Topographical land surveys provide an accurate measured plan of a site and can contain a whole range of different feature information. They are typically used as a base model for design work before a construction project starts. Traditionally the data for these surveys has been collected using land surveyors with GNSS equipment measuring every 3D point individually. Times are changing and UAV s are now an invaluable data capture tool for the survey industry.
Millions of Data Points in a Single Survey
Masses of data points can be captured in one short flight; such a quantity of data by far exceeds that which can be captured by traditional methods. Surveys which would normally take several days for a land surveyor to complete can now be completed in just a few hours, offering major cost savings to clients.
The methods used to collect UAV survey data, mean it is possible to generate highly detailed plans. Furthermore the nature of the survey means a wealth of additional data can be captured at the same time e.g. photo mapping, essentially adding more value to the survey process.
Real World to Desktop
What a UAV survey is able to do is bring the real world to the desk top by gathering a hugely comprehensive data set which can then be manipulated in multiple ways back in the office. With traditional land survey methods it has not been cost or time effective to capture such quantities of data. Using a UAV allows time to be focused on using and analysing data, rather than on gathering it.
Below ground features such as drains and man holes can be included in UAV survey and the technology continues to advance with LiDAR data collection also now possible enabling terrain data to be captured under vegetation too. Accuracy levels vary (as explained below) but it is possible to provide survey data to accuracies better than 10mm if required. On a like for like basis of 40mm accuracy which is similar to a land survey the UAV has proved to be a considerably cheaper method of data collection.
Data from a UAV survey can be supplied in a wide variety of formats since in essence the UAV is merely a means of carrying the data gathering sensors. This means topographical survey data can be supplied for immediate use in AutoCAD, Bentley and GIS formats including .DWG, .DGN and .SHP. as well as a number of other formats. Data can be provided in nearly all co-ordinate reference systems, the most common being British National Grid and Snake grid.
From a cost perspective every job has its varying complexities and requirements however it is safe to say, that, since the data can be collected much more quickly from a UAV versus a land survey, it will always be more cost effective. For example a client requiring a pre-project pipeline survey which would have taken 6 weeks to complete on foot has been completed in 3 days not only saving cost but adding to the overall project efficiency. There are regular examples of a weeks’ worth of data being captured in just a day’s surveying by UAV. In a test scenario comparable topographic survey data which took 4 days for a traditional survey was matched by a UAV in just 1 hour.
Added Value Data
Unlike traditional land surveying techniques, UAV topographical surveys also provide the following data at no extra cost:
- Photo map of survey area
- Geotagged photography of site
- RGB Point cloud
- Digital elevation model
The sheer amount of useable data a UAV can collect sets it apart from traditional surveying techniques. Notably a UAV also requires far fewer people not only to carry out the survey but also to process the data yielding another cost saving.
Land surveyors can face difficult or dangerous areas to survey or those where access is limited. With a UAV not only can they access this type of area, less time is spent on the ground thus improving staff safety by minimising risk to surveying teams especially when measuring difficult sites such as quarries, landslides and rail routes.
A UAV can overcome access problems and deliver data safely although there are locations such as heavily congested areas where extra permissions sometimes need to be sought when operating e.g. in central London. The cost of the survey can also increase if it is in a complex area.
A traditional survey may not be deterred by the weather but for UAVs this may prohibit a survey on a particular day but taking account of weather forecasts is part and parcel of planning a UAV survey. They are generally scheduled when the conditions are likely to be favourable. Since the data is gathered quickly even on a generally poor day, windows of opportunity usually open up allowing the data to be gathered.
Clients who haven't used a UAV before are often concerned about the level of accuracy that can be achieved. When considering the relative accuracies of different types of survey methods it is important to note that there is no such thing as the perfect GIS data.
Accuracy is generally defined as the degree or closeness to which the information the map created from the data captured matches the actual real world. To be clear, accuracy is the proximity of measurement results to the true value as opposed to precision which is the repeatability, or reproducibility of the measurement. Necessarily then, when accuracy is being discussed the conversation will be about the quality of data and about number of errors contained in a particular dataset.
So whatever the source of the data there can and there will be a level of inaccuracy, which raises the question of relativity, in so much as, is data gathered using a UAV more or less accurate than other methods?
If it is at least as good as other sources then it is valid to look at what other benefits can be garnered by collecting the data in this way e.g. it is quicker, safer, more cost effective etc. Of course, fundamentally, without the accuracy of the data the other benefits will be muted.
Not all UAV Surveys are the same
Essentially a UAV is a means of carrying a camera over the area to be surveyed so the quality of the camera used will be a significant influence on the outcome. With digital camera technology approaches differ by manufacturer, such that not all cameras perform the same, as they have different characteristics and are targeted at slightly different capture and output scenarios. It is very important to consider which camera is best to fulfil a specific survey task based on the platform in which it is mounted, the pixel resolution of the camera and the final output applications which are required from the collected and processed data.
The smaller the pixel size and lower flying height the more accurate the final product will be. Since UAVs are able to fly at low altitudes the level of accuracy is therefore necessarily very high. Indeed there have been a number of trials comparing UAV survey methods with traditional routes and the conclusion is always the same; there is an enormous time saving (and concurrent cost saving) on data collection without an appreciable loss in accuracy when using a UAV.
In short, UAVs can offer the most spatially accurate aerial survey data currently available.
Since accuracy is determined in part by pixel resolution & quality, the results are highly dependent on the quality of camera and lens used. Where high quality equipment is used the error of extracted survey points can get down to around 9mm in height and 6mm in plan with good ground control and specialised post-processing techniques. However for most UAV land surveys comparable accuracies to a traditional survey are used which is generally around 40mm in height and 30mm in plan; essentially this is the most cost effective solution. It is important to understand what accuracies are required and the tolerances needed to achieve it as the costs vary with the level of accuracy required.
Photorealistic 3D Modelling
Indeed, research shows that UAV photogrammetry is as accurate as RTK GPS, whilst being much faster and providing a fuller representation of geography. The findings show that UAV aerial photogrammetry offers far richer data than conventional survey vector data consisting of points, text and lines. In terms of representing the landscape, the UAV sourced orthophoto can be combined with a DEM to produce very accurate photorealistic 3D modelling.
Measurements taken from the UAV survey data can get down to sub-millimetre for distances and similar scales for volumes. Given the high levels of accuracy that can be achieved from a UAV survey it is important not to over engineer the problem in order to obtain the most cost effective solution.
The real benefits of UAV surveys are not that they can replicate or indeed exceed the results from conventional methods but in the amount of data that they can acquire in significantly reduced survey time.