The accuracy of the height above sea level for the contours depends on the methods used during data acquisition and processing. Here are the key points:
Using MGA/AHD with RTK or GCPs:
For the best comparability to sea level, it is recommended that outputs are in MGA (Map Grid of Australia) and AHD (Australian Height Datum).
When the data is flown and processed using Real-Time Kinematic (RTK) positioning or Ground Control Points (GCPs), we can achieve high accuracy.
Specifically, if RTK and GCPs are based on AHD, the processing accuracy is within ±40mm.
Without RTK or GCPs:
If neither RTK nor GCPs are used, the data will likely be based on the WGS84 ellipsoid.
In Australia, for example in Sydney, this could result in significant discrepancies. The contours could be around 20 meters off. This means that a contour line representing sea level in AHD could appear approximately 20 meters above the actual sea level.
Additionally, GPS vertical errors could further affect the accuracy.
We hope this information helps. If you need the highest accuracy for your project, we strongly recommend ensuring that RTK and GCPs are used based on the AHD standard.