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.
Below is the policy for Co-ordinate Systems approved by Main Roads Western Australia and a link to our Project Zones page.
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:
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.
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 heighting method and what accuracy is required for your project.