Survey control marks provide a reference framework which enables the position of earth related information to be expressed in a common coordinate system. They provide a practical basis for position fixing for a wide range of users who deal with geographically related information.
- Select survey control marks by name or number
- Identify survey control in an area defined by grid or geographic coordinates
- Download a variety of reports, including one in .csv format that contains an extract of basic coordinate information about all control marks in Tasmania with survey accurate GDA94 horizontal coordinates.
Horizontal Datums in SurCoM
Coordinates on the new Geocentric Datum of Australia 2020 (GDA2020) have been available in SurCoM since 13 April 2018. However Geocentric Datum of Australia 1994 (GDA94) coordinates remain the default coordinates reported to users in accordance with the Land Tasmania GDA2020 implementation plan.
SurCoM includes the ability to select the horizontal datum coordinates
will be reported in, noting that survey marks do not necessarily have a horizontal
coordinate for every datum listed.
Horizontal Datums in SurCoM
There are records for over 16700 marks (existing or destroyed) with
GDA2020 and GDA94 horizontal coordinates but only 15400 records have horizontal AGD66
coordinates. There are no marks with
AGD84, ITRF2000, WGS72 or WGS84 coordinates on mainland Tasmania or the near
In total, approximately 7200 survey control marks have an Australian Height Datum
(Tasmania) 1983 (AHD83) elevation value but approximately 2000 of these marks have GDA2020 or GDA94
horizontal coordinates that have only been SCALED from a map.
GDA2020 and GDA94 ellipsoidal heights for approximately 3400 marks are available upon request from DPIPWE.
Coordinate Quality Indicators
The current Standard for the Australian
Survey Control Network produced by ICSM completes the transition from CLASS and ORDER to UNCERTAINTY as
the basis for evaluating and expressing the quality of measurements and survey
mark coordinates that was introduced in earlier versions of the Standard.
The quality of mark coordinates in SurCom for all datums other than
GDA2020 is still indicated by CLASS and ORDER (as described in the SurCom Instructions page).
Positional Uncertainty or the uncertainty of the coordinates of a survey control mark with respect to datum, expressed in metres in terms of the 95% confidence level of the horizontal circular confidence region, is the only measure used to indicate the quality of horizontal GDA2020 coordinates.
Where the mark is included in the national GDA2020 least squares adjustment these values have been calculated rigorously in accordance with the formula in the Guideline for the Adjustment and Evaluation of Survey Control. If the mark isn't in the national GDA2020 adjustment the Positional Uncertainty is estimated.
GDA2020 Station Summary
In the instance of survey marks with GDA94 coordinates that were derived
from an historic field survey and transformed to GDA2020 applying the NTv2
format Conformal and Distortion transformation grid the estimate is based upon
empirical evaluation of the quality of a range of sample marks. GDA94 coordinates SCALED from a map are generally
assigned an estimated Positional Uncertainty of 30 metres or greater. These coordinates were left unaltered since
the uncertainty is so much greater than the difference between GDA94 and
GDA2020 coordinates, which is around 1.5 metres in Tasmania.
The following table summarises the number of marks in nominal Positional Uncertainty ranges.
|Horizontal Positional Uncertainty (m)||Approximate Number of Marks||Positional Uncertainty Method|
|0.008 – 0.025||3350||Rigorous|
|0.026 – 0.05||1600||Rigorous or Estimated|
|0.051 – 0.15||2950||Rigorous or Estimated|
|0.151 - 1||1400||Rigorous or Estimated|
|1.001 - 200||7500||Estimated|
Survey Control in LISTmap
LISTMap displays the available survey control marks and you can obtain survey control station summaries with GDA94 coordinates for individual control sites. GDA94 will remain the default option for horizontal datum coordinates until LISTmap is on GDA2020.
- Go to LISTmap
- The Survey Control Layer will not display at this initial zoom scale – it will display automatically when you zoom into your area of interest using any of the zooming options
- Click on a survey control mark you are interested in and select the GDA94 Site Summary from the Identify Results box
When using survey control information, you must be aware of the warnings
associated with its use.