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I have a CADD drawing that uses a Local Datum Plane (LDP) scale factor of 0.99991199. When I add the drawing to ArcMap without taking into account the scale factor, the drawing appears to the east of where it should. Is there a way for me to enter this LDP scale factor within ArcMap?

I'm using ArcGIS 10.2.1, and the NAD 1983 HARN StatePlane Oregon North FIPS 3601 Feet Intl projection.

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Depending on which version you have, you can add translation/rotation/scale information to a CAD layer via its layer property page in ArcMap. There's an option to enable an extra transformation.

Another possibility is to incorporate the LDP scale factor (LDPSF) into the existing coordinate reference system. If it's Transverse Mercator-based, multiply the false easting, false northing, and existing scale factor with the LDPSF and use the new values in a custom coordinate reference system. If it's Lambert conformal conic-based (LCC), you can add a scale factor to the definition, plus again multiply false easting/northing by the LDPSF. In this case, you may need to define the new coordinate reference system from scratch in order to get the scale factor to show up in the LCC definition.

Here's a more complete write-up for LCC that I did about five years ago on an Esri forum.

There are 2 main 'styles' of Lambert conic (LCC):

  1. 2 standard parallels
  2. 1 std parallel + scale factor (sf)

In ArcGIS, the math will handle both, and in fact, you can conflate them to:

2 std parallels + scale factor

The math is really:

fe + sf*(LCC x) 
fn + sf*(LCC y) 

where fe/fn are false easting/northing, sf is scale factor, and LCC x/y is the 'raw' coordinates. However, for a LCC-based SPCS zone, sf is 1.0.

If the Combined Adjustment Factor (grid-to-ground) is applied to the final state plane coordinates, you would get:

CAF*(fe + (LCC x)) 
CAF*(fn + (LCC y)) 

So rewrite:

CAF*fe + CAF*(LCC x) 
CAF*fn + CAF*(LCC y) 

Due to how the UI works, you'll need to copy down the existing parameter values. Then when you're modifying the PCS, change the "projection" to something else and then back to "Lambert_conformal_conic". That will add in the scale factor parameter.

Set the sf = CAF, and multiply the existing fe/fn by the CAF for the new values.

  • Thanks, the CADD drawing uses "Oregon State Plane North Zone NAD-83 (91) Datum", so I assigned it NAD 1983 HARN StatePlane Oregon North FIPS 3601 Feet Intl. Would you agree with that? Also, I'm using 10.2.1, and I added the 0.99991199 scale factor in the layer properties like you mention, but it didn't seem to help. Is that scale box in the layer properties the same as setting an LDP scale factor? – ianbroad Jan 8 '15 at 17:18
  • Yes, using the HARN definition should match up. I think it's the same, but I'm not as familiar with CAD data and how we handle it as I should be. – mkennedy Jan 8 '15 at 23:48
  • I had put this project on hold, but now need to come back to it. I don't understand how the Combined Adjustment Factor (CAF) is calculated. Could you provide more information? – ianbroad Aug 26 '15 at 17:42
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    Good explanation here, look for plane-to-surface. A surveyor takes into account the average elevation over the site versus the elevation of the projection plane plus length distortion due to the projection. The CAF compensates for both so you get "ground" distances rather than "projection plane" distances. – mkennedy Aug 26 '15 at 17:50

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