I converted Nad27 drawing to NAD832011 in cad and compared the results to NCAT.?ÿ
X values show a difference of 0.09 and Y of 0.24.?ÿ ?ÿ
Anyone else run into this issue??ÿ?ÿ
It would help people to comment if the type of CAD was specified.
There was enough error and distortion in NAD27 that I'm not surprised if different implementations of a conversion disagreed a tenth or two. What does each implementation hold while it interpolate to your points?
Autocad Map 3d 2019
What transformation model is being used in Autocad Map 3d 2019??ÿ
This may be slightly off topic but.?ÿ Beware of what version of NAD83 you tell Autodesk you're in.?ÿ At least for reprojections, my experience has been that four of the five realizations of NAD83 add their own transformation into the equation.?ÿ For the life of me I can not figure out how to view those parameters.?ÿ I'm sure this applies to the actual transformation from NAD27 to NAD83.
The other thing to remember is that there is no true mathematical transformation between the two systems.?ÿ Both conversion are completed with type of affine transformation or lookup table.
NCAT uses a transformation grid, but I am not familiar with what is used in Autocad. Maybe it is using a 7 or 14 parameter transformation?
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Wow!! I've never seen it get that close, it's usually more like .5' to 1'.
A parcel I plan on staking was a boundary agreement between 2 parties which was settled in court (1957).?ÿ An agreement was reached with NAD27 Coordinate values set as the boundary.?ÿ No survey after the agreement or record of a survey.?ÿ Guess ill use the NCAT coordinates if cant find any monuments.?ÿ
@cordgrass NCAT and previously NADCON are the official transformation between NAD27 and NAD83. This software uses a table of lookup values between known points and then interpolates. It definitely will make a difference as to which realization of NAD83 you are tying to get to from NAD27. I have spot checked the transformation on NGS points that had values published in NAD27 all the way to current NAD83 and it is exact on those points so I assume they were used as input values in the lookup tables.
I have also used NCAT to get from RTN positions in NAD83(2011)(2010.0) back to the HARN, in tat case NAD83(91) and pretty good comparison with record values, sub 0.1'.
I would consider the NCAT to be official for checking other software.
Of course if you want better values, create a transformation if you can find enough old NAD27 points that are solid. Even a couple might work to do a XY shift from the NCAT values, localizing NCAT if you will.
SHG