Skip to main content

Glossary: surveying terms

This page defines surveying and geodetic terms used throughout the Benchmrk documentation. For app-specific terms, see Product terms.

Survey marks​

Survey Control Mark​

A physical monument or point established by a state or territory survey authority at a precisely known location. Survey control marks serve as reference points for surveying, mapping, engineering, and spatial data work. They are typically brass plaques, iron pins, or concrete pillars set into the ground.

PM (Permanent Mark)​

A survey mark intended for long-term use, typically a brass plaque or similar durable monument set in concrete or rock. PMs are established by survey authorities and form the backbone of the geodetic control network.

TS (Trigonometric Station)​

A survey mark established at a prominent location (often a hilltop) for triangulation surveys. Trigonometric stations were historically used to build the national geodetic network through angular observations.

BM (Bench Mark)​

A survey mark with a precisely determined height (elevation) above the Australian Height Datum. Bench marks are used as vertical reference points for levelling surveys and height determination.

SSM (State Survey Mark)​

A survey mark established and maintained by a state survey authority. SSMs form part of the state's geodetic control network and are used for cadastral and engineering surveys.

PSM (Permanent Survey Mark)​

A durable survey mark established for long-term reference, similar to a PM. The term is used in some jurisdictions to distinguish marks intended for permanent retention from temporary marks.

Datums and coordinate systems​

GDA2020​

Geocentric Datum of Australia 2020. The current national geodetic datum for Australia, adopted on 1 January 2020. GDA2020 is based on the International Terrestrial Reference Frame 2014 (ITRF2014) at epoch 2020.0. It replaces GDA94 and accounts for tectonic plate movement since 1994. EPSG code: 7844.

GDA94​

Geocentric Datum of Australia 1994. The previous national geodetic datum, based on ITRF92 at epoch 1994.0. GDA94 coordinates differ from GDA2020 by approximately 1.5 to 1.8 metres across Australia due to tectonic plate movement. EPSG code: 4283.

AHD​

Australian Height Datum. The official vertical datum for Australia, established in 1971 from tide gauge observations at 30 stations around the Australian coast. AHD heights represent elevation above mean sea level. Most survey control marks with vertical data reference AHD.

MGA​

Map Grid of Australia. The UTM (Universal Transverse Mercator) projection system used with GDA2020 and GDA94. Australia is divided into MGA zones (typically zones 49 to 56), each 6 degrees of longitude wide. MGA coordinates are expressed as easting and northing values in metres.

Datum​

A reference system used to define the position of points on the Earth's surface. A horizontal datum (e.g. GDA2020) defines latitude and longitude, while a vertical datum (e.g. AHD) defines height or elevation.

Geoid​

A model of the Earth's gravity field that approximates mean sea level. The geoid is an irregular surface that differs from the mathematical ellipsoid. The difference between the geoid and ellipsoid (geoid-ellipsoid separation or N-value) is used to convert between ellipsoidal heights and AHD heights.

Ellipsoid​

A mathematical model of the Earth's shape, defined by a semi-major axis and flattening. The GRS80 ellipsoid is used with both GDA2020 and GDA94. Ellipsoidal heights differ from AHD heights by the geoid-ellipsoid separation.

Accuracy and quality​

Horizontal Order​

A classification of the accuracy of a survey mark's horizontal (latitude/longitude) coordinates. Higher orders indicate greater accuracy. The classification system varies by jurisdiction but generally ranges from first order (highest accuracy) to lower orders.

Vertical Order​

A classification of the accuracy of a survey mark's vertical (height/elevation) data. Similar to horizontal order, higher vertical orders indicate more precise height determination.

Positional Uncertainty​

A quantitative measure of the accuracy of a survey mark's coordinates, typically expressed in metres at a 95% confidence level. Lower positional uncertainty values indicate more precisely determined positions.

Identifiers​

Nine-Figure Number​

A unique nine-digit identifier assigned to survey marks in Victoria (VIC). The nine-figure number provides a way to reference individual marks by a numeric code.

Jurisdiction​

An Australian state or territory whose survey authority establishes and maintains survey control marks. Each jurisdiction has its own data schemas, classification systems, and mark identifier formats.

Was this page helpful?