understanding co ordinates on planning drawings

[redtop]

trying to make sense of the coordinates used by our architect so can roughly mark on the site where stuff is going.

 

there are a couple of coordinates with E, N and L and then loads of single ones such as 50.880.  Now i can see how they work, increasing from left to right; but when on the site how do i know i am standing in the exact right spot?

[Mr Punter]

If the drawing is based on a topographic survey you should see features like walls, drain covers, trees, kerbs and so on so you have somewhere to start from.

[Russell griffiths]

Are the numbers not height points, not necessarily a longitudinal or latitude point. 

So basically a datum point above sea level. 

[LA3222]

Looks like spot heights to me.

[Jeremy Harris]

As above. 

 

If the topographic survey has been done in the normal way then the spot heights marked will be in metres above Ordnance Datum (the mark on the harbour wall at Newlyn, Cornwall) and the letters by them should refer to the base of a feature, usually something like a tree, post, etc that's on the ground.  There may be a key (with luck) showing what each point reference letter, or letters, refers to.  For example, on our survey TP = Telephone Pole, T = tree, FP = fence post, etc.

 

With luck you should be able to find some fixed reference points on site to work from.  Usually  a topo survey will be done from one or more baselines, with defined end points, and these form the horizontal plane datum from which the other spots are measured, so if you can recreate the baseline then it's easier to set out from it.  Often it seems to be a matter of luck as to whether there are any fixed points left on site to work from, though.  In our case we were lucky, in that we had an Ordnance Survey spot height nail in the lane just by our drive, and this was used by our surveyor as one end of his baseline, with an orthogonal offset from a telephone pole used as the other end of the baseline.  It was then easy to just reference every measurement back to that to set out the site and get everything in the right place.

[epsilonGreedy]

On 28/03/2019 at 17:27, redtop said:

... but when on the site how do i know i am standing in the exact right spot?

 

 

Tricky isn't it. The first question is what level of accuracy is required for your site. In general I reckon the larger the plot and the more rural the location then the lower the accuracy. The other extreme would be a tight urban plot where a 1m error could adversely affect a neighbor or create a manifest aberration to an establish building line along a street.

 

My plot is in a semi rural village position and before doing my own foundations setting out I had a chat with the local planning office and was told they would not act on a positioning error of less than half a meter. In the end I had to use a portfolio of reference points to establish the house position, I had one tree, a road with an indistinct verge and an old cottage 10m beyond my boundary shown on my site plan. In order to cross check my positioning I resorted to celestial navigation after establishing that at 12:44 BST the sun would cast a shadow along the front wall of my house. 

[Temp]

On 28/03/2019 at 17:27, redtop said:

trying to make sense of the coordinates used by our architect so can roughly mark on the site where stuff is going.

 

there are a couple of coordinates with E, N and L and then loads of single ones such as 50.880.  Now i can see how they work, increasing from left to right; but when on the site how do i know i am standing in the exact right spot?

Easting & Northing are coordinates for a point on the plot. Ideally you use a GPS to accurately find that point on the ground (unless the surveyor picked a landmark like a tree or or marked its position with a stake etc).

 

I also recommend buying a 60m tape and double checking the dimensions of the plot vs the survey plan and title deed.

Edited April 1, 2019 by Temp

[Ferdinand]

 

On 31/03/2019 at 11:32, epsilonGreedy said:

On 28/03/2019 at 17:27, redtop said:

... but when on the site how do i know i am standing in the exact right spot?

 

 

Tricky isn't it. The first question is what level of accuracy is required for your site. In general I reckon the larger the plot and the more rural the location then the lower the accuracy. The other extreme would be a tight urban plot where a 1m error could adversely affect a neighbor or create a manifest aberration to an establish building line along a street.

 

That is a good comment.

 

But also remember the difference between a sizing error, and a positioning error.

 

Positioning is wrong place; sizing is too large - and your situation will have different sensitivities to each.

 

And also between error and "error", the latter being slightly deliberate because you might want an unofficial 6" more in the width of your garage. If the 2 sides of your house have convenient measurement "errors" in different directions, then they will be rightly giving you an old-fashioned look. It can be got away with, but you would be playing blind man's buff.

 

My dad had one where somebody had had an Approved Design that was bigger than suitable for the plot, because they had an argument with the chap the other side and tried to move it by stealth so the foundations were 18" away not 3 feet. He made them move them by threatening to get Planning onto it, and it ended up 18" narrower. The cockup was probably because they had not done their homework when buying the plot, but a lot of people building things would have tried to bully their way through it or create facts on the ground. Had it reached eaves height the Council would not have enforced as being "inexpedient".

 

If the change had been say 10ft to 8ft away, there would have been no objection.

 

Ferdinand

Edited April 2, 2019 by Ferdinand

[redtop]

sorry for the delay, thanks for all the responses!