Need DWG modified to ground to display correctly in ArcMap

Question

I have a DWG with these notes:

1. All coordinates are ground & adjusted to NADCON, +- 12 to 18 cm.
2. NAD 1983, NAVD 1988
3. Combine Factor = 0.99969305
4. Benchmark used NGS J-432, Published Elevation 5129.15
5. NM State Plane, Western Region

I want to bring the file in to an existing ArcMap project where the Data Frame coordinate system is NM State Plane West NAD 1983 feet. I have tried to transform the file by adjusting the scale under the layer's Transformations tab but the points aren't moving where I know they should be located. Specifically, I click the Enable Transformations check box, click the Rotate, Scale, Translate radio button, and enter values of 0.99969305 or 1.00030695 in the Scale box. (I have tried both numbers.) What am I doing wrong? I've done it in ArcMap before but it's been a few years and obviously I am missing something.

Aaron Allen
Albuquerque, NM

TomTeater · Answer

You can also apply the scale factor in AutoCad first, then import the dwg file into ArcMap. Use the 'scale' command in AutoCad to do this. I've found that this is much easier than creating a custom projection file in ArcGIS.

MelitaKennedy · Answer

I have a DWG with these notes:

1. All coordinates are ground & adjusted to NADCON, +- 12 to 18 cm.
2. NAD 1983, NAVD 1988
3. Combine Factor = 0.99969305
4. Benchmark used NGS J-432, Published Elevation 5129.15
5. NM State Plane, Western Region

I want to bring the file in to an existing ArcMap project where the Data Frame coordinate system is NM State Plane West NAD 1983 feet. I have tried to transform the file by adjusting the scale under the layer's Transformations tab but the points aren't moving where I know they should be located. Specifically, I click the Enable Transformations check box, click the Rotate, Scale, Translate radio button, and enter values of 0.99969305 or 1.00030695 in the Scale box. (I have tried both numbers.) What am I doing wrong? I've done it in ArcMap before but it's been a few years and obviously I am missing something.

Aaron Allen
Albuquerque, NM

Hi Aaron,

btw, when I inverse your scale values, I'm not matching. My calculator reports 1/0.99969305 is 1.00030704 while 1/1.00030695 is 0.99969314.

Anyway, another way is to incorporate the combined scale factor (CSF) into a custom projected coordinate system. What you'll need is the CSF that converts to ground.

You multiple the false easting, false northing, and scale factor values by the CSF to create new projection parameter values. Thus, the new pcs reflects the 'ground' coordinate system.

I would suggest modifying the pcs name to reflect that the parameters are adjusted.

Melita

:cool: 
Read this section if you're interested in the math:

When a projection is done the final values are calculated by:

X_grid = false easting + sf*(bunch o'math)
Y_grid = false northing + sf*(bunch o'math)

where the other projection spheroid parameters are used to calculate the 'base' XY values.

A CSF is applied to the final values:

X_ground = CSF*X_grid
Y_ground = CSF*Y_grid

substituting in:

X_ground = CSF*(false easting + sf*(bunch o'math))
Y_ground = CSF*(false northing + sf*(bunch o'math))

or

X_ground = CSF*false easting + CSF*sf*(bunch o'math)
Y_ground = CSF*false northing + CSF*sf*(bunch o'math)

So we get:

new false easting = CSF*false easting 
new false northing = CSF*false northing 
new scale factor = CSF*sf