AutoCAD and GPS data misalign in ArcMap

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06-15-2012 07:05 AM
MarcusKaller
New Contributor
I recently converted some AutoCAD Map dwgs into shapefiles for ArcGIS 9.3.1, which is projected in NAD_1983_StatePlane_Utah_Central_FIPS_4302_Feet.  Then I copied over GPS data from the Trimble GeoXT, which has ArcPad 7.1, onto my comp and reprojected it from WGC 84 to the Stae Plane projection to match it with the shapefiles from AutoCAD Map.  Despite all of this, when I bring both on ArcMap, the shapefiles dont mesh together, instead the're far apart.  Upon further examination, their coordinate values are very different from each other.  So my question is which shapefile's coordinate sytem needs to be modified, the GPS or the AutoCAD?  And would modifying the GPS's coordinate system effect the GPS data in terms of georeferencing?  I welcome any tips and suggestions.
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5 Replies
michaelcollins1
Occasional Contributor III
How far apart are they?
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MathewCoyle
Frequent Contributor
How far off are we talking? Meters? Kilometres? Your projection sounds like a local system, do you require a grid loaded to complete a transformation? I recall in 9.3.1 you would not get a warning if you were doing a projection with missing transformation files and would simply project it using whatever default.
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MarcusKaller
New Contributor
The displacement is way off, by 2804 miles (in triangular terms, about 2793 miles north and 260 miles east.  I tried something else, which was to export the CAD features again, this time project it to the WGC 84 system which is the default setting for ArcGIS.  And still, they are far apart.  One thing that did happened was a window popped up about transformation.  Any thoughts?
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curtvprice
MVP Esteemed Contributor
The displacement is way off, by 2804 miles (in triangular terms, about 2793 miles north and 260 miles east.  I tried something else, which was to export the CAD features again, this time project it to the WGC 84 system which is the default setting for ArcGIS.  And still, they are far apart.  One thing that did happened was a window popped up about transformation.  Any thoughts?


The transformation difference between WGS84 and NAD83 will on the order of meters, not kilometers, even when you are working with projected data. My money is on incorrect coordinate system defined for your CAD data.

One thing common with in CAD files is the use of offsets to make the coordinate magnitudes numerically friendly to CAD operations. Make sure that coordinate system in ArcGIS that you have defined accurately reflects any offsets used in the CAD coordinates. If not, you can fix it by using the Define Projection tool to tell ArcGIS about any offsets / shifts.

The way to see which is "wrong" is to plot other reference data, say from Esri data and maps. If one of them is correct, and the other is incorrect, it should be pretty darn obvious.
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HardolphWasteneys
Occasional Contributor III
Marcus,

the offset you describe; 2800 miles south and 260 west of Utah puts the CAD data on the Equator in the Pacific.

That means that either you have not defined the projection in such a way that the embedded coordinates are interpreted as being in Utah State Plane with the correct false origin (lat 38.2 deg N, long -111.3 deg W; 2,000,000 false easting) or they are in a local drawing or mine grid with numbers in the 10s of thousands of feet and the proejction has placed them just that; a few 10s of thousands of feet north of the equator.   With that offset placing them on the equator the effective false origin of the projection is zero north by some value of longitude other than zero or -180, so it looks more likely to be the  UTM zone for western Utah that you have projected them into.  

I expect that you will have to georeference the CAD data; not project it.   For this you use only 2 points and otherwise it works just like a raster georeferencing job.


Hardolph
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