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Topographical surveys


Vijay

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Morning,

 

Are these supposed to be totally accurate showing boundaries and tree locations etc?

 

Would the references found around the edges be ordnance survey references, e.g 251700N or 476100E?

 

Cheers

 

Vijay

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Will depend who did it and what they used. Modern tree surveys use 12 figure GPS references that are only useful really if you use the same kit due to calibration issues. The last unit I had used to "move" even when you've stood still for a minute so they have to be treated with caution. Much better is finding two immovable objects (buildings are good ...) and using good old trigonometry to triangulate the third point on the ground. 

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The topo survey we had done was accurate to within +/- 20mm 2D, +/- 50mm 3D, using an OS reference nail that happened to be in the lane next to where our drive now runs.  I'm not sure how accurate OS spot height reference is, as everything is referenced from that, but I'm sure it's good enough for things like boundary location, as the boundary survey we had done used the same reference point.

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My experience was that the trees on our topo survey weren't that accurately marked, but the base data set we were given (as an AutoCad dxf file) had notes by some points indicating that they were approximate.  I emailed the surveyor and he told me that as this wasn't a tree survey he'd got as close as he could to the two big trees that were on the plot at the time, but couldn't "see" the centre of the tree, because of the canopy and undergrowth, so estimated their location on the data set.  I don't know how big the error was, as the first thing we did when we bought the plot (before applying for planning permission) was get the trees cut down, so that no one had a chance to get a TPO on them once our plans were under consideration.  The boundary locations seemed to be very accurate, as we used the same reference nail in the lane to set out the plot with a Total Station, and everything lined up very well, certainly within about 50mm.

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5 minutes ago, Vijay said:

I'm looking at one that was done about 6 years ago and there is a tree that is 4-4 metres out :o

 

Would the reference round the edges of the survey be OS?

 

Ok ... is this a "proper" topo survey that shows a large number of contour lines and multiple reference points, or is it a tree survey superimposed over the OS location plan ..?

 

Reason I ask is that "some" arbtech surveyors* can be "artistic" in their interpretation of survey points especially when the "survey" is being used for planning permission i.e. if a tree is in the wrong place then planning may be declined, and most tree assessments for planning these days by LAs are desk based and done using Google Earth and the "report" by a "professional".  It pays sometimes for a tree to move slightly so this may be what you are seeing...!! 

 

As with everything, caveat emptor...

 

 

 

* this is no indication that all arbtech surveyors are of the opinion that customers pay the bills not trees, or that "moving" a tree is of benefit to a client knowing it will never be checked. It is based entirely on recent review of a number of planning "tree surveys" and finding 75% containing inaccurate data....  

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9 hours ago, Vijay said:

Are these supposed to be totally accurate showing boundaries and tree locations etc?

For the boundaries the answer is NO. The boundary is only fixed by reference to land registry recorded information and the RED line they draw on the plans when you buy a property are about a meter thick (+/-) and only the notional boundary the land registry holds. Often the LR boundary is based loosely on the OS mapping and as the OS boundary has an error rating based on  a level of confidence. So from a 1:1250 OS map a distance will be accurate to within +/- 900mm 99% of the time but 1% of the time they can / will be out by more and they never give a 100% confidence error. (95% confidence is 800mm). When we had our survey done we asked for +/- 2mm accuracy based on the national grid ref system accurate to +/-20mm. So, sorry Jack, the location could be out by 20mm but all measurements from it are with 2mm. (It is only a 15m x 50m area and level change is 5m max) To get the reference point they used a clever GPS and links to all the local 4G masts. We found that the existing house was about 300mm east and 200mm south of where the OS map had it. Even the relative distances between ours and the adjacent property (both built in 1911), and on OS maps since then, differed from the OS measure. We had all the hedge lines and existing fence lines done as well so we could show where everything was. We learned, off the record IIRC, that the latest OS maps just add features rather than redoing the survey because a new survey will move so many things. Put simply, when you add a conservatory for instance, they just connected it to the existing walls rather than try and work out where it actually is. We also found a distance in the land registry title of our neighbours house stating a distance between ours and theirs. This measurement is the only really fixed thing we have. 

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