# Glossary of terms

This topic explains some of the terms used in this Help.

accuracy

The closeness of a measurement or coordinate value to the actual (true) or accepted value.

almanac

Data, transmitted by a GNSS satellite, that includes orbit information on all the satellites, clock correction, and atmospheric delay parameters. The almanac facilitates rapid satellite acquisition. The orbit information is a subset of the ephemeris data with reduced precision.

Angles and distance

Measurement of horizontal and vertical angles and a slope distance.

Angles only

Measurement of horizontal and vertical angles.

annotation

Markings on images for the purpose of clarification.

attribute

A characteristic or property of a feature in a database. All features have a geographic position as an attribute. Other attributes depend on the type of feature. For example, a road has a name or designation number, surface type, width, number of lanes, and so on. Each attribute has a range of possible values, called a domain. The value chosen to describe a particular feature is called the attribute value.

Autolock

The ability to lock onto and track a target.

automated rounds

The process of automatically measuring a multiple observations to observed points.

autonomous positioning

The least precise form of positioning that a GNSS receiver can produce. The position fix is calculated by one receiver from satellite data alone.

azimuth

Horizontal direction relative to a defined coordinate system.

backsight

Point with known coordinates or known azimuth from the instrument point that is used to orientate the instrument during station setup.

base station

In a GNSS survey, you observe and compute baselines (the position of one receiver relative to another). The base station acts as the position from which all unknown positions are derived. A base station is an antenna and receiver set up on a known location specifically to collect data to be used in differentially correcting rover files.

baud

A unit of data transfer speed (from one binary digital device to another) used in describing serial communications; generally one bit per second.

BIM

Building Information Modeling (BIM) is a process where the planning, design, construction, and maintenance of buildings and other built assets such as roads, bridges or utilities infrastructure is managed using digital 3D models. For information on BIM model file formats supported in Trimble Access, see BIM models.

C/A (Coarse Acquisition) code

A pseudorandom noise (PRN) code modulated onto an L1 signal. This code helps the receiver compute the distance from the satellite.

change face

Refers to when the face of the conventional instrument that is measuring observations changes between face 1 and face 2. On a servo instrument, this happens automatically. On a robotic instrument, this happens when you tap Change face in the Trimble Access software. On a mechanical instrument, you must manually change face on the instrument.

CMR

Compact Measurement Record. A satellite measurement message that is broadcast by the base receiver and used by RTK surveys to calculate an accurate baseline vector from the base to the rover.

constellation

A specific set of satellites used in calculating positions: three satellites for 2D fixes, four satellites for 3D fixes. All satellites visible to a GNSS receiver at one time. The optimum constellation is the constellation with the lowest PDOP. See also PDOP.

construction offsets

A specified horizontal and/or vertical offset distance to enable equipment to operate without disturbing construction stakes.

construction point

A point that is measured using the "quick fix" option in COGO.

control point

A point on the earth that has an accurately known geographic position.

conventional survey

In a conventional survey, the controller is connected to a conventional survey instrument such as a total station.

curvature and refraction

Correction to the measured vertical angle for the curvature of the earth and the refraction caused by the earth's atmosphere.

data message

A message, included in the GNSS signal, that reports on the location and health of the satellites as well as any clock correction. It includes information about the health of other satellites as well as their approximate position.

datum

See geodetic datum and local datum.

design code

The code name given to the design point.

design name

The name given to the design point.

Differential Positioning

Precise measurement of the relative position of two receivers tracking the same satellites simultaneously.

Direct Reflex (DR)

Type of EDM that can measure to non reflective targets.

displacement model

A model of the movement of points on the surface of the Earth due to plate motion, tectonic strain accumulation, seismic/postseismic deformation, glacial isostatic adjustment, and/or other geological or anthropogenic processes that cause significant coordinate change over large areas. Used to propagate coordinates from one epoch (such as the epoch of measurement) to another (such as the reference epoch of the selected global reference datum).

DOP (Dilution of Precision)

An indicator of the quality of a GNSS position. DOP takes into account the location of each satellite relative to other satellites in the constellation, as well as their geometry relative to the GNSS receiver. A low DOP value indicates a higher probability of accuracy. Standard DOPs for GNSS applications are:
– PDOP – Position (three coordinates)
– GDOP – Geometric (three coordinates and time)
– RDOP – Relative (Position, averaged over time)
– HDOP – Horizontal (two horizontal coordinates)
– VDOP – Vertical (height only)
– TDOP – Time (clock offset only)

Doppler shift

The apparent change in frequency of a signal caused by the relative motion of satellites and the receiver.

DRMS

Distance Root Mean Square. In Trimble Access, DRMS is an estimate of the root mean square of the radial distance from the true position to the observed position. DRMS is one of the available options for the display of GNSS precision estimates in the Trimble Access software. See Precision display.

dual‑frequency

GNSS receiver that uses both L1 and L2 signals from GNSS satellites. A dual‑frequency receiver can compute more precise position fixes over longer distances and under more adverse conditions because it compensates for ionospheric delays.

dual‑prism offset

Measurement of horizontal and vertical angles and a slope distance to two prisms located on one prism pole for the purpose of positioning an obstructed point.

DXF file

A DXF file is a 2D or 3D vector graphic file format generated from CAD software such as AutoDesk. DXF stands for Drawing Exchange Format.

Earth‑Centered,
Earth‑Fixed (ECEF)

A cartesian coordinate system expressing coordinates in the Global datum. The center of this coordinate system is at the earth's center of mass. The z axis is coincident with the mean rotational axis of the earth and the x axis passes through 0° N and 0° E. The y axis is perpendicular to the plane of the x and z axes.

eccentric object

Measurement of horizontal and vertical angles and a slope distance to the face of a radial object (for example, power pole). An additional horizontal angle is observed to the side of the object to calculate the radius and thus position the center of the object.

EGNOS

European Geostationary Navigation Overlay Service . A satellite‑based augmentation system (SBAS) that provides a free‑to‑air differential correction service for GNSS.

elevation

Height above mean sea level. Vertical distance above the geoid.

elevation mask

The angle below which Trimble recommends that you do not track satellites. Normally set to 10 degrees to avoid interference from buildings and trees as well as ground multipath errors.

ellipsoid

A mathematical model of the earth formed by rotating an ellipse around its minor axis.

ephemeris

The current satellite position predictions (trajectory), transmitted in the data message.

epoch

The measurement interval of a GNSS receiver. The epoch varies according to the survey type: – for real‑time surveys it is set at one second – for postprocessed surveys it can be set to a rate of between one second and one minute

face 1

Observing position of an instrument where the vertical circle is commonly on the left hand side of the telescope.

face 2

Observing position of an instrument where the vertical circle is commonly on the right hand side of the telescope.

FastStatic survey

A type of GNSS survey. A FastStatic survey is a postprocessed survey using occupations of up to 20 minutes to collect raw GNSS data. The data is postprocessed to achieve subcentimeter precisions.

feature

A representation of a real‑world object on a map. Features can be represented as points, lines, or polygons. Multipoint features consist of more than one point but only reference one set of attributes in the database.

feature codes

Simple descriptive words or abbreviations that describe the features of a point.

fixed solution

Indicates that the integer ambiguities have been resolved and a survey is initialized. This is the most precise type of solution.

float solution

Indicates that the integer ambiguities have not been resolved, and that the survey is not initialized.

FSTD (fast standard)

The method of measuring one distance and one angle to coordinate a point.

GAGAN

GPS Aided Geo Augmented Navigation. A regional satellite‑based augmentation system (SBAS) implemented by the Indian government.

Galileo

Galileo is a global navigation satellite system (GNSS) built by the European Union (EU) and European Space Agency (ESA). Galileo is an alternative and complementary GNSS to the U.S. Global Positioning System (GPS), the Russian GLONASS, and the Japanese Quasi‑Zenith Satellite (QZSS).

GDOP

Geometric Dilution of Precision. The relationship between errors in user position and time, and errors in satellite range. See also DOP.

GENIO

GENeric Input Output file exported by a number of road design software packages that defines a road as a series of strings. See also string.

geodetic datum

A mathematical model designed to fit part or all of the geoid (the physical earth's surface).

geoid

The surface of gravitational equipotential that closely approximates mean sea level.

Global

Global is the short form name to refer to coordinates in the Global reference datum.

Global reference datum

The Global reference datum is the datum of RTK measurements, such as the reference frame of base stations including VRS. The Trimble Access software determines the Global reference datum using the coordinate system and zone you have selected from the coordinate system library.

If you perform an RTK survey in the job, you must make sure the selected real-time correction source is providing GNSS positions in the same datum as that specified in the Global reference datum field in the Select coordinate system screen of the job properties.

Global reference epoch

The Global reference epoch is the epoch of realization of the Global reference datum. The Trimble Access software determines the Global reference epoch using the coordinate system and zone you have selected from the coordinate system library.

GLONASS

GLObal NAvigation Satellite System (GLONASS) is the global navigation satellite system (GNSS) operated for the Russian government by the Russian Space Forces. GLONASS is an alternative and complementary GNSS to the U.S. Global Positioning System (GPS), the European Union Galileo positioning system, and the Japanese Quasi‑Zenith Satellite (QZSS).

GNSS

Global Navigation Satellite System (GNSS). This is the standard generic term for satellite navigation systems that provide geospatial positioning with global coverage.

GNSS survey

In a GNSS survey, the controller is connected to a GNSS receiver.

GPS

Global Positioning System (GPS) is the global navigation satellite system (GNSS) operated U.S. government. GPS is an alternative and complementary GNSS to the GLObal NAvigation Satellite System (GLONASS), the European Union Galileo positioning system, and the Japanese Quasi‑Zenith Satellite (QZSS).

GPS time

A measure of time used by the NAVSTAR GPS system.

H. Angle offset

Measurement of vertical angle and slope distance. Horizontal angle is then measured separately, usually to an obstructed point.

H. Angle only

Measurement of horizontal angle.

HDOP

Horizontal Dilution of Precision. See also DOP.

Helmert adjustment

A Helmert transformation is a coordinate transformation which uses rotation, scaling, and translation. The horizontal adjustment in a GNSS site calibration is a 2D form of Helmert transformation, and it can also be used for computing a resection.

high dynamic range
(HDR)

With high dynamic range turned on, multiple images are captured, each with different exposure settings, each time the camera button is pressed. During HDR processing the images are combined to produce a composite image that has better tonal range and can therefore show more detail than any of the individual images. For images captured using a total station that has Trimble VISION technology, HDR processing can be performed in the Trimble Business Center after importing the data.

horizontal circle

Graduated or digital disc from which the horizontal angle is measured.

instrument height

Height of the instrument above the instrument point.

instrument point

Point that the instrument is occupying.

integer ambiguity

The whole number of cycles in a carrier phase pseudorange between the GNSS satellite and the GNSS receiver.

integrated survey

In an integrated survey, the controller is connected to both a conventional survey instrument and a GNSS receiver at the same time. The Trimble Access software can quickly switch between the two instruments, within the same job.

ionosphere

The band of charged particles 80 to 120 miles above the earth's surface. The ionosphere affects the accuracy of GNSS measurements if you measure long baselines using single‑frequency receivers.

K factor

The K factor is a constant defining a vertical curve in a road definition.
K = L/A. where:
L is the length of the curve
A is the algebraic difference between the incoming and outgoing slopes in %.

L1 signal

The primary L‑band carrier used by GNSS satellites to transmit satellite data.

L2 signal

The secondary L‑band carrier used by GNSS satellites to transmit satellite data. Block IIR‑M and later GPS satellites will transmit an additional signal on L2 called L2C.

L5 signal

The third L‑band carrier used by GNSS satellites to transmit satellite data. This has been added to Block IIF and later GPS satellites.

LandXML file

A LandXML file is an XML file format for civil engineering design and survey measurement data such as points, surfaces, parcels, pipe network data and alignments.

Local datum

The Trimble Access software determines the Local datum using the coordinate system and zone you have selected from the coordinate system library.

measurement modes

Angles are measured and averaged as one distance is measured using one of the following measurement modes: Standard (STD), Fast Standard (FSTD), Tracking (TRK). STD mode is indicated by an S next to the instrument icon on the status bar. One angle and one distance are measured. FSTD mode is indicated by an F next to the instrument icon on the status bar. Angles and distances are continually measured. TRK mode is indicated by a T next to the instrument icon on the status bar.

mechanical instrument

A conventional instrument that must be manually turned to change face or to locate targets. Compare with servo instrument.

MGRS

Military Grid Reference System

MSAS

MTSAT Satellite‑Based Augmentation System. A satellite‑based augmentation system (SBAS) that provides a free‑to‑air differential correction service for GNSS over its coverage area, which is Japan.

multipath

Interference, similar to ghosting on a television screen. Multipath occurs when GNSS signals traverse different paths before arriving at the antenna.

neighborhood adjustment

A coordinate adjustment that is applied to conventional surveys with multiple backsights or jobs with a GNSS site calibration. During station setup plus, resection or GNSS site calibration, residuals are calculated for each observed control point. The calculated distances from each new point to the control points used in the station setup or calibration are used to determine the coordinate adjustment to be applied to the new point.

NMEA

A standard, established by the National Marine Electronics Association (NMEA), that defines electrical signals, data transmission protocol, timing, and sentence formats for communicating navigation data between marine navigation instruments.

NTRIP

Networked Transport of RTCM via Internet Protocol

observation

A measurement made at or between points using surveying equipment, including GNSS receivers and conventional instruments.

OmniSTAR

A satellite‑based system that broadcasts GPS correction information.

P‑code

The "precise" code transmitted by the GPS satellites. Each satellite has a unique code that is modulated onto both the L1 and L2 carrier waves.

parity

A form of error checking used in binary digital data storage and transfer. Options for parity checking include Even, Odd, or None.

PDOP

Position Dilution of Precision, a unitless figure of merit expressing the relationship between the error in user position and the error in satellite position.

PDOP mask

The highest PDOP value at which a receiver will compute positions.

point cloud

A collection of data points in 3D space.

polyline

Polylines are two or more lines or arcs connected together. A line is a single line between two points.

positioning system

A system of instrumental and computational components for determining geographic position.

postprocess

To process satellite data on a computer after it has been collected.

postprocessed kinematic survey

A type of GNSS survey. Postprocessed kinematic surveys store raw stop‑and‑go and continuous observations. The data is postprocessed to achieve centimeter‑level precisions.

PPM

Parts per million correction that is applied to measured slope distances to correct for the affects of the earth's atmosphere. PPM is determined using observed pressure and temperature readings together with specific instrument constants.

precision

A measure of how closely random variables tend to cluster around a computed value, which indicates the repeatability of one or a set of measurements.

prism constant

Distance offset between the center of a prism and the point being measured.

projection

Used to create flat maps that represent the surface of the earth or parts of that surface.

QZSS

Quasi‑Zenith Satellite (QZSS) is a Japanese based satellite system built by the Japan Aerospace Exploration Agency (JAXA). QZSS is a complementary GNSS to the U.S. Global Positioning System (GPS), the Russian GLONASS, and the European Union Galileo positioning system. QZSS is also a satellite‑based augmentation system (SBAS).

RDOP

Relative Dilution of Precision. See also DOP.

real‑time differential survey

A type of GNSS survey. A real‑time differential survey uses the differential corrections transmitted from a land‑based receiver or from SBAS or OmniSTAR satellites to achieve submeter positioning at the rover.

real‑time kinematic and data logging survey

A type of GNSS survey. A real‑time kinematic and data logging survey records raw GNSS data during an RTK survey. The raw data can be postprocessed later, if required.

real‑time kinematic and infill survey

A type of GNSS survey. A real‑time kinematic and infill survey allows you to continue a kinematic survey when radio contact with the base station is lost. The infill data must be postprocessed.

reference station

See base station.

refline

The process of establishing the position of an occupied point relative to a baseline by taking measurements to two known or unknown points.

region

A region contains scan points from one or more .rcwx scan point clouds or from other regions. Create a region to include only the scan points you are most interested in. A region is especially useful when performing a surface inspection.

resection

The process of establishing the position of an occupied point by taking measurements to two or more known points.

RMS

Root Mean Square. This is used to express the accuracy of point measurement. It is the radius of the error circle within which approximately 70% of position fixes are to be found.

RMT

Remote target

robotic survey

A survey in which the controller running the Trimble Access software is connected via radio to a conventional instrument, so that the instrument can be controlled robotically from the Trimble Access software.

rounds

Conventional observation method of multiple observations to multiple points.

rover

Any mobile GNSS receiver and field computer collecting data in the field. The position of a roving receiver can be differentially corrected relative to a stationary base GNSS receiver.

RTCM

Radio Technical Commission for Maritime Services. This is a commission that was established to define a differential data link for the real‑time differential correction of roving GNSS receivers.

RTK

Real‑time kinematic, a type of GNSS survey.

SBAS

Satellite Based Augmentation System. SBAS is based on differential GNSS, but applies to wide area networks of reference stations (for example, (WAAS, EGNOS, MSAS). Corrections and additional information are broadcast using geostationary satellites.

servo instrument

A conventional instrument that is equipped with servo motors, which enables the instrument to change face and turn to track targets automatically. Compare with a mechanical instrument.

If the servo instrument is also equipped with a radio, you can use it in a robotic survey, where the instrument is controlled via the Trimble Access software.

Shapefile

A Shapefile is an ESRI vector data storage format for storing geographic features as points, lines, or polygons as well as attribute information.

single‑frequency

A type of receiver that only uses the L1 GNSS signal. There is no compensation for ionospheric effects.

single distance offset

Measurement of horizontal and vertical angles and a slope distance. Plus additional offset distances to position obstructed points.

SNR

Signal‑to‑Noise Ratio, a measure of the strength of a satellite signal. SNR ranges from 0 (no signal) to 99, where 99 is perfect and 0 means the satellite is not available. A typical good value is 40. A GNSS system typically starts using a satellite when its SNR value is higher than 25.

station

The distance or interval along a line, including an arc, alignment, road, or tunnel.

station setup

The process of defining the instrument occupation point and setting the orientation of the instrument to a backsight point or points.

string

A string is a series of 3D points joined together. Each string represents a single feature such as a curb line or the centerline of a road.

surface

A surface can be a 3D digital representation of a topographical surface (terrain) or a representation of an object or the face of objects in a 3D model or BIM file. Topographical surfaces are typically digital terrain model (DTM) files which represent the surface using a mesh of contiguous triangles.

surface inspection

The Surface inspection Cogo function compares the scan point cloud of an as-built surface with a reference surface and calculates the distance to the reference surface for each scan point to create an inspection point cloud. The selected reference surface can be a horizontal plane, vertical plane, inclined plane, cylinder, another scan, or an existing surface file such as a DTM or BIM model. You can create a region to include in the inspection only the scan points you are interested in.

superelevation

In terms of a road design, superelevation refers to the addition of an extra slope (banking) on curves of the road to assist vehicles negotiating the curves. Adding superelevation assists in achieving the required design speed for the curve. Superelevation is generally defined in conjunction with widening.

SV

Satellite Vehicle (or Space Vehicle).

target height

Height of prism above the point being measured.

TDOP

Time Dilution of Precision. See also DOP.

TOW

Time of Week in seconds, from midnight Saturday night/Sunday morning GPS time.

tracking

The process of receiving and recognizing signals from a satellite.

tracking mode

Used to measure towards moving targets.

Tracklight

A visible light that guides the prism operator on the correct bearing.

traverse

A traverse is formed by surveying a number of points at traverse stations and then linking them into a circuit. A closed traverse is formed when the circuit ends at the starting point. It is useful for surveying large areas which are defined by a boundary. An open traverse is formed when the circuit ends at a different point to the starting point. It is useful for surveying a narrow strip of land such as a coastline or a corridor for a road. A valid traverse station has one or more backsight observations to the previous traverse station, and one or more observations to the next traverse station. To compute a traverse closure, there must be at least one distance measurement between successive points used in the traverse.

Trimble Terrain Model

A Trimble Terrain Model (TTM) file represents a 3D terrain surface model as a mesh of contiguous triangles.

TRK

See tracking mode.

TTM

See Trimble Terrain Model.

USNG

United States National Grid

UTC

Universal Time Coordinated. A time standard based on local solar mean time at the Greenwich meridian. See also GPS time.

VBS

Virtual Base Station.

VDOP

Vertical Dilution of Precision. See also DOP.

vertical circle

Graduated or digital disc from which the vertical angle is measured.

VPI

Vertical Point of Intersection.

WAAS

Wide Area Augmentation System. A satellite‑based augmentation system (SBAS) that improves the accuracy and availability of the basic GNSS signals over its coverage area, which includes the continental United States and outlying parts of Canada and Mexico.

Weight exponent

The Weight exponent is used in the neighborhood adjustment calculation. When the coordinate adjustment to be applied to a new point is calculated, the calculated distances from each new point to the control points used in the station setup are weighted according to the Weight exponent.

widening

In terms of a road design this refers to the widening of the road around a curve to provide extra safety for cars negotiating the curve. Widening is generally defined in conjunction with superelevation.

WGS‑84

World Geodetic System (1984), the mathematical ellipsoid used by GPS since January 1987. See also ellipsoid.