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Thread: Create a custom projection

  1. #1
    michael collins
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    Default Create a custom projection

    How difficult would it be to create a custom projection with a grid shift and a rotation?
    I have shape files from a mine site and they used their own mine grid. I've been given a diagramme (see attached) that shows the grid shift and rotation of their mine grid with respect to a UTM grid (I changed the coordinates for privacy). I need to create a projection based their mine grid so I can use their shape files. Any ideas?
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  2. #2
    Melita Kennedy

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    Default Re: Create a custom projection

    This has come up multiple times, including in the last week:

    http://forums.arcgis.com/threads/11191-Local-Mine-Grid

    Add some base data (known coordinate system) plus some of the local grid data (unknown coordsys) to ArcMap. Set the data frame's coordinate system to use a custom projected coordinate system.

    For your case, I would try to use Rectified Skew Orthomorphic (RSO) Center. Use the same GCS as the UTM zone and set the linear units to whatever they should be.

    It looks like you have the local grid center coordinates, plus the grid coordinates at that point. Use that for the center in RSO, leave scale factor = 1, azimuth = 0. Set the xy plane rotation to the stated rotation value. Set the false easting and false northing equal to grid's center values. OK the dialog.

    You may see some offsets in the data, particularly a rotation. Try to fix the rotation by modifying the XY Plane Rotation value. When you do so, you'll have to also adjust the false easting and northing values (but you can't decide on those values until you check the rotation!). Repeat until satisfied with the results. You may want to periodically add a version to your Favorites so you can step back if the results are getting worse.

    Once finished, that coordinate system is what you want to assign to the local grid datasets. To retrieve the prj from Favorites, go to

    XP: C:\Documents and Settings\your_login\Application Data\ESRI\Desktop10.0\ArcMap\Coordinate Systems

    Win7: C:\Users\your_login\AppData\roaming {or local}\ESRI\Desktop10.0\ArcMap\Coordinate Systems

    Copy to the c:\{arcgis install}\coordinate systems\projected coordinate systems folder.

    Melita
    Melita Kennedy
    Sr. Esri Product Engineer

  3. #3
    michael collins
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    Default Re: Create a custom projection

    Thanks. It's getting there. The mine grid origin at that coordinate is 0, 0, so I used those values as the false easting and northing. Still working on the rotation though. I used -18.36, that seems to be correct. It's all pretty close, I'll have to work with the mine to work it out. Still off by a few hundred metres.

    thanks for the help.
    Last edited by mcollins999; 12-02-2011 at 12:36 PM.

  4. #4
    michael collins
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    Default Re: Create a custom projection

    There isn't a "Coordinate Systems" folder in the location(s) you suggest. I checked both roaming and local. Any ideas?

    Win7: C:\Users\your_login\AppData\roaming {or local}\ESRI\Desktop10.0\ArcMap\Coordinate Systems

  5. #5
    Melita Kennedy

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    Default Re: Create a custom projection

    Quote Originally Posted by mcollinsamec View Post
    There isn't a "Coordinate Systems" folder in the location(s) you suggest. I checked both roaming and local. Any ideas?

    Win7: C:\Users\your_login\AppData\roaming {or local}\ESRI\Desktop10.0\ArcMap\Coordinate Systems
    Hi,

    It might be that no custom coordinate systems have been created yet. Have you saved the prj definition as a favorite in ArcMap yet? Otherwise, I won't have access to the software until Thursday to check.

    Melita
    Melita Kennedy
    Sr. Esri Product Engineer

  6. #6
    michael collins
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    Default Re: Create a custom projection

    Ok, thanks. I managed to save it.

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