These may be required in order to fly over, photograph or process data of sensitive locations, or in order to operate in particular countries. In some cases, this may require an in-air observer in the aircraft during the data capture. The data captured may also need to be processed in the host country.

Some element of air traffic control is required to operate fixed wing aircraft, helicopters and UAVs in congested airspace. Permissions, which are time sensitive, may need to be applied for in advance. Permissions are more difficult to obtain around major airports or over military training areas.

Flying can take place all year round. However, it is recommended to specify that the angle of the sun is at least 15 above the horizon, which, in the northern hemisphere, generally coincides with a flying season of between April and October. A sun angle of >15 is high enough to provide optimal lighting conditions; lower sun angles may result in deep shadows, particularly in urban areas or where there are considerable differences in the height of the terrain. Some clients may also specify that flying should take place during 'leaf-off' conditions, i.e. when there is minimal growth on trees, to provide better visibility of the ground beneath. The timing of this period is variable and should be subject to agreement between the client and the contractor.

Choosing the day(s) on which to fly is crucial in determining the quality of the final image. A balance should be met between flying in suboptimal conditions, risking the client rejecting the photography, and waiting too long for better conditions, lengthening the project acquisition period.

For coastal projects in which it is beneficial for the maximum amount of intertidal area to be exposed during the data capture period, the tide times may be a project constraint. Clients may specify set periods of time before and after low tide during when data can be captured; this is known as the tidal window.

Some clients may require risk assessments to be prepared, for example for low-level flights in sensitive or congested areas, or for flights taking place during unsociable hours.

There may be flying limitations or restrictions relating to observance of religious days or for security reasons.

UAVs tend to operate at lower altitudes, and it is advisable to consider the potential for public interest in the flight scheduling. For example, if operating in the vicinity of a school, it is prudent to inform the school of the planned survey and perhaps even schedule the project for outside of school hours. See Drones: applications and compliance for surveyors, RICS insight paper, for more information.

For all types of imagery, ground control points can either be pre-marked or captured on points of detail after the flying mission is complete.

The advantage of pre-marked points is that there will be no ambiguity in their position when they are used in the aerial triangulation process. The disadvantage of this approach is that they may have been removed before the flight mission takes place. This approach also tends to be more expensive.

Points of detail may be harder to locate or observed in error within the aerial survey data, but they can be captured in the photography soon after the mission is complete, ensuring that up-to-date points are used. Using points of detail is the most common approach.

For full ground control, on the rare occasions where direct geo-referencing of the imagery is not used, points should be positioned in the overlaps between images, every five images along the strip, depending on the terrain. Control should be placed in this manner every third strip within the block. With the use of global positioning system (GPS) and inertial measurement unit (IMU) data, the amount of control can be reduced.

It is good practice to observe two or three points in each ground control location to act as checkpoint for the verification survey or if one point becomes unsuitable to be observed. Each point should be accompanied with a detailed witness and location diagram, including a photograph of the site to aid in the identification of the point on the imagery.

See the current edition of Guidelines for the use of GNSS in surveying and mapping, RICS guidance note, for more information.

For LiDAR surveys, there are two types of control points:

• ground control areas (GCAs): a grid of 121 points to validate the height of the captured data and
• ground control points (GCPs), which are captured on points of detail to validate the plan position.

GCAs and GCPs should ideally be captured in the same locality. The number of points required is dependent on the size of the AOI and the flight line configuration. Ideally, they should be no more than 10-15km apart and located under crossing flight lines if possible.

GCAs do not necessarily need to be completely flat; a slope of <10 is acceptable. They should be established on hard, smooth surfaces within the survey area, away from aerial obstructions such as tall buildings or trees. If there is space, a 5x5m grid should be established with a point roughly every 0.5m, giving a total of 121 points. If this is not possible, an appropriate shape that captures 121 height points should be chosen.

GCPs should be captured on points of detail - on hard surfaces within the project area away from overhead obstructions - that can be located within the LiDAR AOI. Suitable points are along the tops of kerb lines and road markings. 20 points per location are enough for this purpose.

Nadir (near-vertical) photography should be flown in approximately straight and level runs (strips) to achieve full stereoscopic coverage. There is little cost saving to be obtained by reducing the overlaps between exposures to a minimum. Full stereoscopic coverage will maximise opportunities for future use of the data.

Oblique photography should also be flown in approximately straight and level runs to achieve full coverage of the contract area. Modern aerial camera equipment can capture nadir photography simultaneously with oblique imagery. Commonly, four oblique cameras are employed, two along the flight line and two additional cameras either side, inclined at 45. Care should be taken where a nadir camera is flown simultaneously with oblique cameras to ensure that the resulting coverage is compatible.

Contractors commonly prepare flight lines that are aligned either east-west or north-south, unless the client has a specific request for the orientation of the photography, or a more suitable flight line orientation is dictated by:

• the terrain
• air traffic control restrictions
• the avoidance of secure sites or
• the capture of tidal areas.

Coverage of corridor features such as for transport infrastructure can be achieved with the use of additional flight lines to capture the bends in the most economical manner.

Oblique projects are also frequently planned either in a north/south or an east/west direction, rather than 'off-grid'. This means that every spot has coverage from the north-, south-, east- and west-facing cameras, with the camera direction and compass points being maintained.

Once the minimum number of runs required to cover the target area has been calculated, taking account of the terrain, the spacing between flight lines may be adjusted to increase and equalise the lateral overlap between each run.

There should be no duplicate run/strip or frame numbers. The sequence should be maintained even if the target area is subsequently flown in several missions.

Flight plans may be reused where there is a requirement for an ongoing repeat or monitoring survey.

Forward overlap is required to ensure adequate formation of stereoscopic coverage. Lateral overlap (sidelap) is required only to ensure that no areas are missed between adjacent flying strips, and to tie adjacent strips together.

The forward overlap between successive exposures in each run is usually between 60% and 80%. The sidelap between adjacent strips is normally between 15% and 40% for flying heights greater than 1,500m AGL, increasing to between 20% and 40% for lower flying heights. An allowance of ±5% of the selected overlap is permissible.

In urban areas with high-rise buildings, a forward overlap of 80% and a sidelap of 30% is normally used. By increasing the coverage, each tall building will appear in more frames, thus increasing the choice of images that show the building to appear vertical. Exceptionally, the sidelap may be increased to 60% (along with a forward overlap of 80%) by clients who intend to use the imagery to produce 3D city models.

In coastal areas where a run crosses the shoreline, the forward overlap can be increased to 90%. The increase in overlap should include at least three photo centres over land.

In mountainous areas, where it is impossible to maintain the usual sidelap requirements, short infill runs should be flown, parallel to and between the main runs, to fill the gaps. Where ground heights within the area of overlap vary by more than 10% of the flying height, a reasonable variation in the stated overlaps is permitted, provided the forward overlap does not fall below the selected percentage and the sidelap does not fall below 10% or exceed 45%.

In oblique imagery, overlaps are not as crucial as stereo imagery is not expected. However, oblique imagery is still planned with some overlaps to ensure full coverage over the contract area.

Push broom imagery sensors acquire multiple strips of images simultaneously (forward, nadir and backward) as opposed to operating via a series of separate exposures. Stereo viewing is derived from the fixed geometry of the sensor. Scanner-based imagery must therefore be flown in a continuous swathe with a minimum of 20% sidelap (25% in elevated or urban areas).

The following list is intended to act as a set of acceptable quality limits (AQLs) to provide guidance on the subjective topic of image quality. The prevailing weather and atmospheric conditions, which are outside of the control of the contractor, are the most important factors that affect the image quality, and therefore the AQLs. The client and contractor should work closely together to ensure a mutually acceptable result. These guidance notes apply equally to nadir imagery and oblique imagery.

• The photography should be taken at any solar altitude above 15, unless special restrictions are included.
• The imagery should be sharp.
• There should be minimal flare from expanses of glass, water or cars.
• Colour and light balance should be uniform.
• Contrast should be consistent across the block of imagery.
• There should be a good match between flight runs and adjacent images.
• The photography should only be flown in conditions where the visibility does not significantly impair the image quality and detail is not lost because of rain, atmospheric haze, dust, smoke or any other conditions detrimental to the photographic image.
• The photography should be substantially free of cloud, dense shadow, or smoke. Isolated areas of cloud, dense shadow or smoke should not be cause for rejection of the photography, provided the intended use is not impaired. Typical tolerances for cloud and cloud shadows may be less than 5% for a single image and 1% over a contiguous block of images.
• The photography should conform to any specific radiometric values specified by the client, including:
• mean histogram luminosity values
• mean of the individual colour bands or
• standard deviation for each colour band.

The contractor may supply all stereo digital imagery upon request, which enables the client to take advantage of any future improvements in image processing techniques.

The client may specify the image format, image compression and data transfer medium. Stereo imagery data can require large volumes of space, so clients are likely to specify a format that can be easily incorporated into their archive system.

It is rare that stereo imagery is supplied without the accompanying positional information, derived from a GNSS/IMU navigation system. As a minimum, for each image, this should consist of:

• a unique run and frame number
• easting, northing and height position and the three photogrammetric camera rotations - omega, phi and kappa - in the client's choice of coordinate system.

The accompanying positional information is frequently used to georeference the imagery to meet the client's requirements. This should be simply supplied in a text file or database format.

The data should be accompanied by metadata that describes the instruments used to capture the data and any processing applied. The following international standards are relevant to documenting metadata for imagery:

• ISO 19115-1:2014, Geographic information - Metadata - Part 1: Fundamentals
• ISO 19115-2:2019, Geographic information - Metadata - Part 2: Extensions for acquisitions and processing and
• ISO/TS 19115-3:2016, Geographic information - Metadata - Part 3: XML schema implementation for fundamental concepts.

Metadata should be made available to client organisations for onward supply to their customers. It may be specified for digital imagery or for any other digital imagery products described in the following sections.

Metadata standards are normally specified by the client organisation. Positional data derived from a GNSS/IMU navigation system is a good example of metadata.

Other examples of metadata are:

• date and time flown
• GPS time
• geographic reference
• flying height
• coordinate system
• resolution
• file size
• camera system
• oblique image orientation and
• date of production.

Digital stereo imagery is a raw product used to produce more complex geospatial data. This can in turn provide insight into the business proposition for which the imagery was commissioned. For example, supplying imagery with geo-referenced coordinates enables basic plan measurements to be taken and exploited for numerous applications.

Aerial photography can be used to form stereoscopic models, which are created using a mathematical model to replicate the geometry of the image at the time it was captured. This produces a precise geographical location for each individual image at the time of exposure. The position of each image is described using three coordinates (easting, northing, height) and three rotations (omega, phi, and kappa) around the three principal camera axes.

Aerial triangulation can then be used to view the stereoscopic models in 3D. This is a technique in which the imagery is tied together, geo-referenced and verified against independently captured ground control points, within a rigorous least squares adjustment model. The result is a 3D coordinate for every pixel in the image, enabling 3D measurements to be taken from the imagery.

The completion of a rigorous aerial triangulation process significantly increases the utility of the imagery. It provides the basis to:

• create orthophotography (see section 4.4.4)
• generate digital terrain models (DTMs) and digital surface models (DSMs) (see section 4.4.5)
• extract 3D mapping data (see section 4.4.6) and
• create additional specialist products such as:
• 3D building modelling
• tree mapping and
• line of sight analysis.

There are other less rigorous methods of achieving the same result, such as SfM and simultaneous localisation and mapping. These methods are favoured by aerial survey methods that use very small cameras on smaller aerial platforms such as UAVs.

Orthophotos are true-to-scale 2D images. A DTM is used to remove the effects of the aerial camera geometry, tips or tilts in the imagery and the effects of relief displacement. They can be created in any common imagery format and in any specified coordinate system. The measurement of a distance in the image, such as a road width, will be replicated in the terrain, making them an indispensable tool for a wide range of applications.

A terrain model is a digital representation of the ground surface defined by 3D points. Terrain models can be automatically generated from stereo photography. The modelling software takes advantage of the mathematical model established during the aerial triangulation process to calculate a 3D coordinate for each pixel in the stereo imagery.

There are two types of terrain model:

• DTMs, for all points located on the ground and under trees and bridges and
• DSMs, which include all surface features such as the tops of tree canopies and buildings.

Terrain models are used for wide-ranging applications such as:

• 3D city modelling
• 3D visualisation
• forest management
• road, rail, and energy sector engineering
• the management of flood risk and
• preventing coastal erosion.

Mapping is a traditional application of aerial imagery. Stereoscopic models have a long history of being used to update vector databases for national mapping companies and for engineering applications, among others. Where the third dimension is not required, orthophotos have been successfully employed for map update tasks and land use classification.