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

RICS Draft Guidance Note: Earth observation and aerial surveys, 6th edition

Appendix C: Glossary

Accuracy

The degree to which a set of independent measurements are free from defects, consistent with a standard rule.

Aerial triangulation

The process by which stereo photography is tied together to form stereo models and geo-referenced to ground control points.

Active sensor

An instrument that both emits and receives reflected energy from the terrain and the responses recorded at the on-board instrument. LiDAR is an active sensor.

Aerial photography

Photographs taken from an aerial vantage point.

Ambiguity

The unknown integer number of carrier phase cycles in an unbroken set of global navigation satellite system (GNSS) measurements.

Angular misalignment

The misalignment between the survey instrument reference frame and the aircraft reference frame.

Base line

The 3D vector distance between the GNSS receiver on a moving aerial platform and a ground GNSS base station.

Civil Aviation Authority (CAA)

The government organisation responsible for all aspects of aviation in the UK.

Calibration

The procedure by which aerial survey instruments are checked and adjusted to ensure consistency of the resulting measurements.

Charged-coupled device (CCD)

An electronic chip employed in digital cameras to measure light intensity.

Circular error (CE)

A term commonly used to represent the horizontal accuracy of earth observation imagery. A value of CE (90) 5m represents a circular error where a minimum of 90% of the points measured have a horizontal error of less than 5m.

Coordinate reference system

A mathematical definition of a coordinate system, including the origin, scale, position and orientation of the reference ellipsoid.

Cross strip

A strip of aerial photography or LiDAR data flown at 90 to the main block to tie blocks of flight lines together improving block geometry.

Cycle slip

The loss of lock of the satellite signal by a GNSS receiver.

Dense image matching

A photogrammetric technique that enables the extraction of 3D surfaces from images acquired from multiple views.

European Union Aviation Safety Agency (EASA)

The European authority in aviation safety.

Elevation

The height above a defined level datum, e.g. mean sea level.

Elevation mask

The lowest elevation in degrees above the horizon at which a GNSS receiver is set to track a satellite.

Ellipsoid

In geodesy, a mathematical figure formed by revolving an ellipse about its minor axis to describe the shape of the earth. Used interchangeably with spheroid.

Ellipsoidal height

The height between a point on the ellipsoid and the topographic surface.

Ephemeris

A set of data that describes the position of a celestial object as a function of time.

Epoch

A point in time that is the reference for a set of coordinates.

Focal length

The distance between the centre of a lens and its focal point.

Forward motion compensation (FMC)

A technique used to compensate for the forward motion of an aircraft during the capture of an aerial image.

Geo-referencing

Referencing an image, point cloud, vector data or other entity by its geographical coordinates.

Geodetic datum

A precise mathematical model designed to best fit part or all of the geoid.

Geoid

The equipotential surface that most closely approximates to mean sea level. This surface is everywhere perpendicular to the force of gravity.

Geoidal separation

Difference in height between the ellipsoid and the geoid or mean sea level.

Global navigation satellite system (GNSS)

Encompasses all satellite systems that are used for navigation purposes including GPS, GLONASS, Galileo and BeiDou.

Ground resolved distance (GRD)

The minimum detectable distance between two small features on the ground.

Ground sampled distance (GSD)

The distance between the centres of two consecutive pixels on the ground. GSD is a common way to define the resolution of earth observation and aerial imagery.

Gyro-stabilised mount

An aerial camera mount that is held constantly in a horizontal position during flight using a gyroscope and an inertial measurement unit (IMU).

Hyperspectral imagery

An imaging technique operating across the visible and non-visible parts of the spectrum, typically generating 500 separate spectral bands for in-depth analysis

Inertial measurement unit (IMU)

An electronic device capable of measuring the gravitational and acceleration forces on a moving vehicle and reporting its velocity and position.

Kinematic survey

A dynamic method of GNSS positioning using carrier phase observations in which one receiver is moving (typically on an aerial platform) and one or more base station receivers are stationary.

LAS file format

A common LASer point cloud file format specified by the American Society for Photogrammetry and Remote Sensing (ASPRS). LAZ is the compressed version of the same format.

Linear error

A term commonly used to represent the vertical accuracy of earth observation imagery. A value of LE (90) 5m represents a circular error where a minimum of 90% of the points measured have a vertical error of less than 5m.

Metadata

A set of data that describes and stores information about other data.

Metric camera

A camera for which the focal length and radial and tangential distortions are known and calibrated regularly.

Mosaic

An assembly of digital images that have been carefully cut and joined to produce a composite image of an area of terrain larger than could be covered in a single aerial photograph at the same scale.

Multispectral imagery

Imagery captured using sensors operating outside the visible part of the electromagnetic spectrum.

Nadir

The point on the earth directly below the observer

Oblique photography

Aerial photography taken from an off-nadir position

Orthometric height

Height between a point on the geoid and the topographic surface, also known as mean sea level.

Orthophotography

The use of digital rectification and digital elevation models to produce an image that has a consistent scale.

Passive sensor

An instrument that receives and records reflected energy from the sun. A hyperspectral instrument is a passive sensor.

Panchromatic imagery

Images created using an imaging sensor that is sensitive to all radiation in the visible region of the spectrum.

Permission for commercial operations (PfCO)

The legal document needed to operate a drone commercially in UK airspace.

Position dilution of precision (PDOP)

A unitless scalar value expressing the relationship between the error in user position and the error in satellite position.

Photogrammetry

The science of making accurate measurements from digital imagery normally for the measurement of an object, mapping, or geographic information system (GIS) data collection.

Photogrammetric six degrees of freedom

The six values required to position a single aerial image in space - easting, northing and height, (real world coordinates) and omega, phi and kappa (rotations around the X, Y and Z axes).

Precision

The degree to which a set of independent measurements are exact and accurate.

Precise dilution of precision (PDOP)

A computed unitless scalar value which describes the geometric contribution to the uncertainty of a GNSS position solution.

Principal point

The position on the focal plane of a theoretically perfect camera where a perpendicular line passed through the perspective centre.

Point cloud

A 3D visualisation constructed from millions of geo-referenced points.

Point density

A measure of LiDAR resolution, usually expressed in points per square metre (ppm2).

Points of detail

Points that can be identified in both the terrain and in aerial imagery for the purpose of geo-referencing the imagery or for the determination of geometric accuracy.

Polar orbit

A satellite orbit that passes within 20o of both poles.

Pre-marked points

Points of detail that are established before the imagery is captured.

Push broom scanner

A scanning action that uses a fixed linear array of detectors located at the focal plane to build up an image line by line along the direction of flight of the aerial platform. These passive sensors acquire multiple strips of images simultaneously (forward, nadir and backward) as opposed to a series of separate exposures. Stereo imagery is collected in-track (or from the same orbit) and is derived from viewing the forward, nadir, and backward imagery in combination. A push broom scanner is also known as an along track scanner.

Radiometric

The measurement of radiant energy across the whole electromagnetic spectrum.

Relief displacement

The shift in an object's image position caused by its elevation above a particular datum. For vertical or near vertical photography, the shift occurs radially from the nadir point.

Resolution

A measure of the level of detail that can be detected.

Receiver Independent EXchange format (RINEX)

A common GNSS data file format.

Root mean square error (RMSE)

The standard deviation of the predicted errors.

Spectral signature

The variation in the electromagnetic reflectance from a homogeneous target.

Stereoscopic photography

Pairs of photographs that can give a visual impression of depth, or a 3D representation.

Sun-synchronous orbit

A satellite orbit in which the satellite passes overhead at the same local time every day, tracking the sun.

Short wave infrared (SWIR)

Part of the electromagnetic spectrum.

Tasked imagery

Earth observation imagery that is specifically targeted for the client by the satellite imagery providers to meet the client's specific requirements. The alternative is to purchase existing imagery inventory of the same area that has been routinely captured.

Tidal window

A period when the coastal tides are at their lowest, usually twice a day

Thermal imagery

Imagery created from detecting and recording the infrared part of the electromagnetic spectrum.

Unmanned aerial system (UAS)

This includes the ground control segment, the unmanned aerial vehicle (UAV) and all equipment, network, and personnel necessary to control the UAV.

Unmanned aerial vehicle (UAV)

Also known as a drone.

Virtual reference station (VRS)

A specialised processing technique that generates a virtual base station for a GNSS survey from a network of other fixed real base stations.

Whisk broom scanner

A scanning action that uses a rotating mirror to scan across the satellites track or orbit reflecting the energy emitted from the earth's surface onto a single detector. The imagery is built up one pixel at a time. Whisk broom scanners are also known as cross track scanners.

Witness diagram

A diagram used to locate a point in the terrain such as a ground control point.

World Geodetic System (1984) (WGS 84)

World Geodetic System (1984). The geocentric datum used by GNSS since January 19847. It has its own reference ellipsoid.

Acronyms

AGL

Above ground level

AOI

Area of interest

AQL

Acceptable quality limit

ASPRS

American Society for Photogrammetry and Remote Sensing

ATC

Air traffic control

BVLOS

Beyond visual line of sight

CIR

Colour infrared photography, includes the near infra-red band

DSM

Digital surface model

DTM

Digital terrain model

ETRS89

European terrestrial reference system 1989.

FOV

LiDAR field of view, usually expressed as an angle

GCA

Ground control areas

GCP

Ground control points

HAPS

High altitude pseudo satellite

InSAR

Interferometric synthetic aperture radar

ISO

International Organization for Standardization

NIR

Near infrared (part of the electromagnetic spectrum)

PRF

Pulse repetition frequency (also known as pulse repetition rate, PRR)

RADAR

Radio detection and ranging

RGB

Red, green, blue or visible imagery

SfM

Structure from motion

SLAM

Simultaneous localisation and mapping

UTC

Coordinated universal time

VNIR

Visible near infrared (part of the electromagnetic spectrum)

VTOL

Vertical take-off and Landing