A physical
marker that is used as a stable basis for survey measurement. The mark is
normally coordinated either horizontally and/or vertically using a recognised
coordinate system. The mark can be used to coordinate surveys in the
surrounding area. Survey Control Marks are displayed in ACTmapi by mark
type. X and Y values should be treated as easting and northing. Z value
uses AHD value and not elipsoid height.
Major
Control (MC):
Identifier – district names, old surveyors’
names (e.g. Tennent, Goodwin)
Trigonometrical stations (Trigs) – Generally
consist of a ground mark with a quadripod above the ground mark. Most Trigs
were part of the ACT Precision Zone, which in turn formed part of the national
geodetic survey and adjustment for the Australian Geodetic Datum 1966 (AGD66).
The ACT Precision Zone and its associated marks have been the primary survey control
for all new development in the ACT since the early 1970s. The accuracy of ACT
Precision Zone marks is 1 in 250,000 with respect to AGD66. In the early 1990s,
40 of the Precision Zone Trigs were surveyed with geodetic quality GPS observations
as part of the GDA94. More recently, all the ACT trigonometrical network was
re-surveyed with geodetic quality Global Navigation Satellite Systems (GNSS) observations
for inclusion in the GDA2020 national adjustment.
Continuously Operating GNSS Reference
Stations (CORS) – In
partnership with NSW DCS Spatial Services, OSGLI has established a network of
permanent GNSS tracking stations which form part of CORSnet-NSW.
This network is a key component of GDA2020 and provides centimetre-level
real-time positioning for a range of scientific and spatial industry purposes.
SGC values were also established on each CORS from the passive trigonometrical
network.
Sectional Control Marks (SC):
Identifier – two alpha plus numerals (e.g. ST61)
These marks are fixed directly from the ACT Precision Zone
at approximately one-kilometre intervals. They usually consist of a deep driven
rod protected by yellow timber or concrete posts. Some SC marks are beaconed (e.g.
YA90, TG84). The accuracy of Sectional Control marks is 1 in 100,000.
Subdivision
or Neighbourhood Control Marks (SRM):
Identifier
– SRM plus
numerals (e.g. SRM363)
These marks are coordinated from Sectional Control at
intervals of between 200m and 1000m. They usually consist of a galvanised pipe
set in concrete or a galvanised star picket with a concrete collar and
protected by two steel droppers, painted red and white. There may be an
aluminium tag with the mark's identifier either set in the concrete of the mark
or wired to one of the steel droppers. SRMs identifiers greater than 1000
consist of DH&Ws with an accompanying aluminium tag with the mark's
identifier. The accuracy of Subdivision Control is 1 in 30,000.
The
subdivisions in Woden Valley (early 1960s) were the first to have Subdivision
Control. In suburbs constructed before Gungahlin (pre-1990), SRMs were 50m to
100m apart. Some of these marks survived development and can be found in open
spaces in the suburbs. Only those SRMs established since the 1990s have been
entered in the survey control mark database ‘Control Base’. Information on earlier
SRMs is contained on control plans.
Coordinated Reference
Marks (CRM)
Identifier – CRM plus numerals (e.g. CRM7381)
In today's subdivisions, numbered CRM plaques are placed in
the kerb and coordinated by survey networks that include Sectional Control,
Subdivisional Control marks or other previously coordinated CRMs/SRs. These
marks are positioned up to 200m apart.
The early style (flat) CRMs do not have a height attributed
to them (e.g. CRMs numbered 1 to 9999). Since April 1998, a new style of CRM is
used that has a raised nipple for accurate levelling. These are numbered from
10,001 onwards.
Steel Rod (SR) Marks:
Identifier – SR plus numerals (e.g. SR1691)
SR marks are placed in new subdivisions, one for every 100
blocks. The mark consists of a deep driven steel rod contained within a steel
cover box marked "SURVEY MARK". These SRs are coordinated as part of the
CRM network. Not all SR marks are levelled. Conversely, some SRs have been
levelled, but are yet to be surveyed with accurate horizontal coordinates.
Kerb
Bench Mark (KBM):
Identifier – KBM plus numerals (e.g KBM5203)
Kerb Bench Marks were placed in concrete kerbs throughout
urban Canberra prior to April 1998. They consist of a rectangular brass casting
with a nipple and stamped number and were generally levelled to Class LC Order
L3 standard of accuracy. (Note: many kerbs have moved over time and differences
of more than 0.03m have been found). All KBM reduced levels are on the
Australian Height Datum 1971 (AHD71). Since 1998, new style CRMs are placed
instead of KBMs.
Precise Bench Marks (PBMs) are generally buried precast
concrete blocks that were part of an Imperial levelling network on an early
Canberra datum known as Precise Datum. Refer to A Short History of Heights in the ACT (2022) for information to convert imperial heights
(feet) to AHD71 (metres). PBMs are listed as KBMs in ACTmapi (e.g. PBM13
is KBM13 in ACTmapi).
Rural
Bench Marks (RBM):
Identifier – R plus numerals (e.g. R804)
Rural Bench Marks were placed in the 1960s and 1970s, at
half mile (approx. 800 m) intervals, along many roads in the ACT and
surrounding NSW as supplementary levelling to the AHD. They usually consist of
a star-iron picket driven to its full length into the ground and surrounded by
a concrete collar containing a brass plaque with the mark's identifier. One
yellow concrete post may be protecting it. Many of these marks have been
coordinated by GNSS surveys. Other rural bench marks exist with identifiers
"P", "NE", "NW" and "C".
Photo
Control Marks (PC):
Identifier – Alphas plus numerals (e.g. GCP27)
Photo Control marks may consist of a GI Pipe in concrete
protected by a timber post painted red and white. More recently, Photo Control
Marks may also consist of DH&Ws in a concreate structures, such as a
footpath, with painted wings. Many older Photo Control marks are for "level
only" and their accuracy depends on the scale of the photography they are
controlling.
Alpha Marks (AM):
Identifier – Three alphas, starting at AAA (e.g. FFC)
Alpha Marks usually consists of a GI Pipe in concrete
protected by two steel droppers. They are placed as a control mark for a
specific survey, usually an engineering survey. The accuracy of these marks
depends on the nature of the survey they are controlling. Many are of an accuracy
that does not warrant nominating the ACT coordinate system.
Recovery Marks (RECM):
Identifier – Main mark it recovers, plus RM, plus
numerals (e.g. Painter RM2)
Recovery Marks are placed as supplementary control to
high-order marks such as trigonometrical stations. Alternatively, Recovery
Marks may be identified with “ECCE” (i.e. a mark eccentric to the main mark).
Miscellaneous
Marks (MISC):
Identifier – Not specified
Miscellaneous marks are any mark not contained in the
previous categories. Some examples are old radial blocks (e.g. RA18), Control
Marks placed before Sectional Control series commenced, dam deformation marks
(e.g. P05) and towers (e.g. BLACK MOUNTAIN TOWER), etc.
