Australian Height Datum - levelling survey?

[Op Ivy]

I'm trying to get my head around the concept of datums and pretty much get it but am a little confused about how the Australian Height Datum was established. Hoping someone here might know something about it as this is the only place where I know anyone with any geographical interest....

On the Geoscience Australia website it says:

On 5 May 1971, Geoscience Australia, on behalf of the National Mapping Council of Australia, carried out a simultaneous adjustment of 97 230 kilometres of two-way levelling. Mean sea level for 1966-1968 was assigned the value of zero on the Australian Height Datum at thirty tide gauges [GIF 6KB] around the coast of the Australian continent.

What I don't understand is what does "a simultaneous adjustment of 97 230km of 2-way levelling" mean? I've done a bit of digging around on other websites but most just quote the ga.gov.au site. I seem to have established there was a levelling survey which collected "spirit-levelling observations". Were they setting benchmarks from the tide guages? Or, were they adjusting pre-existing benchmarks according to the tide guages? Or, something else?

[ian-and-penny]

I'm trying to get my head around the concept of datums and pretty much get it but am a little confused about how the Australian Height Datum was established. Hoping someone here might know something about it as this is the only place where I know anyone with any geographical interest....

On the Geoscience Australia website it says:

On 5 May 1971, Geoscience Australia, on behalf of the National Mapping Council of Australia, carried out a simultaneous adjustment of 97 230 kilometres of two-way levelling. Mean sea level for 1966-1968 was assigned the value of zero on the Australian Height Datum at thirty tide gauges [GIF 6KB] around the coast of the Australian continent.

What I don't understand is what does "a simultaneous adjustment of 97 230km of 2-way levelling" mean? I've done a bit of digging around on other websites but most just quote the ga.gov.au site. I seem to have established there was a levelling survey which collected "spirit-levelling observations". Were they setting benchmarks from the tide guages? Or, were they adjusting pre-existing benchmarks according to the tide guages? Or, something else?

Seems to me that they adjusted all the tide gauges to read zero at the pre established "mean sea level" which was calculated from data between 1966 & 1968

(I think) Elevations are calculated/defined as height above seal level.

[Kerry]

Basically the entire network of existing levelling was adjusted over the entire country holding the mean sea level value to zero (0) at 30 tide gauges. This in affect then provided what became known as AHD71 values on the pre-existing bench marks as referenced to this defined mean sea level "datum surface".

Now today we know there is actually a difference of around 1 metre in mean sea level primarily due to water density between the warmer northern and cooler southern waters plus some other issues.

[Op Ivy]

Thank you. Thank you. Its making much more sense to me now. Any ideas on what surveying activities were undertaken? Keeping in mind there were 30 tide guages around the coast of Australia involved. The Geoscience Australia website says the coastline of Australia is 35877km for the mainland or 59736 including islands. To set the AHD they carried out "97230 km of two way-levelling". Just trying to picture what that means... I was thinking they might have carried out surveys between tide guages to compare the zero levels and check they agree (or do some adjustment) - which seems to me like a huge undertaking to be done in one day. But then when I realised the distance they are talking is significantly bigger than the coastline I was wondering if they also surveyed between the tide guages and existing benchmarks inland to set them to zero too. What I am picturing would have been a HUGE surveying project and must have involved every surveyor in the country... wondering if I'm imagining something close to what actually happened?

[PesceVerde]

I'd guess it would've taken many months to compile the data, and the correction implemented on 5th May '71?
Why not shoot an email to the horse's mouth (possibly at http://www.icsm.gov.au/ )?

[mundoo]

How do you find out what the height datum is for a particular location?

[Kerry]

This levelling didn't all occur over one day but was built up over quite some time. Two-way levelling is basically starting from bench mark 1 levelling to the bench mark 2 then levelling back to bench mark 1, compare the difference and if out of tolerance then do it all again.

Much of this levelling is classed as high order so all sorts of procedures and practices had to be used, level checked (and maybe adjusted every day before starting), use of Invar staffs, equal sighting distances (measured and recorded) between level and staff, parallel plate levels, levelling only done during specific times of the day.

Basically the cycle is staff on the starting BM (back sight), sight/read/book from the level, staff man moves the same distance the other side of the level (fore sight) and sight/read/book from level, level man then moves the same distance on the other side of the staff and reads back, staff man moves forward, level man reads foresight .... level moves and so on and so on until the next BM is reached then they will have to level back to the BM they started from but this is generally done on a another day.

So basically it's one foot after the other.

The network is a maze of levelling runs between node points that criss cross the country. The major junction points can have at least three (3) permanent marks within 30-50 metres of each other and generally different types of marks depending on the ground conditions. With 1 mark you never know if it has moved, with 2 marks you can determine there's been movement but which one?, with 3 marks then hopefully two will agree if 1 has moved.

Some of the bench marks depending on ground conditions are classed as deep driven which are stainless steel rod sections that are added (screw together) and kept driving until they can't be driven any further or hit rock.

[Kerry]

How do you find out what the height datum is for a particular location?

Depends on the accuracy you require? All topographical maps are typically AHD (contours/spot heights), physically level from an AHD Bench mark, by GPS means and AusGeoid09.

And also be aware that (the biggest majority of) flood warning heights are not AHD but hopefully in the future common sense will prevail out of all the flood enquiries and the whole flood height gauging scheme will be turned upside down and fall in line with meaningfull heights (such as AHD).

[Op Ivy]

Thanks Kerry - that's a really helpful explanation! I can actually picture the two way levelling proces now.

So, referring back to the paragraph from the ga.gov.au website in my first post - would this be the order of things?. At some point in time prior to 5 May 1971 they started going to the tide gauges and setting their zero at the mean sea level derived from readings taken 1966-68. They then worked off the tide gauges to set up a network of benchmarks across the country with two-way levelling covering a total distance of 97,230km. Then, on 5 May 1971 a simultaneous adjustment was carried out - meaning (???) there must have been points in the network that didn't agree so some of them were adjusted to create a uniform system??? Or (???) maybe there were pre-existing benchmarks from a previous vertical datum (?) and the zero was shifted uniformly on all benchmarks to meet with the new zero being applied to the tide gauges?

[Kerry]

The levelling runs provide basically just height differences between subsequent marks. On a very small scale take four marks being the corners of a square, we could level between each corner along the sides of the square as well as accross the diagonals to give six (6) height differences between four (4) points which gives redundancies in determining the Most Probable Values of the remaining points if say one point is held fixed (say at zero).

Expand this to a country wide least squares adjustment comprising thousands of level runs (height differences) holding 30 tide gauge stations at zero and you can imagine it's not just an ordinary mathematical problem. There probably would have been some level runs that would have been flagged as exceeding acceptable tolerances, so these may have been re-levelled or possibly weighted so as not to affect more reliable data. This would not have been a simple process in 1971.

Somewhere around this office I have a copy of Special Publication 8 (Adjustment of the Australian Height Datum 1971), just need to find where I archived it. It is history probably worth a re-read.

[Op Ivy]

Thanks for your patient explanations Kerry. I've got a hold of "The Adjustment of the Australian Levelling Survey 1970-1971" since I last posted and its been interesting to have a flip through. The intro gives a rundown of the process to establish AHD71 and the adjustment of further supplementary levelling. Think the whole process is an interesting story.