How to Find AHD?

[wander]

Is there a simple way to get a AHD for any given locations coordinates or is it work out from the contours or use a location with a spot height only?

[taswegian]

Apart from a GPS or other suitable survey routine I'd suggest the contours but take into account the constraints on that arising from the accuracy of the contours.
Hand held GPS are useless for elevations and whilst you may fluke an elevation that resembles true AHD its just that - a fluke.

Google earth does give elevations, but I'd take them with due caution in hilly areas where elevation differences are great. Google earth doesn't always use true orthorectified images and both horizontal and vertical accuracies are to be found wanting at time.

Some maps give spot heights but they are usually few and far between.

Tree cover can give anomalies in heights. Newer mapping procedures do better than older aerial photography in some cases but the areas you are interested in may not have been remapped?

If around town then there are other options but I assume you mean in the bush.

How accurate information are you after?

[frenchy_84]

Comes down to wether your GPS can use a geoid model. I dont have alot of experince with handheld GPS's but I wouldnt think many if any can handle a geoid model. A geoid, is a model which appoximates the difference (N-Value) in ellipsoidal heights (what a GPS gives) and AHD (relative to gravity). And this varys all across Aus, in some areas the N-value can be pretty consitent in others it can vary signifcantly (I also guess that depends on what you consider significant) particulary in hilly terrain. And its all relevative to what kind of accuracy you are expecting, if your using a handeld GPS that has a vertical accuracy of 5m well then just use your map contours because your GPS isnt going to get much better results. On a side note, Geoscience have recently released a new Geiod model (AusGeoid09) which is a huge advancement over the previous incarnation.

[wander]

I was asking to pre-load my GPS (Fenix) with as many AHDs as practical.

I found that over 5 days on the Picton Range the GSP was typically within 10M or better with the altitude where I was able to check against the map. I was very surprised at this accuracy.

On Picton itself the GPS was only 6M out. And all that after only providing a correct AHD at home and calibrating and having the GPS running through the Adelaide Melbourne flight then 5 days wandering with GPS running often.

[wildwalks]

Hi
You can use a DEM (Digital Elevation Model), this is a list of locations with altitude points. With software you can pass in a lat,long and get a alt back. Use use a DEM for stuff like drawing terrain profiles for walks. Google provides a API to look up altitiude against their DEM used for Google Earth terrain.
Matt

[taswegian]

The Fenix uses atmospheric pressure and as such is an altimeter so one could expect far better results than a handheld recreational GPS.

You would want to calibrate that against known height datum somewhere close to your place of interest.

[Rob A]

Altimeter if a topo map doesnt give enough detail. Contours are interpretations, and dont equal profiles.
Talking to the pilots who have double redundant back up systems, they go altimeter, altimeter, altimeter, radar, something else(?) look out the window, GPS. Something like that. Kind of says something about GPS elevations.

[Davo1]

I have no idea how you would apply it to the likes of WoJ but any local council should be able to give you a reference point as they use AHD when working out flood mitigation etc.
As an example a new house in a new subdivision has a requirement of having the floor "x" mts above AHD.
If they are no help go have a talk to your local surveyor.
Cheers.