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

RICS Draft Guidance Note: Earth observation and aerial surveys, 6th edition

3 Project planning

All aerial survey projects have some key items in common regardless of location, extent, data capture platform, type of data, ground control, accuracy or resolution. These items should be addressed systematically during the project planning stage.

In the fast-moving area of UAV surveys, the International Organization for Standardization (ISO) have developed standards covering the general specification, product systems and operational procedures.

3.1 Area of interest

The starting point for any survey should always be a geo-referenced digital file supplied by the client showing their area of interest (AOI). In many cases, this is accompanied by a specification document providing details of the target imagery GSD, accuracy requirements and the deliverable products to be created.

However, the contractor can sometimes simply be supplied with a geo-referenced AOI file and a description of the potential use case for the survey data. An experienced contractor should be able to advise on the specification of the data, the data formats, a schedule of data deliverables and the best sensor to capture the geospatial data to meet the client requirements.

3.2 Project start and end date

Every aerial survey project has a start date and an end date. There are a number of factors that influence the rate at which an aerial survey project proceeds, including:

  • the prevailing weather conditions
  • time of year (see section 3.3.3) and
  • the limitations associated with working in congested airspace.

It is important that the client and contractor work closely to define the project start and end dates, taking these factors into consideration. The need for adherence to the contract requirements and a completion date must be tempered by the need to produce an acceptable product.

3.3 Project constraints

The project constraints listed in the following subsections should be identified and addressed at the earliest opportunity.

3.3.1 Military or civil security clearances

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.

3.3.2 Air traffic control requirements

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.

3.3.3 Time of year

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.

3.3.4 Tidal constraints

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.

3.3.5 Health and safety/environmental requirements

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.

3.3.6 Special limitations

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.

3.4 Ground control requirements

Independent ground control points are required to support the aerial triangulation process and verify the accuracy of the final product. To be effective, these ground control points should be captured to a standard of accuracy that is three times higher than that required by the final mapping product using professional land survey techniques. Capturing the ground control points to a higher standard of accuracy enables a buffer for additional random error propagation that may occur during the triangulation and product creation process.

If very high accuracies are required that are beyond the technical limits of a global navigation satellite system (GNSS), total station and precision levelling techniques may be required. This is most commonly the case with UAV surveys on discrete sites, such as in the rail environment.

The ground control points should be specified in the coordinate system for the final imagery product output. The contractor should be able to advise on the number of verification points to be surveyed to provide adequate evidence to support the final accuracy claim.

3.4.1 Control points for imagery surveys

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.

3.4.2 Control points for LiDAR surveys

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.

3.5 Project reports

The contractor should submit a brief progress report at regular intervals on the acquisition of the data and the production of the derivative products.

This should detail:

  • each flight sortie
  • location
  • ground control locations and
  • verification of the accuracy of the survey. This is best achieved by observing additional ground control points to act as checkpoints during the verification process (see section 3.4).

Clients may also request that they are informed of progress at significant milestones, for example, immediately prior to the commencement of data acquisition, on completion of data acquisition and during the data production and deliverable stages.

The time between the data acquisition and quality acceptance by the client of the derived products should be kept to a minimum to reduce the risks involved with additional rework.

3.6 Form of contract

Where the form of contract is not specified by the client, it is recommended that this guidance note is used along with the project information requirements set out in section 1 of the current edition of Measured surveys of land, buildings and utilities, RICS guidance note. Appendix A contains sample specifications for aerial photography, LiDAR, thermal and earth observation data.

3.7 Data ownership and copyright

The client should decide whether they wish to retain the ownership and copyright of the captured data and the derived products. The options for data ownership and copyright are as follows.

  • The data and products become the property of the client upon completion and final payment of the contract.
  • Under an agreement with the client, the contractor has the right to resell the data under a separate agreement for royalty payments.
  • The data and products remain the property and copyright of the contractor.

4 Aerial photography

Aerial survey cameras come in two forms:

  • frame cameras, where the imagery is captured frame by frame in a traditional sense and
  • push broom scanners (also known as along track scanners), where the image is built up line by line.

Frame cameras are the most widespread cameras used for aerial surveys.

The term 'metric' is used to identify a camera that has been specifically made or modified for photogrammetry. They are factory-calibrated, and the calibration parameters are used to remove distortions in the assembled digital images. This is particularly important during the data processing stages where the successful removal of image distortions will have a positive effect on the subsequent 3D photogrammetric data products. The use of a metric camera designed for photogrammetric data capture should increase the utility of the data captured.

UAVs typically utilise non-metric cameras. It is therefore important that the surveyor is sufficiently competent to correctly calibrate the camera themselves to avoid adversely affecting the achievable accuracy of a project.

4.1 Key considerations

4.1.1 Footprint area

It is recommended that the camera CCD array/lens combination is selected to minimise the number of images to cover the contract area

The size of a digital camera's charged coupled device (CCD) determines the footprint area covered by a single frame on the ground. All other factors being the same - such as the lens and the flying altitude - the larger the CCD array, the larger the camera footprint coverage. Normally, digital frame cameras use multiple CCDs in their construction and the images from the individual CCDs are 'stitched' together by processing software to form a larger image or footprint on the ground. A larger camera footprint requires fewer images to cover the contract area, improving the efficiency of both the data capture and subsequent data processing. Medium or small format cameras use a reduced number of CCDs, making them impractical for large survey tasks due to their smaller footprint and therefore the greater number of strips of photography required to be flown for the same area.

4.1.2 Focal length

A typical multi-purpose lens will have a focal length of up to 210mm. A shorter focal length of 80mm will cover a wider area and produce height measurements that are more precise. A longer focal length of 210mm will improve the GSD of the imagery (and reduce relief displacement in the imagery) from the same flying height. For this reason, longer focal lengths are advantageous for the capture of data in urban areas.

In the case of UAVs, a shorter focal length of between 20mm and 80mm is generally recommended because of the smaller sensor sizes available on drones compared to manned aircraft. Typically, full frame complementary metal-oxide-semiconductor (CMOS) sensors are 25 to 35mm across. However, medium format sensors are now small and light enough to be carried on a UAV. These larger focal length sensors have greater image footprints at a given GSD - leading to more efficient data capturing - and enjoy improved low-light performance.

4.1.3 GSD

The GSD of the imagery is determined by:

  • the choice of camera lens focal length
  • the resolution of the camera sensor and
  • the altitude at which the survey is flown.

The relationship between focal length, GSD, flying height and coverage is explored in Table 2.

Focal length (mm)






GSD (m)






Flying height (m)






Cross-track coverage (m)






Along-track coverage (m)






Footprint (km2)






Table 2: Relationship between focal length, GSD, flying height and coverage

Halving the GSD and maintaining the focal length will reduce the flying height and consequently the footprint of a single image by a factor of four. This in turn will increase the number of flight lines and time in the air.

Increasing the focal length will enable the GSD and image coverage to be maintained while flying at a higher height, improving data capture efficiency.

Long focal length lenses are suited for data capture at higher altitudes, with maximum efficiency. Shorter focal lengths will provide wider coverage from the same altitude, but not necessarily an improved GSD.

It is normal for imagery output to be quoted as a GSD rather than the traditional photographic scale.

4.1.4 Restriction of image movement

Restricting the amount of image movement during exposure and keeping the camera as level as possible during random instances of air turbulence can significantly improve the image quality, particularly at large scales. The image movement is determined by:

  • the speed of the aircraft
  • the camera exposure time and
  • the GSD of the imagery.

Image movement should not exceed 25µm over three or more consecutive exposures. This requires the use of a camera with:

  • forward motion compensation (FMC) and
  • a gyro-stabilised mount during turbulent conditions.

FMC, offered by most current air survey cameras, is a digital compensation technique that reduces the amount of the image movement due to the speed of the aircraft over the ground in the direction of flight.

Gyro-stabilised camera mounts are designed to reduce camera tilts and sudden rotations along three axes and are frequently controlled using GNSS and IMU data. Maintaining the camera in a flat and level position less than 2 from the horizontal is desirable. An occasional exposure with up to 4 may be permitted, provided the minimum forward and lateral overlaps are maintained (see section 4.2.2). Rotation in the z-axis (known as crab) should not exceed 5 as measured between the direction of flight and a line parallel to the image frame.

The contractor should select the equipment necessary to provide the product quality required under the flying conditions at the time of photography.

4.1.5 Directly geo-referencing aerial imagery

Using GNSS coupled with an IMU during the capture of vertical and oblique aerial imagery significantly increases the utility of that imagery. GNSS and IMU systems enable significant savings to be made in the amount of ground control and aerial triangulation effort required to provide accurate positioning of imagery both for subsequent mapping projects (see section 4.4.6) and to produce orthophotography (see section 4.4.4).

These systems usually require access to GNSS base station data, which can either be captured specifically for the project or data that comes from a continuously operating network. However, companies specialising in navigation are now developing solutions to remove errors resulting from orbital and atmospheric delays without the need for base stations.3

Evidence should be provided by the contractor that the GNSS and associated IMU and camera are calibrated on a test area at regular intervals. This is particularly true where the camera components have been removed and reinstalled on the aerial platform. The calibration procedure calculates the angular misalignments, which are then applied to the data when producing positioning and exterior orientation files.

The collection and supply of GNSS/IMU data is considered a standard practice. The camera positions can be outputted in the coordinate system of the client's choice.

4.1.6 Calibration

Each camera lens unit to be used on the contract should be calibrated, cleaned, tested and certified by the camera manufacturer or by a calibration centre that is recognised internationally or approved by the camera manufacturer. This should be carried out less than two years prior to the date of the photography. The two-year period is in recognition of the fact that modern lenses remain stable and that the cost of an annual recalibration in countries without local laboratories may be considered excessive.

The calibration certificate should contain the following information:

  • name and address of the calibration centre and name of authorised signatory
  • date of calibration
  • camera manufacturer's serial number of the lens unit
  • calibrated focal length of the lens unit in accordance with the manufacturer's recommendations and
  • radial and tangential distortion parameters in micrometres. The measured distortion should fall within the limit defined by the manufacturer for the lens type.

If the contractor becomes aware of anything that may affect the calibration of the camera, the client should be informed immediately.

Digital cameras are calibrated and 'electronically adjusted' by the manufacturer to bring the system to a 'zero' state or within the manufactured tolerances as stated on the calibration certificate. The calibration data file produced during the calibration process is used in the first stages of image data processing. This data file would not normally be provided to the client unless the supply of raw data is specified as part of the contract.

Consumer-grade cameras should be calibrated as above where possible. However, camera self-calibration methods do exist, particularly for short focal length configurations, based on the analysis of imagery captured coincidently with the project data. Frequently, structure from motion (SfM) algorithms favoured by cameras carried by UAV platforms incorporate some elements of camera calibration.

4.2 Flying and coverage

4.2.1 Flight lines

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.

4.2.2 Overlap

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).

4.2.3 Acceptable quality limits

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.

4.3 Aerial photography accuracy and resolution table

The relationship between aerial photography scale, GSD, mapping scale and potential photogrammetric accuracies is well known. Table 3 has been reproduced from the previous edition of this guidance note.

Photo scale


Mapping scale

Horizontal RMSE
























Table 3: Relationship between photo scale, GSD, mapping scale and potential photogrammetric mapping accuracy

The following table is an expanded version of Table 3 that includes high-altitude fixed wing aerial photography, photography captured from a helicopter and photography captured from UAVs.

It is worth noting that Table 4 is based on high-end equipment on the market today and that many factors may affect accuracy and resolution. Therefore, the values quoted can only be referenced as achievable.


Height AGL

Achievable accuracy (m) (RMSE figures, at 1 sigma)

Achievable resolution - GSD (m)



Plan X,Y

Height Z




+/- 0.01

+/- 0.015





+/- 0.04

+/- 0.06





+/- 0.05

+/- 0.03





+/- 0.10

+/- 0.05


Fixed wing



+/- 0.10

+/- 0.05


Fixed wing



+/- 0.19

+/- 0.09


Fixed wing



+/- 0.38

+/- 0.19


Fixed wing



+/- 0.63

+/- 0.31


Table 4: Achievable accuracy and resolution values for aerial photography

The UAV flying altitude of 400ft represents the highest altitude at which a UAV can be operated in the UK without the approval of an operational safety case.

For comparison purposes, Table 4 is based on a camera focal length of 120mm for the fixed wing aircraft, an 80mm lens on a helicopter-based medium format camera system and a 35mm UAV camera lens. The miniaturisation of camera technologies has meant that medium format cameras (with a focal length of around 80mm) are now small and light enough to be carried on UAV platforms.

Table 4 shows that mapping accuracies of 2.5 to 3x the GSD can be achieved horizontally and 1.25 to 2x vertically. This range in values is a reflection on the quality of the camera and navigation equipment that can currently by carried by each platform.

When using dense image matching photogrammetric techniques, a well-known rule of thumb for calculating likely achievable relative accuracies is:

Plan = 1 to 2x GSD

Height = 2 to 3x GSD

Similarly, the absolute accuracy follows a similar pattern in which horizontal accuracy is better than vertical accuracy (in contrast to the pattern shown in Table 4). Dense image matching processing is particularly popular with UAV workflows as it is more forgiving on external camera geometry.4

The accuracy achieved at an altitude of 1,200m is roughly equivalent to band G in the survey detail accuracy band table given in section 2.3 of the current edition of the RICS guidance note Measured surveys of land buildings and utilities. Altitudes of 2,250 and 4,500m are roughly equivalent to bands H and I.

4.4 Digital imagery deliverables and products

4.4.1 Digital imagery

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.

4.4.2 Metadata

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:

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.

4.4.3 Stereo imagery

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.

4.4.4 Orthophotos

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.

4.4.5 Digital terrain/surface models

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.

4.4.6 Mapping

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.