CAD Map scale off in Arcmap

[BradHoffer]

i'm sure the answer is simple and maybe i just need more coffee but I have georeferenced a cad jpeg of a blueprint into arcmap.  Once I have digitized the features the total length of, lets call it a rectangle, measures out to 188 feet.  The CAD drawing is at 205.  I'm using a projected coordinate system and have been rattling my brain for hours.  Any advice out there?  Thanks all.

Brad

[NICOLAPEDDIS]

Are you sure your georeferenced jpg is good..??? I'm not using arcgis to georeference images because is not very good. I prefer external tools like envi, erdas or global mapper, more suitable to manage large images

[BradHoffer]

I thought the same thing. I tested that idea by just pulling the the image into arc and measure it before georeferencing and it still measure out short.

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[WesMiller]

It's far to easy to georeference an image incorrectly if quality is important, ask for the "dwg" file in a known coordinate system

[BradHoffer]

I have no known coordinate system for the dwg

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[NICOLAPEDDIS]

Brad, the coordinate system is not a problem, you can easly assign the coordinate system after you georeferenced a image, but you have to sure the initial scale of the original drawing to apply correctly the georeference

[BradHoffer]

Ok. So both drawings need to be to the same scale to georeferenc correctly then?

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[NICOLAPEDDIS]

Yes the same scale its an important prerequisite to do a good job.

After you have both files at the same scale, and you are sure the scale reflect the real space between the objects you can digitize your new objects.

You can use google earth  to see the real scale of the dwg, if you know really where the dwg come from