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Defining projection of Shapefile and CAD

09-09-2011 07:18 AM
New Contributor
Hi everyone,

I have a Shapefile that has been created from CAD file (using Surfer) with a local coordinate system.
Basically this local coordinate system has the same characteristics of the OSGB 1936 but it is defined in meters and the coordinates are shifted as follow:
the x coordinates are shifted by -500000
the y coordinates are shifted by -200000

For example:
In the local coordinate system I have:
x=124600 m
y=107400 m
whereas in the OSGB 1936 the same point will be:
x=624600 m
y=307400 m

When I import the shapefile into ArcGis of course it results that it doesn´t have a coordinate system.

Now what I want to do is:

1) shifting the shapefile so that it will result aligned with the OSGB 1936 System. I would like to perform this task with ArcGis (although I know how to do it with other software). Is it possible?

2) Attributing the OSGB 1936 projection to the Shapefile.
Now what I tried to do is shifting the original cad map in surfer so that it was perfectly aligned (as I said I would like to learn how to do this operation with ArcGis), then I exported the cad map as shapefile. The units were meters.
In ArcGis I then defined the projection of the Shapefile with the Define Projection tool but when I reopened the file I got this error message:

Probably because my original CAD file was in meters while the OSGB 1936 System works in feet (is it right?).
How can I work around this problem? I found the help quite a bit confusing. I can see that my shapefile has unknown units but I don't know how to transform them such that they will result in feet, as the OSGB 1936 System.

I would really appreciate any help since I am really struggling with this urgent problem.

Thank you very much.

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2 Replies
Regular Contributor II
You know all about the coordinate system of the data. It is based on a system you know with modifications you also know. Most of the battle is already won.
Simply define your data in the coodinate sytem you know it has, even though you have to create the system yourself.
Create a new coordinate system definition, based on the known one, with the appropriate modifications (meter units, custom false origin), and give it a new, appropriate name. Asign that new definition to the data set.
You should then be able to project it into any other defined system you fancy, or project other data into that custom system.

Mark Denil
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New Contributor
Thank you so much Mark, I sorted everything out.
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