction project layout
- Infrastructure project layout
- To identify boundaries and neighbouring property details to support disputes/planning applications
- To record ‘real’ or legal property boundaries
- As an aid in the sale or procurement of land
- Mapping an area in preparation for alterations or improvements
- For the purpose of providing an historical record
- To pinpoint the location of manmade or natural features in the survey area, e.g. trees, rivers
How is it done?
The type of equipment used for a topographical survey depends very much on the scope of the job and the preferences of the individual surveyor. Location, terrain and environmental conditions must also be taken into account when selecting the most suitable tools to achieve the required level of accuracy.
For instance, if the survey is to take place on an area of wide open space, with little in the way of large obstacles and a decent GPS signal then a single GPS receiver with RTK (a type of satellite navigation) and might be used with the option of switching to a radio signal if GPS is lost or interrupted. In built-up areas, such as towns or housing estates, it is likely that a GPS system may be less suitable. In such cases the surveyor may opt for a total station instead. Whilst a somewhat simplified version of events, when using a total station measurements can be taken by aiming the instrument either at a prism pole held by an assistant or, if working alone or seeking to measure certain more inaccessible features, at a particular object.
Using either a GPS or total station allows all the data to be recorded and stored electronically. The coded data is then downloaded and processed on a computer to produce the final drawing / report.
Whilst this is an incredibly quick and efficient way of producing topographical surveys, sometimes more basic, traditional survey instruments are required. These could include an optical (or ‘dumpy’) level or even a tape measure. Again, this depends almost entirely on the survey scope, the level of accuracy required and the environment (and any restrictions it might impose).
Topographical surveys: key considerations
When undertaking a topographical survey, a surveyor takes into account several different criteria to ensure a client receives exactly the data that is required. However, while certain factors may vary from project to project, the following considerations define all surveys:
- The survey area
- The level of detail needed
- The level of accuracy needed
- Grid layout and reference points
- Future surveys
1. The survey area
There are several things to consider in addition to the precise area of land to be surveyed. Access to the site, the location of naturally-occurring and manmade features as well as features just outside the boundary of the survey site; all of these should be considered before commencing a survey as failing to may adversely affect the results.
The clearer a client can be about the area to be surveyed, along with its environs, the better the resulting topographic survey. To assist, there are several online mapping websites that clients can use to create a plan, simply search “online mapping tool”.
2. The level of detail needed
This will affect both the time needed to do the survey and, as a result, the overall cost of the project. Therefore, this is an extremely important thing to get right in advance, especially if the project is time critical or you are working to a budget.
Your surveyor may be able to give you some guidance as to the level of detail needed but it is useful to understand how it can affect the outcome of the survey.
For a flat, fairly featureless stretch of land, it is quite acceptable to space the survey points further apart, whilst the opposite is true for undulating or feature-packed areas.
The further apart the points, the quicker it will be to survey the area and, in theory, it will cost less too; but, remember, spacing will vary from area to area and survey to survey. It is easy to see that a survey with points spaced 20m apart will be significantly less detailed and less costly than a survey of 5m spacing.
Unless a client orders a highly-detailed survey, items of street furniture such as manhole covers, streetlights and hydrants, may end up being represented on the final drawings as a generic symbol only. Therefore, if you need a precise description of these items on the survey you should make this abundantly clear in the specification. It is crucial that you are clear on the level of detail needed for your survey as it could make the difference between certain key features being shown or completely ignored on the final drawings. Any survey company is completely within their rights to refuse a repeat visit to site (at their own cost) to pick up information/features that were not originally specified.
Always consider the implications a miscalculation like this could have on the overall project before you set the level of detail required. The advice is; don’t scrimp or you could end up having to commission extra surveys further down the line.
3. The level of accuracy needed
The level of detail needed for a survey will, naturally, dictate the applications for which the survey can be used. Survey software, such as CAD, generates beautifully formatted, scaled drawings but the actual level of accuracy depends on the client’s exact requirements for the topographical survey.
Industry guidelines state that topographical reports should be accurate to within 0.3mm RMSE (root-mean-square error), that is to say that the fine detail on the drawing will be within this level of tolerance.
Tolerances are checked for accuracy against the nearest permanent survey control station or PID (Permanent Identifier). These are permanent points on the earth’s surface listed for reference in a world-wide database.
4. Grid layout and reference points
Most surveys can be conducted using a scale factor of 1 - where horizontal ground and map distances are the same - with a northern alignment. It is often advisable to relate the survey grid to the Ordnance Survey (OS) National Grid but this is only really essential for very large scale surveys incorporating OS surveyed data.
Survey levels (vertical measurements), however, are best related to the Ordnance Datum Newlyn (ODN) where measurements are taken against Mean Sea Level (MSL); for mainland UK the benchmark is Newlyn in Cornwall. This can be done by using either Ordnance Survey markers or GPS as a point of reference but it’s worth noting that OS no longer actively maintain survey markers so if the topographical survey data is heavily reliant on precise levels, GPS should be used instead.
Some surveys require a custom level as a starting point. If this is the case a surveyor should ensure there is a temporary benchmark (a permanent feature such as a tree marked with a nail or a wooden post driven into the ground) at the site that can be easily located should future survey work need to be undertaken.
5. Future surveys
Some projects may involve future survey work so it’s useful to have an idea beforehand whether or not your surveyor might be returning to site at some point in the future. If this is a possibility then it’s useful to set-up some permanent survey markers on and around the site, allowing a surveyor to pick-up where he left off but, more importantly, to maintain accurate reference points so that future surveys which can be linked back to the original.
Topographical surveys: preparing a specification for tendering
As with any large-scale undertaking it is best practise to obtain several quotations before selecting the most appropriate surveyor for your topographical survey. To ensure like-for-like quotations are received and so that a fair comparison can take place you should put together a specification of the work needed. Your specification - your expectations of exactly what you want to receive - will form an integral part of the brief you present to a surveyor for a quotation.
This should be as comprehensive as possible, even if your knowledge is limited. As a starting point, you should include all of the factors discussed above, making sure to add in anything which makes this particular survey unique.
If you intend to use an older or previously-used specification, make sure it is still fit for purpose. If in doubt, a competent, experienced and responsible surveyor will be able to provide advice and guidance on what should be included in the specification.
Next, create an overarching brief for the survey. As a bare minimum, it should include the following:
- The purpose of the survey
- A plan outlining the area of land to be surveyed
- The specification (as discussed above)
- Deliverables - how the survey will be presented
- How the survey will be delivered (i.e. one-off job, or in phases)
- Practicalities, including access and security
Choosing the right surveyor
Whilst it is useful to have a specification and brief for a project, you should always discuss your requirements with a highly experienced, responsible surveyor at the earliest possible opportunity. As professionals and experts, they have a wealth of knowledge that can help you hone your brief to perfection.
Having done it many times before, they should have a good idea of what can and can’t be done. They have a better idea of timescales and may also highlight restrictions or criteria you hadn’t considered.
In short, a good surveyor will guide you in your brief. For many clients, the inclination is to over-specify, requesting things that simply aren’t needed. Input from an experienced surveyor will not only result in a better project brief but it could also lead to time and cost savings for the client.
Why use City Surveys?
One of the largest, multi-disciplinary survey firms in the UK, City Surveys have an enviable reputation for delivering precision surveys on time and on budget. What's more, we add real, measurable value to our clients' projects, helping them understand their sites and the data they generate.
Want to know how The City Surveys Group could add value to your project? Here are just a few pointers...
- Nationwide coverage - One of the largest firms of our type. we operate throughout the UK delivering measurement and engineering services across a range of sectors.
- Proven track record - Every year we deliver hundreds of projects on time and on budget to a loyal client base. Over 70% of our work is based upon repeat business.
- We save you money - By working harder, longer and more intelligently, over the years we’ve actually saved our clients millions in unnecessary costs and delays.
- Honest, transparent pricing - The price we quote is the price you pay. If your brief is accurate and the scope remains the same there will be no additional costs whatsoever.
- Training and technology - We’re constantly investing in the latest technology, training, software and systems. Our equipment and people are at the top of their game.
- Fast, efficient service - Our business is based around service quality, flexibility and speed. We work with you to deliver the results you need, when you need them.
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