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Converting global coordinated grid to a local grid

B525 (Civil/Environmental) (OP)
8 May 10 14:02
Hi, I need to set out foundations, bolt boxes etc - I have some coordinated stations around site but i want to convert these coordinates to set up a local grid with coordinates so i can work out any coordinates/points within the building. Does anybody know the calculations for doing this or know of any programmes i can use??    
underhill (Civil/Environmental)
10 May 10 18:05
You may need to describe your problem in more detail, for a clear answer.

Do you already know the relationship (rotation & shift) between your global and local coordinates? If so, you should be able to easily do a transformation of coordinates, by parameters.

Do you know both the global and the local coordinates for a few control points?  If so, you need be estimate, perhaps by least-squares, the transformation parameters between the two coordinate systems.

Or maybe you already have your cartesian (or rectangular) coordinates and need to convert them to radial (or polar) coordinates from one station. That requires "just" a set of inverse calculations.

If any of the above are the case, then there are several land surveyor's COGO (coordinate geometry) programs around -- Copan by UGL being a free example.
B525 (Civil/Environmental) (OP)
11 May 10 14:47
Thanks for your reply. I already have the global coordinates for some stations on site that the architect has provided. What i want to do is convert these global coordinates for these stations to local coordinates so i can set up a local grid so i can work out my own coordinates using the dimensions off the architects drawing. So for example i may give the bottom left corner on the drawing (say Gridline A1)a coordinate of 500.000 E and 100.000 N, then from this if i wanted to set out a point which is 1 metre to the right and 1 metre up then i know the coordinate would be 501.000 E and 101.000N. It's the actual calculation that i need to do to convert these global coordinates into local coordinates/grid. Is it what you would call rotating these stations/coordinates?? Sorry if i sound patronising or stupid but if fairly new to setting out/engineering! Thanks again.
IRstuff (Aerospace)
11 May 10 14:55
There appears to be plenty of resources online to do what you're asking: http://www.uwgb.edu/dutchs/usefuldata/utmformulas.htm#Spreadsheet

TTFN

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underhill (Civil/Environmental)
11 May 10 17:01
It's a bit of a Catch-22: if you know exactly how to describe the problem, you know enough that you needn't ask the question! smile

The resource suggested by IRstuff is for converting between UTM and latitude-longitude -- something i didn't address before, but, i assume is not applicable to you. I assume your "global" coordinates are already in UTM or State Plane or some other planar system. True? If not, ignore the following...

The procedure i describe assumes you're using Copan (see http://www.underhill.ca/Software/Copan/Windows/CopanWin.php):

1. Create a new coordinate file
2. Add or import the global coordinates
3. Assume you'd like one of the global points (call it 1001) to be at local coords 500.0, 500.0
4. Add a new local point (call it 11) at local coords 500.0, 500.0
5. Assume you'd like another of the global points (call it 1002) to be at a local bearing of 00 (ie, local north)
6. Use Cogo to inverse calc a global bearing from 1001 to 1002 (call it bng B)
7. Using Transform by Parameters:
    a) Rotate about point 11 by negative bng B
    b) Scale by the combined scale factor (if you don't know what that is just keep it at 1.0)
    c) Shift from 1001 to 11
    d) Transform the points
8. Delete point 11
All original points are now in the local system.

You'd be well advised to contact a local survey engineer/technician for help.

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