Architects Drawing has Incorrect Coordinates

[SBroster]

Hi, I am quite new to Autocad and was hoping somebody could help.

I have received a dwg of an architect which has the correct coordinates of the points written on the drawing but when you ID the point on CAD it is in coordinates I have never seen before.

I require help orientating that drawing to the correct coordinates so that I can upload it on to my total station for setting out purposes.

For reference, the coordinates that the architect has written on the drawing are in British OS grid system.

Many Thanks

[rkmcswain]

I've seen a lot of drawings that may be dimension-ed or labeled with some values that do not match where the CAD objects lie spatially.

What does your architect say? Is it supposed to be, or did they just draw it at some random point in space?

[SBroster]

This is the problem, the architect is very elusive and takes forever to respond to queries and we haven't got the time to wait.

I know that the coordinates they have wrote on the drawing are correct, I just need a way of getting the drawing lined up with them in cad so I can use it in my total station on site.

Cheers

[rkmcswain]

What do you have in the arch drawing that is known? Points, line bearings, distances?

[SBroster]

It is a grid setting out drawing and he has wrote on the drawing the coordinate for each grid intersection and the distances between the grid lines.

I can screen shot later on so you can visually see what I mean.

[ChicagoLooper]

Hi @SBroster 

What direction is north? Is the north orientation pointing straight up toward the top of your monitor? If north is not pointing straight up then there's a possibility your UCS is not set to World Coordinates  and that may mess up your coordinates.

To verify whether UCS is set to 'World' look at the UCS icon in lower left corner of modelspace. A small square at the intersection of the X and Y axes indicates UCS=World. If a small square is not shown at intersection, then you're not in World.

To set USC to world, enter UCS on command line=>W=>Enter. 

Most importantly, coordinates are typically in feet (or meters). Architects, however, draw in feet-inches so coordinates won't be accurate unless he switched to drawing in feet, and I doubt he did that. 

[Kent1Cooper]

Would this work?  Assuming the compass directions agree....  Say you have a location that has coordinates X=100, Y=50, that you know are correct numerically.  Draw a LINE from that location to @-100,-50 [note the minus signs].  The other end of that line will be 0,0 in terms of those numerical coordinates.  Then MOVE the entire drawing contents from that other-end-of-Line to your 0,0 coordinates.

[Pointdump]

Hi Sean,
"...getting the drawing lined up..."
More details please. Screen shot(s) would definitely help to show the problem.
Are you going to load a DXF of the project into your data collector?
Dave

[richard_387]

I think that you will find that the Architect has the correct coordinates, but you are not experienced enough to deal with them.

Whatever you do, DO NOT ALTER the architect's drawing. First of all, find out what units they have used. Open the drawing and look for dimensions, then check by measuring in AutoCAD what the distance is. Contrary to other thought British architects work with the metric system of units and commonly use the millimetre as a drawing unit. Ordnance Survey usually use metres as a drawing unit. Check to see the exact units of each.

What are the points that the Architect has written OS coordinates on? Are they grid intersections or points on the building?

To proceed you should set out the OS points first using GPS, then with the Architect's drawing loaded in your total station, you must set up a local grid using these given points in the Architect's grid. Now you are set to set out all, after you have checked any pinch point dimensions to boundaries. OS linework on their mapping drawings may be +/- 1m, so there may not be enough room for the project on the ground.

Architects usually ignore North when drawing, just get the project on paper the easy way up

[SBroster]

Hi,

I've attached a couple of screenshots to try and make what i'm asking a bit more coherent.

The first screen shot is of the drawing showing where the architect has labelled the coordinates of the grid intersection. The second is a screenshot of what is shown when i ID  the same point.

Cheers

[richard_387]

I would see nothing unusual in those pictures. Instead of drawing everything at an angle of 15° and getting a crick in the neck from trying to read everything at that angle, the Architect has drawn the project orthogonally. They seem not to be very particular where they start a drawing in CAD coordinates.

They have kindly labelled with Ordnance Survey coordinates, but have put the Northing first which is not the norm for those who know.

You should set out each end of Grid line 1 (for example) with the OS coordinates. Then use those points as control for the Architect's grid and off you go.

If you start trying to put the Architect's drawing to Ordnance Survey coordinates, then you should also scale the Architect's drawing by the Local Scale Factor of 0.99961656. Remember that the OS coordinate system (Transverse Mercator) is for mapping a spherical shape onto a flat sheet of paper, and the Architect's system is "flat earth" where 100m on the drawing is 100m on the ground.

[ChicagoLooper]

Hi @SBroster 

Thanks for uploading the images. 

The coordinates displayed in green indicate millimeters while the OSGB system is in meters. This does NOT mean the architect’s coordinates are wrong, it just means if you input millimeters into your Total Station when its expecting meters, you’ll get bad results. 

Also, a detailed analysis of the northings and eastings of the coordinates reveals the architects has rotated the building. He has NOT rotated the UCS. The drawing is NOT georeferenced.

To get what you want, you’ll need to do three things:

1. SCALE your drawing from millimeters to meters.
2. Rotate the building so it’s properly oriented to the equator and north pole.
3. Move the building (linework) so the X,Y’s are consistent with the ordinance survey grid. 

To get a step-by-step procedure to make your drawing geospatially correct, it’ll be a lot easier if you share your drawing.

When your dwg is properly scaled, rotated and georeferenced, you’ll be able to turn on Bing Map to see the linework against real world imagery. 

[Pointdump]

Sean,
Kinda depends on the control points you've set up on the building site. If your control points are OSGB1936.NationalGrid, then you'll need to use Command ALIGN to situate the building. (From a copy of the drawing. As @richard_387 says, don't alter the original.)
This is best done with Civil 3D. It's going to be challenging if all you have is Vanilla AutoCAD.
Dave

[Kent1Cooper]

[Down to the hundredth of a millimeter?  Inconceivable....]

[richard_387]

I have found this with several Architects that I have set out for. They seem to be coordinate blind, and so do not notice that they leave the length precision at the default of 4 whilst working with millimetres.