Glossary of Terms

Term
Description
ASCII
A 7-bit wide serial code describing numbers, upper and lower case characters, special and non-printing characters. Typically used for textual data
Acquisition
The process of locking onto a satellite's C/A code and P code. A receiver acquires all available satellites when it is first powered up, then acquires additional satellites as they become available and continues tracking them until they become unavailable
Almanac

A set of orbit parameters that allows calculation of approximate GPS satellite positions and velocities. The almanac is used by a GPS receiver to determine satellite visibility and as an aid during acquisition of GPS satellite signals.

Anti Spoofing

Denial of the P-code by the Control Segment is called Anti-Spoofing. It is normally replaced by encrypted Y-code.

Azimuth

The horizontal direction of a celestial point from a terrestrial point, expressed as the angular distance from 000° (reference) clockwise through 360°. The reference point is generally True North, but may be Magnetic North, or Relative (ship's head).

Aerial Photograph
The grouping together of a selected=‘selected‘ set of like entities to form one entity. For example, grouping sets of adjacent area units to form larger units, often as part of a spatial unit hierarchy such as wards grouped into districts. Any attribute data is also grouped or is summarised to give statistics for the new spatial unit.
Alphanumeric
Information using both letters and numbers.
Analogue
Continuously variable signals or data.
Application Service Provider
A company that offers individuals or enterprises access over the Internet to application programmes (e.g. GIS) and related services that would otherwise have to be located in their own personal or enterprise computers. This may also include access to relevant related data.

B

Term
Description
Bearing

The horizontal direction of one terrestrial point from another terrestrial point, expressed as the angular distance from a reference direction, usually measured from 000° at the reference direction clockwise through 360°. The reference point may be True North, Magnetic North, or Relative (ship's head).

Base Mapping
Usually associated with topographic mapping covering country or region at different scales.
Basic Scale
The scale at which the survey is undertaken. For Ordnance Survey mapping, three scales (1:1250 - urban, 1:2500 - urban and rural, 1:10 000 - mountain and moorland) are used.
Baud
Bits per second. A measurement used in data transfers via telephone lines.
Bearing
Bearings are angles measured against the National Grid in degrees. They are measured clockwise from grid north.
Bench Mark
A survey point on a fixed object, the altitude of which has been surveyed in relation to a Datum or the applicable local datum, which in turn is related to mean sea level.
Bitmap
A digitised image that is mapped into a grid of pixels. The colour of each pixel is defined by a specific number of bits.
Browser
An application which gives the user the ability to view a graphic representation of mapping data. The application would provide tools (e.g. pan, zoom) to aid this viewing. It provides a visual representation of the mapping data, which may displayed at a variety of resolutions dependent on the size of area being displayed.

C

Term
Description
CAD
See Computer Aided Design.
Cadastral Survey
A survey of the boundaries of land parcels.
Cadastre
A public register of land recording the extent and value of land parcels for the purposes A dataset containing information related to land ownership and rights. This usually takes the form of maps and descriptions of uniquely identifiable land parcels.
Calibration
The act or process of comparing certain specific measurements in an instrument with a standard.
Cartesian Coordinates
Numbers expressing the location of a point in two or three dimensions as the perpendicular distances from two or three orthogonal axes.
Cartography
The organisation and communication of geographically related information in either graphic or digital form. It can include all stages from data acquisition to presentation and use.
Carrier

The steady transmitted RF signal whose amplitude, frequency, or phase may be modulated to carry information.

Carrier Phase Ambiguity

The number of integer carrier phase cycles between the user and the satellite at the start of tracking. (Sometimes ambiguity for short)

Carrier Phase Measurements

These are "accumulated doppler range" (ADR) measurements. They contain
Measurements the instantaneous phase of the signal (modulo 1 cycle) plus some arbitrary number of integer cycles. Once the receiver is tracking the satellite, the integer number of cycles correctly accumulates the change in range seen by the receiver. When a "lock break" occurs, this accumulated value can jump an arbitrary integer number of cycles (this is called a cycle slip).

Course Acquisition C/A Code

A pseudorandom string of bits that is used primarily by commercial GPS receivers to determine the range to the transmitting GPS satellite. The 1023 chip C/A code repeats every 1 ms giving a code chip length of 300 m which, is very easy to lock onto.

Coordinated Universal Time (UTC)

This time system uses the second-defined true angular rotation of the Earth measured as if the Earth rotated about its Conventional Terrestrial Pole. However, UTC is adjusted only in increments of one second. The time zone of UTC is that of Greenwich Mean Time (GMT).

Cycle Slip

When the carrier phase measurement jumps by an arbitrary number of integer cycles. It is generally caused by a break in the signal tracking due to shading or some similar occurrence.

Coordinate Transformation
The computational process of converting an image or map from one co-ordinate system to another. Is also known as a transformation.
Coordinates
Pairs of numbers expressing horizontal distances along orthogonal axes, or triplets of numbers measuring horizontal and vertical distances.

D

Term
Description
Differential GPS (DGPS)

A technique to improve GPS accuracy that uses pseudorange errors at a known location to improve the measurements made by other GPS receivers within the same general geographic area.

Dillution of Precision (DOP)

A numerical value expressing the confidence factor of the position solution based on current satellite geometry. The lower the value, the greater the confidence in the solution. DOP can be expressed in the following forms.

 

GDOP

-uncertainty of all parameters (latitude, longitude, height, clock offset)

 

PDOP

-uncertainty of 3D parameters (latitude, longitude, height)

 

HTDOP

-uncertainty of 2D and time parameters (latitude, longitude, time)

 

HDOP

-uncertainty of 2D parameters (latitude, longitude)

 

VDOP

-uncertainty of height parameter

 

TDOP

-uncertainty of clock offset parameter
Doppler

The change in frequency of sound, light or other wave caused by movement of its source relative to the observer.

Double-Difference

A mathematical technique comparing observations by differencing between receiver channels and then between the reference and rover receivers.

Double-Difference Carrier Phase Ambiguity
Carrier phase ambiguities which are differenced between receiver channels and between the reference and rover receivers. They are estimated when a double-difference mechanism is used for carrier phase positioning. (Sometimes double-difference ambiguity or ambiguity, for short)
Digital Terrain Model
A 3D representation of the Earth's surface. It's construction includes a height model (i.e. a DEM) and overlaid with map data relating to features on the surface (i.e. Map Data or Aerial Photograph).

E

Term
Description
Eart-Centred-Earth Fixed (ECEF)

This is a coordinate-ordinate system which has the X-coordinate in the earth's equatorial plane pointing to the Greenwich prime meridian, the Z-axis pointing to the north pole, and the Y-axis in the equatorial plane 90° from the X-axis with an orientation which forms a right-handed XYZ system.

Ellipsoid
A smooth mathematical surface which represents the earth's shape and very closely approximates the geoid. It is used as a reference surface for geodetic surveys
Ellipsoid Height
Height above a defined ellipsoid approximating the surface of the earth
Ephemeris

A set of satellite orbit parameters that are used by a GPS receiver to calculate precise GPS satellite positions and velocities. The ephemeris is used in the determination of the navigation solution and is updated periodically by the satellite to maintain the accuracy of GPS receivers.

Epoch
Strictly a specific point in time. Typically when an observation is made

F

Term
Description
Fixed Ambguity Estimates
Carrier phase ambiguity estimates which are set to a given number and held constant. Usually they are set to integers or values derived from linear combinations of integers
Floating Ambiguity Estimates
Ambiguity estimates which are not held to a constant value, but are allowed to gradually converge to the correct solution.
File Transfer Protocol (FTP)
A protocol which allows a user on one computer to transfer files to and from another computer over a TCP/IP network (e.g. Internet). This will be the method by which OS MasterMap data is made available for online supply.
Firewall
Used as a security measure to protect intranets from the traffic that passes in and out of them. A combination of hardware and software that manages authorisation of uses trying to access the intranet. The firewall bars unauthorised data packets from entering or leaving a network. Firewall software specifies which data packets are authorised. It typically resides on routers or dedicated servers.
FTP
See File Transfer Protocol.

G

Term
Description
Geoid

The shape of the earth if it were considered as a sea level surface extended continuously through the continents. The geoid is an equipotential surface coincident with mean sea level to which at every point the plumb line (direction in which gravity acts) is perpendicular. The geoid, affected by local gravity disturbances, has an irregular shape.

Geostationary

A satellite orbit along the equator that results in a constant fixed position over a particular reference point on the earth's surface. (GPS satellites are not geostationary.)

Global Positioning System (GPS)
Full name is NAVSTAR Global Positioning System. A space-based radio Positioning system which provides suitably equipped users with accurate position, velocity and time data. GPS provides this data free of direct user charge worldwide, continuously, and under all weather conditions. The GPS constellation consists of 24 orbiting satellites, four equally spaced around each of six different orbital planes. The system is being developed by the Department of Defence under U.S. Air Force management
Glonass
GLObal NAvigation Satellite System: A system of satellites operated by the Russian government.
Geographic Information
Information about objects or phenomena that are associated with a location relative to the surface of the earth. A special case of spatial information.
Geographical Information System
A system for capturing, storing, checking, integrating, analysing and displaying data that is spatially referenced to the earth. This is normally considered to involve a spatially referenced computer database and appropriate applications software.
Geography Mark-up Language
The subject of a recommendation paper from OpenGIS, which describes a markup language based on XML, used to store and transfer geographic data over the Internet (specifically geographic data which conforms to the OpenGIS simple features specification).
Geospatial Data
Another term used to describe Map Data but commonly isn't directly associated with a map e.g. An address has a spatial reference associated with it but not in map coordinates.
GI
See Geographic Information.
GIS
See Geographical Information System.
Grid
The planimetric frame of reference e.g. The National Grid.
Grid Goordinates
A plane-rectangular coordinate system based on and mathematically adjusted to a map projection in order that geographic positions (latitudes and longitudes) may be readily transformed into plane coordinates and the computations relating to them made by the ordinary methods of plane surveying.
Ground Height
The height where the building wall intersects the ground.
Ground Station
A facility capable of receiving signals from earth observation satellites such as LANDSAT, SPOT, ERS, JERS AND MOS.
GUI
Graphical User Interface.

H

Term
Description
HTML
See HyperText Markup Language.
HTTP
See HyperText Transfer Protocol.
Hydrographic Surveying
The measurement and description of the physical features offshore and adjoining coastal areas with special reference to their use for the purpose of navigation.
Hypertext Link
These allow the user to jump from one page/Web site/document to another by generating a browser request simply by clicking on the link. These links are often depicted in blue, underlined text.
HyperText Markup Language
An open format for sharing electronic text documents with hypertext extensions via the Internet/Intranet.
HyperText Transfer Protocol (HTTP)
The simple request/response protocol that runs on top of TCP/IP that allows Web browsers to access files on any Web server.

I

Term
Description
Integer Ambiguity Estimates
Carrier phase ambiguity estimates which are only allowed to take on integer values
Iono free Carrier Phase Observations
A linear combination of L1 and L2 carrier phase measurements which provides an estimate of the carrier phase observation on one frequency with the effects of the ionosphere removed. It provides a different ambiguity value (non-integer) than a simple measurement on that frequency
Internet Service Provider (ISP)
Organisations that offer commercial connection to the Internet network.
Interpolation
The estimation of values of a surface at an unsampled point based on the known values of surrounding points.
ISDN
Integrated Services Digital Network.
ISO
International Standards Organisation.

K

Term
Description
Kinematic
The user's GPS antenna is moving. In GPS, this term is typically used with precise carrier phase positioning, and the term dynamic is used with pseudorange positioning

L

Term
Description
L1 Frequency

The 1575.42 MHz GPS carrier frequency which contains the course acquisition (C/A) code, as well as encrypted P-code, and navigation messages used by commercial GPS receivers.

L2 Frequency
The 1227.60 MHz. secondary GPS carrier frequency, containing only encrypted P-code, used primarily to calculate signal delays caused by the ionosphere
Land Terrier

The range of radio frequencies that includes the GPS carrier frequencies L1 and L2 and the OmniSTAR satellite broadcast signal.

.
L Band
The term used to refer to a map or map data that concentrates on small areas. It is called large scale because the features that appear on the map will look larger than those at a small scale. For example a building may appear as its outline (or footprint).
Latitude
The angular distance north or south between a point on the earth's surface and the equator. The distance is measured with reference to an idealised spheroid shape of the earth.
Low Latency Solution
A position solution which is based on a prediction. A model (based on previous reference station observations) is used to estimate what the observations will be at a given time epoch. These estimated reference station observations are combined with actual measurements taken at the remote station to provide a position solution
Longitude
The angular distance of a point east or west of an arbitrarily defined meridian, usually taken to be the Greenwich meridian. The distance is measured with reference to an idealised shape of the earth.

M

Term
Description
Map Data
Digital data that has a spatial component. Typically these are digital maps but can also include data that has some form of spatial attribute that can be linked to a real world location - i.e an address.
Mask Angle

The minimum GPS satellite elevation angle permitted by a particular receiver design. Satellites below this angle will not be used in position solution.

Metadata
Graphical or textual information about the content, quality, condition, origins and characteristics of data.

N

Term
Description
National GPS/GNSS Network
The infrastructure of active and passive GPS/GNSS reference stations which allow surveyors to determine precise coordinates in GPS and National Grid spatial reference systems.
Northings
Rectangular Coordinates.

O

Term
Description
Observation
Any measurement. The two observations used in NovAtel's RTK algorithms are the pseudorange measurement and the carrier phase measurement.
Observation Set

A set of GPS measurements taken at a given time which includes one time for all measurements, and the following for each satellite tracked: PRN number, pseudorange or carrier phase or both, lock time count, signal strength, and tracking status. Either L1 only or L1 and L2 measurements are included in the set. The observation set is assumed to contain information indicating how many satellites it contains and which ones have L1-only and which ones have L1/L2 pairs.

Orientation
Orientation of a point or a text feature measured in degrees anticlockwise from grid east.
Origin
The zero point in a system of rectangular Cartesian coordinates.
Orthogonal
At right angles to each other.

P

Term
Description
Parrellel Receiver
A receiver that monitors four or more satellites simultaneously with independent channels.
Parity
The even or odd quality of the number of ones or zeroes in a binary code. Parity is often used to determine the integrity of data especially after transmission
P-Code
Precise code or protected code. A pseudorandom string of bits that is used by GPS receivers to determine the range to the transmitting GPS satellite. P-code is replaced by an encrypted Y-code when Anti-Spoofing is active. Y-code is intended to be available only to authorized (primarily military) users
PDOP
Position Dilution of Precision
Precise Positioning Service (PPS)

The GPS positioning, velocity, and time service which is available on a continuous, worldwide basis to users authorized by the U.S. Department of Defence (typically using P-Code).

PRN Number

A number assigned by the GPS system designers to a given set of pseudorandom codes. Typically, a particular satellite will keep its PRN (and hence its code assignment) indefinitely, or at least for a long period of time. It is commonly used as a way to label a particular satellite..

Pseudolite
An Earth-based transmitter designed to mimic a satellite. May be used to transmit differential corrections.
Pseudorange
The calculated range from the GPS receiver to the satellite determined by taking the difference between the measured satellite transmit time and the receiver time of measurement, and multiplying by the speed of light. Contains several sources of error
Pseudorange Measurements

Measurements made using one of the pseudorandom codes on the GPS signals. They provide an unambiguous measure of the range to the satellite including the effect of the satellite and user clock biases.

.
Photogrammetry
The science, art and technology of obtaining reliable measurements and maps from aerial photographs.
Point
A zero-dimensional spatial abstraction of an object represented by a coordinate pair.
Point and Line Data Structure.
A form of vector data structure designed for map production in which all map features are designated as points or lines or text. Point and line data does not carry the topological relationships between features.
Point Feature
A zero-dimensional spatial abstraction of an object with its position defined by a coordinate pair. Points may also be represented by symbols which may have attributes such as orientation and size.
Polygon
Polygons are representations of areas. A polygon is defined as a closed line or perimter which completely encloses a contiguous space and is made up of one or more links. At least one node occurs on the perimeter of a polygon where the bounding link completes the enclosure of the area. There may be many nodes connecting the bounding links of a polygon. Links may be shared between polygons. Polygons ma wholly contain other polygons; or be contained within other polygons.
Polygon Boundary
The link or links which enclose a polygon, projected into the horizontal plane.
Polyline
A line made up of a sequence of line segments.
Portal
A web-site or service that offers a window into a broad array of resources and services. A portal also allows the provider and/or user to customise the content of the web-site to meet individual needs.
Positional Accuracy
The degree to which the coordinates define a point's true position in the world, directly related to the spheroid and/or projection on which the coordinates system is based.
Protocol
A formally agreed standard for communication between computers.
Proxy Server
A server that acts as an intermediary between a client and the server the client wants to access. It is the proxy server that makes the request to the server of interest and passes back the response to the client.

Q

Term
Description
Query
The process of selecting features in a database. Can be acheived through a query language directly on the data or, in a GIS, by interactively selecting features.

R

Term
Description
Reference Satellite

n a double difference implementation, measurements are differenced between different satellites on one receiver in order to cancel the correlated errors. Usually one satellite is chosen as the "reference, and all others are differenced with it.

Reference Station
The GPS receiver which is acting as the stationary reference. It has a known position and transmits messages for the rover receiver to use to calculate its position.
Rover Receiver
The GPS receiver which does not know its position and needs to receive measurements from a reference station to calculate differential GPS positions.
Residual

In the context of measurement, the residual is the misclosure between the calculated measurements, using the position solution and actual measurements.

Receiver Independent Exchange (RINEX)
Exchange format for GPS observations. Provision is made for pseudorange, carrier phase and Doppler observations.
Rectangular Coordinates
Also known as x-y coordinates and as eastings and northings. These are two-dimensional coordinates which measure the position of any point relative to an arbitrary origin on a plane surface
Relative Accuracy
Relative accuracy compares the distance between features measured on the ground to the corresponding information contained in the map data. When measuring between data points represented on the map it is worth noting that some distortion may occur due to the materials and process used to produce the map.
Relative Coordinates
A coordinate pair or triplet measured relative to another point in the coordinate system in which it lies, rather than from the origin. (see also absolute coordinates).
Remote Sensing
The process of obtaining information about an object whilst separated by some distance from the subject. Practically, this is a term used to describe the process of using sensors mounted on satellites to observe the Earth's geology, surface and atmosphere.
RINEX
See Receiver Independent Exchange.
Root Mean Square (RMS)

A probability level of 68%.

Radio Technical Commision for Maritime Services (RTCM)
An organization which developed and defined the SC-104 message format for differential positioning
Router
In a message switching system, the portion of a node or exchange that examines incoming messages, interprets the address information and determines which of the outgoing links can be used. Usually a computer program that chooses messages from incoming buffers and places them into outgoing message queues.
Real Time Kinematic (RTK)

A type of differential positioning based on observations of carrier phase.

S

Term
Description
Satellite Based Augmentation System (SBAS)

A type of geo-stationary satellite system that improves the accuracy, integrity, and availability of the basic GPS signals. This includes WAAS, EGNOS, and MSAS.

Selected Availability (SA)

The method used by the United States Department of Defence to control access to the full accuracy achievable by civilian GPS equipment (generally by introducing timing and ephemeris errors).

Schema

Sometimes known as ellipsoid; a perfect mathematical figure which very closely approximates the geoid. Used as a surface of reference for geodetic surveys.

Server
A program that runs on a networked computer that responds to client programs running on other networked computers.
Service Level Agreement
Collective Agreement.
SLA
See Service Level Agreement.
Spatial Data
Another term used to describe Geospatial Data.
Space Vehicle (SV)
Sometimes used as SVID. A unique number assigned to each satellite for identification purposes. The 'space vehicle' is a GPS satellite
Spheroid
A surface generated by revolving an ellipse about one of its axes. Specifically used for a surface whose dimensions are chosen to approximate to the shape of the earth.
Spot Height
A point on the earth's surface for which the height above a reference datum is known and which has been fixed by observation.
SQL
Structured Query Language.
Surveying
The determination of the absolute and relative positions of points, on or near the earth's surface , by means of measurement in the three elements of space; distance, direction and elevation, and hence their subsequent representation onto a plane surface, exhibiting them in their correct horizontal and vertical relationships.

T

Term
Description
TCP/IP
See Transmission Control Protocol / Internet Protocol.
Time To First Fix (TTFF)
The actual time required by a GPS receiver to achieve a position solution. This specification will vary with the operating state of the receiver, the length of time since the last position fix, the location of the last fix, and the specific receiver design
Topography
The study of the physical features of the earth.
The physical medium on which digital data is transferred from one computer system to another e.g. CD-ROM.
Transmission Control Protocol / Internet Protocol
The common communication protocol of the Internet. Each computer connected to the Internet has a TCP/IP address which identifies it. Other protocols such as HTTP are built on top of TCP/IP.
Triangulation Station
A permanently marked and fully documented control station whose position on the earth's surface has been established to a high accuracy both absolutely and in relative terms to other adjacent stations by means of angular or electronic distance measrement. Triangulation stations form the framework on which all survey and mapping techniques are based.

U

Term
Description
Update Rate
The GPS receiver specification which indicates the solution rate provided by the receiver when operating normally

V

Term
Description
Vector
A straight line joining two data points.
Vector Data
Positional data in the form of coordinates of the ends of line segments, points, text positions and so on.
Vertex
An intermediate point in a polyline.
Virtual Private Network (VPN)
A set of nodes on a public network such as the Internet that communicate among themselves using encryption technology to protect their messages from being intercepted and understood by unauthorised users.

W

Term
Description
World Geodetic System 1984 (WGS84)

An ellipsoid designed to fit the shape of the entire Earth as well as possible with a single ellipsoid. It is often used as a reference on a worldwide basis, while other ellipsoids are used locally to provide a better fit to the Earth in a local region. GPS uses the centre of the WGS-84 ellipsoid as the centre of the GPS ECEF reference frame.

Waypoint

A reference point on a track.

Wide Lane

A particular integer ambiguity value on one carrier phase range measurement or double difference carrier phase observation when the difference of the L1 and L2 measurements is used. It is a carrier phase observable formed by subtracting L2 from L1 carrier phase data: Φ' = Φ1 - Φ2. The corresponding wavelength is 86.2 cm

Y

Term
Description
Y-Code
An encrypted form of P-Code. Satellites transmit Y-Code in replace of P-Code when Anti-Spoofing is in effect

Z

Term
Description
Zoom
To increase or decrease the scale of the map view window. Zoom out to view a smaller scale map and zoom in to view at a larger scale.