What you need to know about contours on a property development site

This week I went to an eye specialist. I’ve decided it was time to consider laser surgery as I’m short-sighted and need glasses for driving and now I also need glasses to read (yes, I know, a sign of ageing!).  I spent about an hour peering into various machines while my eyes were scanned, measured and flashed at.  At one point I thought I was in a disco with strobe lighting, but unfortunately no John Travolta materialised.

Afterwards the specialist reviewed the enlarged computer images taken of my eyes; I asked him if the measurements were showing the contours of my eyes. He looked at me and said yes and was a little curious to know how I knew this, apparently not many people ask.  I told him I was a property developer and it reminded me of the contour survey that we need before we start developing our sites (pardon the pun... get it sight / site?!).

So eyes and land both need surveying before any work can be done. It makes sense, really.  The surgeon needs detail such as the height of my cornea, and the builder needs to know the fall of the land.

This made me reflect on how important it is to understand the contours of land. I find that many of our clients do not understand what a contour survey is and why it’s necessary, so I thought I’d focus on this today.

In the property world, a contour is an outline of a mass of land. A contour or detailed survey is done by a surveyor and required before you can start on the design of your buildings. Contour lines will be marked on the survey.  Below are some tips* that can help explain the marking you’ll see:

  • All points on a contour line are of the same elevation.
  • No two contour lines can meet or cross each other except in the rare case of an overhanging vertical cliff or wall
  • Closely spaced contour lines indicate steep slope
  • Widely spaced contour lines indicate gentle slope
  • Equally spaced contour lines indicate uniform slope
  • Closed contour lines with higher elevation towards the centre indicate hills
  • Closed contour lines with reducing levels towards the centre indicate pond or other depression.
  • All contour lines must close either within the map boundary or outside.

*Reference: engineeringcivil.com and Ramasesh Iyer

Once the architect or draftsperson has a contour survey then they can begin work on the plans, and the survey will indicate how much site works may be required.  For example, a sloping site will need to be levelled if you are using a slab construction and therefore the approximate height of retaining walls and volume of spoil to be removed need to be calculated.  These costs need to be allowed for by the builder and he depends on the measurements on the contour survey for this data.


 

Click to enlarge

On this survey above, if you look at the first number on the top right-hand side of the survey it reads 1.06.0 then the next number down reads 105.5, this tells us there is a half metre fall from the rear boundary to this contour line. The surveyor has used 500mm intervals to the next contour line.  The last measurement at the front boundary reads 99.0.  So this tells me the site has about a seven-metre fall, so it’s a steep site. The interesting thing showing on this survey is the area in the middle of the survey where there is a break in contour lines. The surveyor has marked the survey with broken lines to indicate a change in grade. Why would this be?

I can tell you why. There was an old house built right in the middle but it had been demolished, so this part of the land had been levelled. This site, while steep, was purchased for a pier, bearers and joist style project, and the view from the top of the site is spectacular out over the valley. So it’s certainly not an ideal development site, but we made it work nicely as a dual-occupancy project.

Accuracy of a survey is extremely important. On one site we found the surveyor (not mine, as this was a development application-approved site purchased) had taken a measurement from one metre in from the rear boundary instead of right on the rear boundary. This was not picked up until after site preparations started and the site was cut. As the site sloped from rear to front, the retaining required on this rear boundary was much more than the builder had allowed for based on the plans. So it is important if you are buying DA-approved sites, to have your own surveyor double-check the plans and proposed levels that are based on the original contour survey.

I’ll keep you updated on whether I have my sight fixed but in the meantime, set your sights on the contour survey before you start developing property.

Jo Chivers is director of Property Bloom, which manages property development.

Jo Chivers

Jo Chivers

Jo Chivers is director of Property Bloom, which manages property development.

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