The mysterious art of cadastral land surveying

By on 21 March, 2018

Where does the historic discipline of cadastral surveying fit in the era of GIS, datums and big data?

This article was written by Dr. Craig Roberts and appeared in the February/March issue of Position magazine. Dr. Roberts is a Senior Lecturer in Surveying and Geospatial Engineering at the School of Civil and Environmental Engineering, UNSW. Colleagues Dr. Bruce Harvey, also senior lecturer at UNSW and registered land surveyor and Peter Baxter, registered land surveyor and member of the Board of Surveying and Spatial Information, are warmly acknowledged for their valuable feedback.

I’m not a registered/licensed land surveyor, but I am an educator. In my pursuit of teaching the next generation the wonderful world of surveying and geospatial information, I find myself having to give a broad overview of the noble art of cadastral surveying to students. Having recently attended numerous conferences focused more on the spatial world than surveying, I realised that an explainer about this mysterious art might be useful to a growing cohort of geospatial professionals.

The fundamental role of the professional cadastral surveyor is to re-establish existing property boundaries, as per the intent of the original surveyor. This is extended to establishing new property boundaries as per new land development designs.

Let’s think about that for a moment. The largest investment that most of us will ever make is in property. Imagine the farmer, who in 1888 met the surveyor on site and watched him place the boundary peg at the corner of his property. Ably assisted by his grandson, the farmer then replaces the peg with a brand new strainer post in exactly the spot where the peg came out.  The surveyor also places a reference mark (maybe a metal pin, or a bottle or a blaze in a tree) just a few links[1] from the strainer and shows it on their plan of survey. This reference mark provides evidence of the position of the boundary corner.

As the generations pass, the grandson inherits the property and passes it down the family line. As this farmer ages, so too does the strainer post. Five generations on, in 2018, the family decide they want to subdivide portions of their property and need a registered/licensed land surveyor to define the location of the boundary. Registered/licensed land surveyors are the only professionals who can legally redefine property boundaries.

Along comes the modern cadastral surveyor, with her brand new GNSS kit and robotic total station. She’s done a search on all the plans surrounding the property to help her redefine the original intention of the first surveyor to define the land. Therefore she must refer back to the plan from 1888. If she’s lucky, she’ll find the reference mark placed by the original surveyor and after some checks she could simply re-establish the boundary – but there on the ground, after 130 years – the reference mark is gone. In fact, there is very little evidence of the boundary’s whereabouts at all. It’s not even clear which is the correct strainer post marking the corner. In urban surveys, this search for evidence is often compounded as reference marks like drill holes and permanent marks often get destroyed, even in recent decades.

Stay up to date by getting stories like this delivered to your mailbox.
Sign up to receive our free weekly Spatial Source newsletter.
But despite all the technology in the back of her truck, the modern surveyor is acutely aware of the hierarchy of evidence, often referred to as ’monuments over measurements’. Highest on the hierarchy are the original survey marks, but if they are gone then the surveyor refers to occupation, in this case, the strainer post. If such evidence is hard to find, the surveyor uses the dimensions on the plan and makes some assumptions in the field as to what constitutes boundary evidence, either on the property or on neighbouring properties – and so the exploration begins. Some surveyors place more emphasis on dimensions than the pure hierarchy rules specify, and usually consider each distance or angle, not just a transformation of a lot boundary.

Back to our case study, and there could have been other more modern surveys on neighbouring properties in the intervening 130 years. These surveys might provide clues as to the alignment of the south boundary with the west boundary, for instance. Was the internal angle of the original survey preserved? Is the road width correct and parallel – as per the original intention? If the surveyor finds two pieces of evidence in the field and measures between them and the measurement differs from the original plan dimensions, which is correct? The modern gear can measure a distance much more accurately, but this only applies if they are measuring between the original marks – and the original intention of the first surveyor should be preserved, even if they got it wrong. If different surveyors have tried to infer the original position of each corner from a variety of evidence over the period since the original survey, it is likely that subsequent surveyors will have different opinions about where the corners are.  It can be this aspect, rather than the quality of measurements, that causes the most complication.

The modern surveyor is acutely aware that if they pull out their brand new GNSS device and set out a coordinate that is not over the old strainer post – or what’s left of it – then the family will know that this bright young surveyor has got it wrong. The family know where their boundary should be, because they know the story that dates back to 1888.

Dr. Roberts in the field with some of his UNSW surveying cohort.

And here’s the rub. In Surveying 101, I teach the concept of ‘working from the whole to the part’. That’s like geodesy. The International Terrestrial Reference Frame is the biggest ‘whole’ you can get and cascading down from this Continuously Operating Reference Station (CORS) network is the regional Asia Pacific Reference Frame (APREF), followed by the newly gazetted Australian Fiducial Network (AFN), which connects to the various jurisdictional networks and down to Permanent Marks (PM) and State Survey Marks (SSM) used by cadastral surveyors when required.

But cadastral surveying, by circumstance of evolution, ‘works from the part to the whole’ – for example, the outcome of court decisions, or a surveyor trying to avoid court conflicts and choosing the least harm. How well does this property align with the neighbouring property on all boundaries? It can be like a big jigsaw puzzle of land parcels, but because the pieces have been defined over two and a half centuries – by different surveyors, using different equipment and perhaps only aligning to the boundaries that their clients pay them to,[2] the jigsaw looks a little like the dog has chewed it – it doesn’t always fit neatly. This is the skill of the modern registered/licensed land surveyor, to try to sort out these issues which have mounted up over so many years.

From the perspective of the geospatial professional, they want to create a database of all land parcels in their various jurisdictions. Indeed they have. It is called the Digital Cadastral Database (DCDB), but it is referred to as a ‘parcel fabric’. It carries no legal weight. Why? Because of the way it has been created. Initially, by digitising large scale plans, it might be 0.2 metres accurate in the cities and up to hundreds of metres out in remote rural areas. Government authorities have been putting in herculean efforts to improve the representation of the DCDB and aspiring to a survey-accurate cadastre, but even if this is achieved, fundamentally this is still a measurement and lowest on the hierarchy of evidence.

Cadastral surveyors don’t actually care where the land is in the geodetic reference network, rather where it is on the ground.”

 

But that said, they love the various reference networks as they help to find evidence on the ground. GNSS is great for helping to locate the monument, but thereafter the position of the monument on the ground takes precedence.

We still live in a monumented cadastre. Our farming family can walk outside and point to the strainer post that their ancestors placed, knowing that this is the extent of their land and the database coordinate must reflect this, not the other way around. Surveyors still look for reference marks first to redefine the boundary. If the dimensions do not agree, but the surveyor is convinced they have found the monument, then the vector shown on the original plan defines the boundary; monuments over measurements.

Further challenges arise for the geospatial professional because some survey plans do not close perfectly – that is, if you add up all the vector dimensions around a land parcel, when you come back to the starting point, it does not equal zero – there is misclose in the measurements. Surveyors are very comfortable with this contention as they know that no measurement is perfect, and as long as their misclose fits within the tolerances specified in regulations, everything is fine. For the geospatial professional, dealing with a computer program that requires perfect geometry and zero misclose, something has to give. Databases cannot handle uncertainty.

This also does not consider the fact that survey plans show dimensions measured on the ground surface, yet coordinates attained from CORS networks are on the grid. If the surveyor is working on the central meridian of the zone, the difference between a grid and ground distance is 400ppm; which translates to a distance of 100 metres on the ground being equivalent to 99.96m on the grid. If the job is at 1000 metres elevation, then the so-called combined scale factor inflates this discrepancy even further. How do geospatial databases handle this extra complication?

Herein lies the challenge: when geodesy meets cadastral, when grid meets ground, when the surveyor and the geospatial professional try to combine their work, when the whole and the part must merge.”

 

And this is where we are at now, in 2018. We are challenged by exciting new developments such as Cadastre2034, LandXML and ePlan, eGeodesy, 3D cadastres, BIM modelling, point clouds, RPA derived DEMs and datum modernisation. But for all this excitement and reaching toward a utopian digital future, fundamental land equity must not be compromised to accommodate a new technology. Our economy relies on a stable system of land tenure[3] and it is registered/licensed land surveyors who are charged with preserving the integrity of the cadastre. It is not impossible to combine these two schools of thought – but we must respect each other’s expertise and work together to achieve a workable solution. Let’s keep talking.

[1] 1 link = 0.201268 metres. This is an old surveying measurement unit. 100 links = chain. 1 chain road = 20.12m wide. Perhaps you’ve seen a one chain road on a country drive.

[2] Some surveyors will put more time into trying to find all the evidence to solve the jigsaw, but if the client won’t pay for such a thorough survey they might be restricted to a more limited survey.

[3] $5.7 trillion is locked up in residential real estate (Core Logic, 2016)

You may also like to read:


, , , , , , , ,

No comments yet.

Leave a Reply


Newsletter

Sign up now to stay up to date about all the news from Spatial Source. You will get a newsletter every week with the latest news.

  • This field is for validation purposes and should be left unchanged.
Where ethics and geospatial sciences meet
Efforts are being made to ensure that geospatial and locatio...
Tackling the surveying and spatial skills shortage
There’s a shortage of surveying and spatial professionals,...
Vale: Bruce Thompson, visionary spatial leader
The Australian spatial community has lost one of its giants ...
AAUS to hold inaugural NSW Drone Expo
The event will be held in partnership with Wings Over Illawa...
Dial Before You Dig launches next-gen platform
Delivered in partnership with Esri Australia and Deloitte, t...