RICS Draft Global Guidance Note: Earth observation and aerial surveys, 6th edition

RICS Draft Guidance Note: Earth observation and aerial surveys, 6th edition

2 Data capture platform

2.1 Data type

The starting point when commissioning earth observation or aerial survey data is to consider what the data will be used for. In many cases, this will be self-evident, for example:

  • national mapping
  • heritage recording
  • forestry
  • land classification
  • rail transportation or
  • precision agriculture.

The use case for the aerial survey will determine the type of data that will be most appropriate:

  • Aerial photography is the most common type of data as it aligns most closely with the client's view of reality, with features such as trees and buildings instantly recognisable, albeit from a different perspective. The science of photogrammetry was built around the use of stereo aerial photography from which precise measurements can be made, increasing the utility of aerial photography as an aerial survey technique. Today, dense image matching photogrammetric techniques are also used, particularly using photography captured from UAV platforms.
  • LiDAR is a good source of height information and has found numerous applications in modelling the built and natural environments.
  • Multispectral, hyperspectral and thermal data, sensing outside of the visible part of the electromagnetic spectrum, are particularly useful in detecting the health of different plant species.

It is frequently the case that more than one data type is commissioned to meet a particular use case. For example, aerial photography and LiDAR are frequently commissioned together, with the photography providing a view of reality and the LiDAR data the third dimension. Another common combination is aerial photography and hyperspectral imagery, with the latter used to accurately determine plant species.

2.2 Accuracy and resolution

The accuracy of an aerial survey is traditionally expressed by specifying an absolute root mean square error (RMSE) for the positioning of the survey data. Ground checkpoints should be used as an independent check on the mapping accuracy (see section 3.4).

The resolution of aerial photography and multispectral, thermal and hyperspectral imagery is expressed as a ground sampled distance (GSD). The resolution of a LiDAR survey is expressed as the number of points recorded per square metre on the ground, or points per square metre (ppm2).

Higher resolution imagery enables more detail to be observed within the scene and thus finer detail to be mapped. GSD is not synonymous with accuracy, but an aerial survey cannot be more accurate than the GSD.

It is important to specify realistic accuracy and resolution requirements that are appropriate for the use case of the aerial survey. This can directly influence the cost of a project. To achieve both a highly accurate and high-resolution dataset:

  • specialist (and therefore more expensive) sensors and workflows should be employed
  • data should be captured at as low an altitude as possible, which increases the number of flight lines, time in the air and therefore cost and
  • a coordinated ground control survey is required, further increasing costs.

Over-specifying a survey can bring about undue cost. For instance, there would be little advantage in specifying high accuracies for a land cover classification of a river basin.

Appendix B contains a combined accuracy and resolution table for each data capture platform.

2.3 Project extent

The project extent will influence the choice of data capture platform. The basic premise is that large project areas are captured more efficiently at higher altitudes and at higher data capture speeds:

  • Satellite-based sensors are best for capturing areas of hundreds of thousands of square kilometres.
  • Fixed wing aircraft can efficiently capture projects in the tens of thousands of square kilometres. They can carry larger and more complex instrumentation, such as a large format camera and a gyro-stabilised mount, so they can therefore achieve better system accuracy and resolution relative to the flight altitude.
  • Helicopters can fly lower and slower, are much more manoeuvrable than fixed wing aircraft and are more efficient at capturing aerial survey data along corridors such as roads or railway lines. The lower altitudes at which they can fly enable them to capture higher resolution and potentially more accurate data, even considering the limitations of their payloads.
  • UAVs that are part of an unmanned aerial system (UAS) can carry various aerial sensors and are suited to sites of a relatively small extent in the tens of square kilometres. The recent miniaturisation of LiDAR instruments and cameras has enabled very high-resolution surveys in the order of <10mm GSD to be conducted economically over smaller sites owing to the platform's proximity to the ground, which - under standard permissions - is <400ft above ground level (AGL).

2.4 Selecting a data capture platform

The use case, project extent and data type have the biggest impact on the accuracy, resolution and data capture platform for an aerial survey. More than one platform and/or data type can be used for different use cases.

Table 1 is an example of matching data types and data capture platforms in six common use cases. In practice, there are many more successful combinations.

Use case

Project extent

Data type

Accuracy

Resolution

Data capture platform

National mapping

Large

Aerial photography

High

Medium

Fixed wing

Heritage recording

Small

Aerial photography

High

High

UAV

Forestry

Medium

LiDAR

Medium

Medium

Fixed wing

Land classification

Large

Multispectral imagery

Low

Low

Satellite

Rail transportation

Medium

LiDAR

High

High

Helicopter

Precision agriculture

Small

Multispectral imagery

High

High

UAV

Table 1: The relationship between use case, project extent, data type, accuracy, resolution, and data capture platform

2.5 Data capture platform restrictions

Each data capture platform has its own restrictions.

Satellites are on fixed, sun-synchronous orbits passing over the equator at the same time every day, which may or may not suit the project's requirements.

Fixed wing, helicopter and UAV operations are governed by two global bodies: the International Air Transport Association (IATA) and the International Civil Aviation Organization (ICAO). The European Union Aviation Safety Agency (EASA) governs operations within the EU. Practitioners should consult the national aviation organisations of their individual country for specific operational details.

For example, the Civil Aviation Authority (CAA) is the statutory body that oversees and regulates all aspects of civil aviation in the UK. Fixed wing aircraft are restricted to a height of 10,000ft in the UK, above which an aircraft with a pressurised cabin or oxygen for the flight crew can be used. Below this altitude, fixed wing aircraft must comply with restrictions on flight zones set by the CAA, which are more prevalent and restrictive around dense urban areas and sensitive sites such as prisons or airports. UAV pilots are required to hold permissions for commercial operations (PfCO) accreditation, for which mandatory training is required. The drone operators are obliged to write an operations manual that must be approved by the CAA.

Under standard permissions from the CAA, UAVs are restricted to operating outside of controlled airspace - unless they attain permission from the relevant authority - maintaining:

  • a distance of 50m from people, vehicles, vessels and structures not under the control of the pilot
  • a distance of 150m from crowds of 1,000 people and
  • a visual line of sight at all times.

They are also restricted to a height of <400ft AGL. To operate outside of these standard permissions and to operate in congested, restricted or sensitive areas, UAV operators need to have their operational safety case approved by the CAA and possibly third parties with a legitimate interest in the activity of the flight. Clients are advised to review the operator's paperwork before commissioning a survey to check the permissions they hold. See Drones: applications and compliance for surveyors, RICS insight paper, for more information.

In the EU, EASA registration of UAV operators became mandatory on 1 July 2020. All drone operators in applicable jurisdictions operating a drone with a weight of more than 250g are required to register, as well as any drone less than 250g that is not a toy and is equipped with a sensor able to capture personal data. The registration number is required to be displayed on the drone. The training requirements and distances to which pilots can operate are set to change significantly to standardise the legislation across the EU.