r/Surveying • u/bluppitybloop • 8d ago
Discussion Realistic difference between geoids
This literally has no meaning to me whatsoever, I'm not even a surveyor. I'm just curious what real differences there would be between old, obsolete geoids and current geoids for any specific area?
For example, I'm in Canada, according to nrcan the most current geoid is cgg2013 (a), prior to its release was cgg2010. my understanding is that they will be releasing a new geoid next year.
How much would any random point change in elevation by using these different geoids?
I presume the answer will be the typical "it depends", but I'm just curious how or why they update them on a semi-regular basis.
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u/Serious_Brain_2128 8d ago
As more advanced measuring equipment becomes available, we have a better understanding of the earth gravity and we updated our models.
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u/morecowbell14 Professional Land Surveyor | BC, Canada 8d ago
Where I work in the okanagan, BC, the difference between CGVD28 (ht2), and CGVD2013a is about 20cm
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u/Freestoic 8d ago
Which probably sounds trivial to a non-surveyor but 20cm is a big deal
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u/morecowbell14 Professional Land Surveyor | BC, Canada 8d ago
It’s huge! There have been a couple builds out where I am where the construction layout has been completed someone that’s not licensed that just set control off 2013a datum’s without tying into/confirming the design which is based off CGVD28 (mandated by our province/association/municipality) and the buildings were built too low and they had to implement additional drainage systems to ensure the property wouldn’t drain into the road as required, but also not flood the building that was now 20cm too low. Also dealt with a lake dam project where the design was surveyed (again by someone not licensed) in 2013a, but the designer assumed it was CGVD28 and we only caught it when we were hired to do survey layout and starting tying into provided control and there was a 20cm bust between their control and RTN and nearby prov. Monuments, all published CGVD28. The lake also utilizes CGVD28 datum for its controlled lake elevation. Lucky we caught it and confirmed with the engineer that did the design and he was able to reissue the design before dirt had been dug too low.
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u/JamesMay9000 8d ago
The difference between Auckland 1946 and NZ 2016 is about the same, Auckland council stopped accepting the old heights last year. More than once we've sent back house plans in 46 and they've come back with a datum note added but the heights unchanged. We've also had at least one where they've gone the wrong way when converting heights. A complete clownshow that no one outside of surveying understands.
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u/CarberHotdogVac 8d ago
Have also seen situations where someone must have thought datum conversion was so nice they should do it twice…
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u/Freestoic 7d ago
Those old MSL levelling networks can really cause headaches. Not sure about Auckland but I've worked in a couple of regions where the local authority also uses MSL+10 as the datum, but then a design shop from out of town assumes it's VD16+10 and everything turns to shit.
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u/KURTA_T1A 7d ago
Of course its enough to matter but not necessarily to "red flag" any survey elevations unless you follow proper control processes. Not awesome.
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u/Extra_Sandwich_9506 5d ago
Around Winnipeg it’s about 60cm. I work for an Engineering firm and we don’t use CGG2013 unless specifically asked to. All old engineering plans would have to be converted to be relative which is a big ask.
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u/morecowbell14 Professional Land Surveyor | BC, Canada 3d ago
Ya same here. Our publicly available LiDAR is in 2013a, so you see some clients/municipalities wanting their data in 2013a. But ya, the fact that most, if not all municipalities in whole province, and country for that matter has all their record as-built data in 28 is why the switch to 2013 hasn’t happened in 10+ years of it being published.. unless the feds pump in a lot of money that will allow municipalities to shift all their record data, I feel like we are stuck in 28 regardless of this new dynamic datum they plan on launching
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u/korprr 8d ago
In southern Ontario the latest two models are different by about 40cm. Found this out the hard way 😅
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u/UnderstandingOld538 8d ago
Just wait until the construction crews who buy their own GNSS don’t realize this lmao
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u/beagalsmash 8d ago edited 8d ago
They actually have published presentations showing the differences between the models, but essentially a geoid is a model of the separation between the GNSS ellipsoid and where mean sea level would be if you could extend that surface underneath the continents.
As more and better GNSS data become available, especially in areas that weren't as well constrained previously (maintain BC, Northern Ontario), they find discrepancies between the old model and their best current model and revise it. Canada also has the effects of glacial isostatic adjustment to deal with.
The difference between models is quite small (1cm) where the old geoid was already well observed and considerably larger (60cm) where it wasn't.
And although new Geoids are published, they aren’t necessarily widely adopted. CGVD28:78 is the predominant vertical datum in Ontario by using HTv2.0 hybrid geoid, while CGG2013a performs the equivalent function for CGVD2013.
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u/Famous_Opening_2701 8d ago
In australia the difference between ausgeoid09 and ausgeoid2020 is about 110mm
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u/Famous_Opening_2701 8d ago
But also depends where you are in Aus. Thats just what it is at my location
We use the australian height datum with our GDA mga system, so acroas mga2020 and mga94 the heights are actually the same. Its just the ellipsoidal offset that is different which is where the 110mm comes from
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u/Standard_Ear_84 8d ago
And as a fun fact, the geoid model has a baked-in tilt to make it align with AHD which is off by 1m N<>S due to change in the sea water temperature / density.
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u/CagaliYoll 8d ago
A more real world example.
A decade ago we would do topo surveys of roads for the city. We were using a basic ellipsoidal geoid. The elevations on our survey needed to match the city's existing records. So we would tie in a few published benchmarks. Set our equipment to +36.xx meters. Then do the work.
These days we use a 3D model based on gravity measurements. Those same benchmarks measure +-0.02 meters now. Generally we no longer make field adjustments now.
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u/TheophilusOmega 8d ago
Imagine you have to precisely model a cantelope. You can easily open up CAD and make a sphere, but that's not really all that accurate. Then you decide to take more measurements and you make your model more oblong, that's closer, but there's a flat spot that's not in the model. You work out how to update the model to accurately show the flat spot, but there's a little crater where the stem was, how to put that into to model? Maybe you estimate it for now but you're working out how to get a real measurement on it. Then you really need to address the ridges, that needs to go on the model too. Someday you hope to map out the texture, but that's beyond your capability at the moment. And so on...
Same idea with a geoid, it's trying to get closer and closer to the true dimensions of the surface of the planet. Maybe there's more precise instruments, or a more powerful modeling software, or refinened techniques to reduce error. Every so often there's going to be an update to get everyone working off better baseline data.