Glossary of Survey Terms

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Absolute Positioning
Mode in which a position is determined, using a single receiver, with respect to a well-defined coordinate system, typically a Geocentric system (i.e., a system whose point of origin coincides with the centre of mass of the earth). Also referred to as Point Positioning, or Single Receiver Positioning.

Aerial photography
Photography of part of the earth's surface, but is not rectified to account for differences in scale throughout the photograph.

A data file that contains the approximate orbit information of all satellites, which is transmitted by each satellite within its Navigation Message. It is transmitted by a GPS satellite to a GPS receiver, where it facilitates rapid satellite signal acquisition within GPS receivers. Almanac data is kept current within a GPS receiver to facilitate "hot starts" by permitting the Doppler Shift of each satellite signal to be determined and configuring each tracking channel for this Doppler-shifted carrier frequency.

The vertical angle between the plane of the horizon and the line to the object which is observed. In photogrammetry, altitude applies to elevation above a datum of points in space.

That part of the GPS receiver hardware which receives (and sometimes amplifies) the incoming L-Band signal. Antennas come in all shapes and sizes, but most these days use so-called "microstrip" or "patch" antenna elements. The geodetic antennas, on the other hand, may use a "choke-ring" to mitigate any multipath signals.

Antenna Splitter
An attachment which can be used to split the antenna signal into two, so that it may be fed to two GPS receivers. Such a configuration forms the basis of a Zero Baseline test.

Anti-Spoofing (AS)
Is a policy of the U.S. Department of Defense by which the P-Code is encrypted (by the additional modulation of a so-called W-Code to generate a new "Y-Code"), to protect the militarily important P-Code signals from being "spoofed" through the transmission of false GPS signals by an adversary during times of war. Hence civilian GPS receivers are unable to make direct P-Code pseudo-range measurements and must use proprietary (indirect) signal tracking techniques to make measurements on the L2 carrier wave (for both pseudo-range and carrier phase). All dual-frequency instrumentation must therefore overcome AS using these special signal tracking and measurement techniques.

Anywhere Fix
The ability of a receiver to start position calculations without being given an approximate location and time.

Carrier phase measurements can only be made in relation to a cycle or wavelength of the L1 or L2 carrier waves because it is impossible to discriminate different carrier cycles (they are all "sine waves" if one ignores the modulated messages and PRN codes). Integrated carrier phase measurements may be made by those receivers intended for carrier phase-based positioning. In this case the change in receiver-satellite distance can be measured by counting the number of whole wavelengths since initial signal lock-on and adding the instantaneous fractional phase measurement. However, such a measurement is a biased range or distance measurement because the initial number of whole (integer) wavelengths in the receiver-satellite distance is unknown. This unknown value is referred to as the "ambiguity". It is different for the different satellites, and different for the L1 and L2 measurements. It is, however, a constant if signal tracking continues uninterrupted through an observation session. If there is signal blockage, then a "cycle slip" occurs, causing the new ambiguity after the cycle slip to be different from the value before. Cycle slip repair therefore restores the continuity of carrier cycle counts and ensures that there is only one ambiguity for each satellite-receiver pair.

Ambiguity Resolution
If the initial integer ambiguity value for each satellite-receiver pair could be determined, then the ambiguous integrated carrier phase measurement can be corrected to create an unambiguous, but very precise (millimetre observation accuracy), receiver-satellite distance measurement. A solution using the corrected carrier phase observations is known as an "ambiguity-fixed" or "bias-fixed" solution. The mathematical process or algorithm for determining the value for the ambiguities is Ambiguity Resolution. Tremendous progress has been made in AR techniques, making today's carrier phase-based GPS systems very efficient by cutting down the length of observation data needed (resulting in so-called "rapid static surveying" techniques) and even allowing this process to occur while the receiver is itself in motion (in so-called "on-the-fly" AR techniques). (In practice, the AR process and the ambiguity-fixed solutions are carried out on the double-differenced carrier phase observables, not on the one-way satellite-receiver measurements.)

Aneroid barometer
An instrument used to obtain heights above sea level by measuring atmospheric pressure. Since atmospheric pressure varies with the height above or below sea level, the height can be read directly from the height scale on the barometer.

A characteristic which describes a Feature. Attributes can be thought of as questions which are asked about the Feature. Typically associated with geospatial data gathering for inclusion within Geographic Information Systems (GIS).

May refer to a field survey, construction drawing, 3D model, or other descriptive representation of an engineered project or a design. Derived from usage of the adjective asbuilt.

When a diferential GPS system looses communication or cannot get a marching signal to lock on to, the signal is autonomous. Each GPS receiver is now working independantly.

The number of hours per day that a particular location has sufficient satellites (above the specified elevation angle, and perhaps less than some specified PDOP value) to make a GPS position determination possible.

The horizontal angle measured from the meridian planes (a plane which contains the polar axis, being true north).

A surveyed line usually several kilometres long. It is established with the utmost precision available at the time. Surveys refer to the baseline for coordination and correlation. The baseline accumulates distances throughout a triangulation network, extending to other baselines, providing further integrated control.

A Baseline consists of a pair of stations for which simultaneous GPS data have been collected. Mathematically expressed as a vector of coordinate differences between the two stations, or an expression of the coordinates of one station with respect to the other (whose coordinates are assumed known, and is typically referred to as a "Base" or "Reference" Station).

Base Station
Also called a Reference Station. In GPS navigation, this is a receiver that is set up on a known location specifically to collect data for differentially correcting data files of another receiver (which may be referred to as the "mobile" or "rover" receiver). In the case of pseudo-range-based Differential GPS (DGPS) the base station calculates the error for each satellite and, through differential correction, improves the accuracy of GPS positions collected at unknown locations by another (roving) GPS receiver. For GPS Surveying techniques, the receiver data from the base station is combined with the data from the other receiver to form double-differenced observations, from which the baseline vector is determined.

Beam compass
A drafting instrument used for drawing circles with a long radius. The point and scribe are separate units, mounted to slide and clamp on a long beam.

An angle measured clockwise from a north line of 0° to a given surveyed line.
Also referred to as the Azimuth. The compass direction from a position to a destination. The "north" direction is "zero bearing", and the angle is measured clockwise through 360°. May be referred to a number of "north" directions, including magnetic north, (projection) grid north, or geographic north.

An angle measured clockwise from a north line of 0° to a given surveyed line.
Also referred to as the Azimuth. The compass direction from a position to a destination. The "north" direction is "zero bearing", and the angle is measured clockwise through 360°. May be referred to a number of "north" directions, including magnetic north, (projection) grid north, or geographic north.

Pronounced Bay-doe, BeiDou is the Chinese Navigation Satellite System, just as GPS is the American Navigation Satellite System.

All GPS measurements are affected by biases and errors. Their combined magnitudes will affect the accuracy of the positioning results (they will bias the position or baseline solution). Biases may be defined as being those systematic errors that cause the true measurements to be different from observed measurements by a "constant, predictable or systematic amount", such as, for example, all distances being measured too short, or too long. Biases must somehow be accounted for in the measurement model used for data processing if high accuracy is sought. There are several sources of biases with varying characteristics, such as magnitude, periodicity, satellite or receiver dependency, etc. Biases may have physical bases, such as the atmosphere effects on signal propagation or ambiguities in the carrier phase measurements, but may also enter at the data processing stage through imperfect knowledge of constants, for example any "fixed" parameters such as the satellite ephemeris information, station coordinates, velocity of light, antenna height errors, etc. Random errors will not bias a solution. However, outlier measurements, or measurements significantly affected by multipath disturbance (which may be considered a transient, unmodelled bias), will bias a solution if the proportion of affected measurements is relatively high compared to the number of unaffected measurements. For this reason, long period static GPS Surveying is more accurate (less likely to be biased) than "rapid static surveying" or kinematic (single-epoch) positioning.

Binary Shift-Key (BSK) Modulation
BSK is a modulation technique by which a binary message, such the Navigation Message or the PRN codes (consisting of 0's and 1's), is imprinted on the carrier wave. Unlike Amplitude Modulation (AM) and Frequency Modulation (FM), BSK Modulation does not alter the signal level (the "amplitude") or the carrier wavelength (the "frequency"). At a change in value of the message from 0 or 1, or from 1 to 0, the carrier wave is reversed (the phase is "flipped" by 180°). All reversals take place at the zero-crossings of the carrier (sine) wave (i.e., where the phase is zero).

A mark carved in a tree trunk at about chest height, signifying close proximity of a survey line

The standard (Clear/Acquisition) GPS PRN code, also known as the Civilian Code or S-Code. Only modulated on the L1 carrier. Used by the GPS receiver to acquire and decode the L1 satellite signal, and from which the L1 pseudo-range measurement is made.

A Latin term from 'cadastre' referring to a registry of lands. Cadastral surveying is the process of determining and defining land ownership and boundaries.

Cadastral map
A map depicting land parcels and associated nomenclature.

A radio wave having at least one characteristic (e.g., frequency, amplitude, phase) that can be varied from a known reference value by modulation. In the case of GPS there are two transmitted carrier waves: (a) L1 at 1575.42MHz, (b) L2 at 1227.60MHz, modulated by the Navigation Message (both L1 and L2), the P-Code (both L1 and L2) and the C/A-Code (L1).

The art and science of the production of maps. This includes the construction of projections, design, compilation, drafting and reproduction.

Carrier Phase
GPS measurements made on the L1 or L2 carrier signal. May refer to the fractional part of the L1 or L2 carrier wavelength (approximately 19cm for L1, 24cm for L2), expressed in units of metres, cycles, fraction of a wavelength or angle. (One cycle of L1 is equivalent to one wavelength, and similarly for L2.) In carrier phase-based positioning, such as employed in GPS Surveying techniques, carrier phase may also refer to the accumulated or integrated measurement which consists of the fractional part plus the whole number of wavelengths (or cycles) since signal lock-on.

Carrier-Aided Tracking
A signal processing strategy that uses the GPS carrier signal to achieve an exact lock-on the PRN code. More efficient and accurate than the standard approach.

Special purpose navigation maps chiefly used for nautical, aeronautical and mapping of the cosmos.

Circular Error Probable (CEP)
A statistical measure of the horizontal precision. The CEP value is defined as a circle's radius, when centred at the true position, encloses 50% of the data points in a horizontal scatter plot. Thus, half the data points are within a 2-D CEP circle and half are outside the circle.

Class of Survey
Class of Survey is a means of categorising the internal quality, or precision of a survey. The number of categories, the notation applied, and the accuracy tolerances defining the transition from one class to another are defined by individual nations. Typically they are based on traditional geodetic surveying categories, supplemented by several extra categories of higher precision applicable to GPS Surveying and GPS Geodesy techniques, and may be different for horizontal surveys and vertical surveys. The attachment of a particular Class "label" (e.g. A, B, etc.) to a survey, comprising a few or many points within a "network", carried out using GPS or any other technique, is performed as part of the process of "network adjustment" in which the relative error ellipses (in the horizontal case) between coordinated stations are computed and compared with the accuracy standards that must be met for various categories of Class. See Minimally Constrained.

An instrument used to determine the angle of elevation or depression. A De Lisle's Pendent Clinometer was used by surveyors and engineers to set out slopes and gradients in the construction of paths, tracks and roads.

Clock Bias
The difference between the receiver or satellite clock's indicated time and a well-defined time scale reference such as UTC (Coordinated Universal Time), TAI (International Atomic Time) or GPST (GPS Time).

Coarse Acquisition (C/A)
See also C/A-Code. A spread spectrum direct sequence code that is used primarily by commercial GPS receivers to determine the pseudo-range to a transmitting GPS satellite, modulated on the L1 carrier.

Code Phase
GPS measurements based on the C/A-Code. The term is sometimes restricted to the C/A- or P-Code pseudo-range measurement when expressed in units of cycles.

The magnetic compass has a pivoting magnetised needle that always points to magnetic north (geological features may influence readings). The compass circumference is divided into degrees from which a of a chosen direction from magnetic north can be determined. A compass magnetic bearing must be converted to a grid bearing for plotting on a map.

Refers to either the specific set of satellites used in calculating a position, or all the satellites visible to a GPS receiver at one time, or the entire ensemble of GPS satellites comprising the Space Segment.

Contour interval
The difference in elevation between adjacent contours as delineated on a map.

Lines joining points of equal height as shown on a topographic map. Contour lines that are relatively close together depict an area of steep terrain on the earth's surface.

Control Point
Also called a Control Station or Geodetic Control Station. A monumented point to which coordinates have been assigned by the use of terrestrial or satellite surveying techniques. The coordinates may be expressed in terms of a satellite reference coordinate system (such as with respect to WGS84, or to ITRS), or a local geodetic datum.

Control Segment
A world-wide network of GPS monitoring and upload telemetry stations operated by, or on behalf of, the US Department of Defense. The tracking data is used by the Master Control Station at Colorado Springs to calculate the satellites' positions (or "broadcast ephemerides") and their clock biases. These are formatted into the Navigation Message which is uploaded on a daily (perhaps more frequently) basis by the Control Segment stations.

The GPS receiver "software" or electronic means, implemented in some fashion (either analogue or digital) within a Tracking Channel, used to shift or compare the incoming signal with an internally generated signal. This operation is performed on the PRN codes, but may be used for more "exotic" mixed signals in the case of L2 measurements, where under the policy of Anti-Spoofing (AS) the L2 PRN code is not known. Correlator design may be influenced such that it is optimised for accuracy, mitigation of multipath, acquisition of signal under foliage, etc.

In the USA, National Geodetic Survey (NGS), an office of NOAA's National Ocean Service, manages a network of Continuously Operating Reference Stations (CORS) that provide Global Navigation Satellite System (GNSS) data consisting of carrier phase and code range measurements in support of three dimensional positioning.

Course-Made-Good (CMG)
The bearing from your starting point to your present position. Commonly used in marine or air navigation.

Crosstrack Error (XTE)
The distance you are off a desired course in either direction. Commonly used in marine or air navigation.

Crown land
Land belonging to the reigning sovereign.

Cutoff Angle
The minimum acceptable satellite elevation angle (above the horizon) to avoid blockage of line-of-sight, multipath errors or too high Tropospheric or Ionospheric Delay values. May be preset in the receiver, or applied during data post-processing. For navigation receivers may be set as low as 5°, while for GPS Surveying typically a cutoff angle of 15° is used.

Cut Sheet

A report of the horizontal distance and elevation difference between points and a design, used for Cut and Fill. The design elevation can be defined by a grid file, triangulation file, 3D polyline, section file, note file, road template file, runway airway clearance or design points. The station and offset of the points can also be reported when a centerline is applied.  The data for the cut sheet is shown in a spreadsheet.

Cycle Slip
A discontinuity of an integer number of cycles in the measured (integrated) carrier phase resulting from a temporary loss-of-lock in the carrier tracking loop of a GPS receiver. This corrupts the carrier phase measurement, causing the unknown Ambiguity value to be different after the cycle slip compared with its value before the slip. It must be "repaired" (the unknown number of "missing" cycles determined and the carrier observation subsequent to the cycle slip all corrected by this amount) before the phase data is processed in double-differenced observables for GPS Surveying techniques.

A mathematical representation that best fits the shape of the earth. Accurate mapping and coordinate systems must be based on a datum. A new datum known as the Geocentric Datum of Australia (GDA) was introduced in 2000 to bring Australia in line with the rest of the world's coordinate systems. GDA is also totally compatible with satellite based navigation systems, for example Global Positioning Systems (GPS). The previous datum used in Australia was known as the Australian Geodetic Datum (AGD). However, this was restricted because it was defined to best fit the shape of the earth in the Australian region only. The change in datums had a major consequence to all coordinates. Both latitudes/longitudes and eastings/northings were shifted by approximately 200 metres in a north-easterly direction.

Dilution Of Precision or DOP
An indicator of the quality of a GPS position, which takes account of each satellite's location relative to the other satellites in the constellation, and their geometry in relation to the GPS receiver. A low DOP value indicates a higher probability of accuracy.

Standard DOPs for GPS applications are:
PDOP – Position (three coordinates)
HDOP – Horizontal (two horizontal coordinates)
VDOP – Vertical (height only)
TDOP – Time (clock offset only)
GDOP – Same meaning as PDOP

See Dilution Of Precision

Double run levelling
The land surveyor’s method of differential leveling over a kilometer of generally level ground from Benchmark (a point of known elevation) A to Point K is to divide the distance into a series of roughly-equal segments of a hundred meters or so, setting Turning Points (which can be the tops of stakes driven into the ground) B, C, D, E, F, G, H, I, and J.
The level instrument is first set up midway between Benchmark A and Turning Point B, the telescope pointed to the rod set on Benchmark A, leveled, and the rod reading recorded. The rod reading is added to the known elevation of Benchmark A to determine the Height of Instrument (HI 1) at its first setup. The rod is then moved to Turning Point B, the telescope pointed and leveled, and the rod reading recorded. The elevation of Turning Point B is calculated by subtracting the Turning Point B rod reading from HI 1.
Next, the instrument is picked up and carried to a location midway between Turning Point B and Turning Point C. The telescope is leveled and a “backsight” is taken of the rod on Turning Point B. The new Height of Instrument (HI 2) is calculated as the sum of the elevation of Turning Point B and the rod reading. The rod is then moved to Turning Point C, the telescope turned and leveled, and a “foresight” rod reading taken. The elevation of Turning Point C can then be calculated by subtracting the Turning Point C rod reading from HI 2.
This process continues until the rod is held on Point K, when the elevation of Point K is calculated as HI 10 – rod reading on Point K. That’s single-run leveling.
For double-run leveling, the surveyor would then make another 10 instrument setups to work backward from Point K to Benchmark A.
In the perfect world, the sum of all backsights would equal the sum of all foresights, and the calculated elevation of Benchmark A at the end of the measurement process would be exactly equal to the known elevation. The difference between the measured and known elevations of Benchmark A is the “closure error of 1 kilometer of double-run levelling”.

Electronic Distance Measure. This instrument measures distances using light or sound waves.

The height above mean sea level.

Earth Resources Technology Satellite. This was later renamed Landsat.

Fix or Fixed solution
RTK GPS term - a single position with latitude, longitude (or grid position), altitude (or height), time, and date.

Float or Floating solution
RTK GPS term - when a GPS system has established low accuracy readings, before full RTK fixed solution has been establised.

Geocentric datum
A datum which has its origin at the Earth's centre of mass. This datum can therefore be used anywhere on the planet and be compatible with the same datum anywhere else on the planet.

The science and mathematical calculations of the shape and size of the Earth.

Geographical coordinates
A point on a map given as latitude and longitude readings. The values are given as degrees, minutes and seconds.

Geographic Information Systems
GIS is the spatial capture of themed data layers and the storing, analysing and displaying of the geographically referenced information. A GIS also includes the procedures, software, hardware, operating personnel and spatial data associated with the system.

See Geographic Information Systems

Global Positioning System
GPS is a satellite based navigation system originally developed by the United State's Department of Defence. A GPS receiver calculates a position by measuring distances to four or more satellites of a possible 24. These orbit the Earth at all times.

GLONASS is an acronym, which stands for Globalnaya Navigazionnaya Sputnikovaya Sistema, or Global Navigation Satellite System. GLONASS is Russia's version of GPS (Global Positioning System).

GNSS is an acronym, which stands for Global Navigation Satellite System.

See Global Positioning System

A network of crossing lines on a map representing parallels of latitude and meridians of longitude as defined by the projection.

A group of parallel lines that run perpendicular to another group of parallel lines to form a map coverage of squares.

Grid coordinates
A point on a map given as an easting and northing reading. The values are given in metres.

Grid north
The direction of the vertical grid lines shown on a topographic map. The difference between grid north and true north is referred to as grid convergence.

Gunter's chain
A distance measuring device composed of 100 metal links fastened together with rings. The length of the chain is 66 feet. It was invented in about 1620 by English astronomer, Edmund Gunter.
See Dilution Of Precision

See Standard Deviation

Features including rivers, streams, lakes, swamps and other water related features.

Hypsometric tinting
The use of different colours to signify changing elevations on a topographic map.

The angular distance along a meridian measured from the Equator, either north or south.

LeClanche Cell
The cell consists of a glass vessel into which a zinc rod and a cylindrical pot of porous earthenware is placed. The earthenware pot holds a carbon plate. A mixture of equal parts of carbon and needle binoxide of manganese is packed around this plate. To set the cell into action, the glass vessel is nearly filled with a saturated solution of sal-ammoniac. A reaction takes place and a voltage of 1.46 volts is generated.

Levelling (US spelling Levelling)
“Leveling” is a general term used in land surveying that applies to vertical measurements. Vertical measurements are made and referenced to datums, as elevations. The reference datum might be an arbitrary elevation chosen for convenience or a very precise value determined after lengthy studies.

A term often used within the discipline of archaeology and denotes a customary way of living, or a way of life among people.

This is based on the principle that water and grease don't mix. After an image is drawn on limestone with a greasy medium, the stone is dampened and ink is applied with a roller. The greasy image repels the water and retains the ink. Paper is then pressed onto the surface.

Log tables
A set of tables used to abridge arithmetical calculations, by the use of addition and subtraction rather than multiplication and division.

The angular distance measured from a reference meridian, Greenwich, either east or west.

Magnetic north
The direction from a point on the earth's surface to the north magnetic pole. The difference between magnetic north and true north is referred to as magnetic declination.

A representation of the earth's surface where constituancies and related nomenclature are portrayed to a specific format.

Map projection
A means of systematically representing the meridians and parallels of the earth onto a plane surface.

Map scale
The relationship between a distance on a map and the corresponding distance on the earth's surface.

An object, for example an imprinted metal disc, used to designate a survey point. It is usually associated with terms such as reference mark, azimuth mark or bench mark.

Measuring scales
Measuring scales allow the user to represent a subject or drawing to a recognisable reduction or constant ratio of the actual or proposed size. Many early scales were made of silver, ivory, bone or boxwood.

Mercator projection
A conformal cylindrical projection tangential to the Equator. Rhumb lines on this projection are represented as straight lines.

A straight line connecting the North and South Poles and traversing points of equal longitude.

Metes and bounds
The oldest known form of describing the perimeter of a parcel of land. The method of describing the boundary of a parcel of land in which the bearing and length of each successive line is given. Lines may also be described as following some apparent line, for example the bank of a stream.

A number of continuous aerial photographs overlapped and joined together by way of 'best fit' to form a single non-rectified image.

New Zealand Height Datum
The datum used to determine elevations in New Zealand. The NZHD is based on mean sea level being zero elevation.

Aerial photograph images transformed using an 'orthophoto verification' process to remove distortions and capable of registering perfectly with cadastral data.

Information recorded on a transparent medium, superimposed and registered to one or more other records.

See Dilution Of Precision

Used to mark survey corners on smaller portions or acreage. The size of the peg was determined by the 'Rules and Directions for the Guidance of Surveyors' editions. These referred to various land acts of the time from the 1860s onwards.

The science of obtaining reliable measurements by photography.

The process used in a semiconductor operation, which transfers the pattern of an image held on a photomask, onto a flat substrate surface. It follows similar principles to conventional lithography.

The Property Location Index is a database which provides a link between the parcel identifier (lot on plan) and its location address. It is considered the point of truth for location addresses.

The mathematical and calculated correction made to an aerial photograph to show its true ground position at a consistent scale.

Rhumb line
A straight line connecting two points on the earth's surface which cuts all meridians at the same angle. The line maintains a constant bearing.

A large area of land in which squatters could depasture their stock without a lot of fencing necessary. Employed shepherds looked after various areas of the runs. Runs became consolidated pastoral holdings. Many of the runs were about 25 sq miles in area and later became parishes.

The Survey Control Database is a computerised record of the State's geodetic survey control data. Surveyors place and connect to these survey control points. The geodetic network provides a spatial reference framework for all surveys.

Runs were subdivided into selections for farming, agriculture and grazing homesteads. After a period of yearly rental payments, the selector could often obtain freehold ownership.

SmartMap Information Services is an electronic application that accesses, integrates and delivers (through the SmartMap interface) data available from many land-related datasets. These include ATS, DCDB, CISP, PLI, SCDB, Place Names and Aerial Photography Databases.

Spatial information
Data that has a geographical reference to a location on the earth's surface. This includes latitude and longitude co-ordinates, street address and lot number on plan.
Standard Deviation 
ssentially, an indication of the predicted horizontal error (HSDV) or the predicted vertical error (HSDV) 

See Dilution Of Precision

Instrument used by a surveyor for measuring horizontal and vertical angles.

Topographic map
A detailed representation of cultural, hydrographic relief and vegetation features. These are depicted on a map on a designated projection and at a designated scale.

Transverse Mercator Projection
A projection similar to the Mercator projection, but has the cylinder tangent at a particular meridian rather than at the equator.

Trigonometrical survey
A concise method of surveying in which the stations are points on the ground located at vertices of a chain or network of triangles. The angles of the triangles are measured instrumentally and the sides are derived by computation from selected sides termed as baselines.

True north
The direction to the Earth's geographic North Pole.

See Dilution Of Precision

See Standard Deviation

Weichsel glaciation
The last glaciation of the ice age. An ice age is known as a period of low temperatures in the earth's climate causing an expansion of the earth's polar ice sheets and mountain glaciers. There have been approximately four distinct ice ages during the earth's history.