Technical Advice

Document No:  D12#434765
Revision:  5
Date amended:  15-Nov-2019
The information below is intended to reflect the preferred practice of Main Roads Western Australia ("Main Roads"). Main Roads reserves the right to update this information at any time without notice. If you have any questions or comments please contact Rod Stone by e-mail or on ph (08) 9323-6381.

To the extent permitted by law, Main Roads, its employees, agents, authors and contributors are not liable for any loss resulting from any action taken or reliance made by you on the information herein displayed.


Revision Register


Ed/Version Number Clause Number Description of Revision Date
1 All  Guideline Developed  30-Jun-2001 
2 All  Ownership changed. 20-Jun-2006
2A Header Contact Person changed. 05-Mar-2009
​3 ​Header ​Document Number Changed ​16-Jun-2014
​3A​Header​Guideline Introduction Amended​14-May-2015
​4​All​Guideline Review​10-May-2017
​5​Header​Updated contact​15-Nov-2019

Table of Content


Geodesy is the science which deals with the determination of the exact size and shape of the Earth and the precise location of points on the surface.   The Senior Geodetic Surveyor will provide technical advice for all geodetic matters to enable projects to be completed effectively, efficiently and ensure spatial data requirements conform to Main Roads policies and standards. Below are the most common types of enquiries:


1.1 Co-ordinate Systems

Co-ordinate systems are based on map projections so that geographic positions (latitudes/longitudes or eastings/northings) can be determined.

Below is the policy for Co-ordinate Systems approved by Main Roads Western Australia and a link to our Project Zones page.

1.2 Global Positioning System (GPS) Applications

The Global Positioning System is a satellite based navigation system developed by the United States Department of Defence. It is widely used for civilian navigation and positioning, surveying and scientific applications.

All surveying methods have intrinsic weaknesses and errors and GPS is no exception.  GPS requires line of sight with a large portion of the sky so in city areas with tall buildings or locations with tall trees the use of high accuracy GPS (sub metre) is limited.  The geometry of the GPS satellite configuration results in a lesser accuracy height component as compared with the horizontal position.  For example, if your horizontal accuracy was 1 metre then the height accuracy would equate to about 1.75 meters (these figures can be proportioned to your horizontal accuracies).  This is of particular concern for Main Roads construction projects where the height tolerances are generally greater and more critical than the horizontal component.

Below are some examples of GPS survey methodologies and their applications:

Handheld GPS: +/- 20 m for horizontal and +/- 30 m for vertical - ideal for navigation and approximate location.

GPS Vehicle Tracking Systems: To monitor vehicle locations 24 hours/day, 365 days/year.  Used for monitoring vehicle location and progress.

Differential GPS: Accuracy Tolerances are +/- 1.5 m for horizontal and +/- 4 m for height.  Suited for route location, asset management and road centreline capture from a moving vehicle.

Real Time Kinematic GPS: Accuracy tolerances are +/- 20 mm for horizontal and +/- 30 mm for height.  RTK GPS has specific applications in data collection for Digital Terrain Models.  MRWA has completed specific research into the accuracy of RTK GPS.  

All GPS queries should be directed to the Senior Geodetic Surveyor to determine whether a method of GPS surveying can be used efficiently and effectively on your project.

1.3 Australian Height Datum (AHD)

A national levelling survey completed in 1971 resulted in the datum surface for height being established and termed the Australian Height Datum (AHD).   This was adopted by all states and territories as the datum to which all vertical control for survey and mapping is to be referred. The datum surface is that which passes through mean sea level at the thirty tide gauges throughout Australia.

All Main Roads survey and mapping applications are connected to the AHD unless a unique project specific situation occurs.  It should be noted that heights obtained directly from the GPS satellite systems do not refer to the AHD, but to a mathematical reference surface (the ellipsoid). The difference between these two surfaces is known as the geoid ellipsoid separation and can be modelled to calculate identifiable AHD heights.

As with all forms of survey there a many varying techniques all with different resultant accuracies.  The Senior Geodetic Surveyor can provide technical advice to determine which height method and what accuracy is required for your project.