**Defining a dataset in a surface state plane coordinate system**

April 06 2011 |
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Categories:
ArcGIS Methods,
Map Data

I am working with a CAD file that was created using the North Central Texas State Plane Surface Coordinate System. If I were to define this dataset in ArcGIS, what would be the equivalent coordinate system?

### Mapping Center Answer:

We could use a little more information from you but we will take a stab at this anyway! From what you have stated there are two possibilities that we can determine. The first is that you are just asking where to find the State Plane definitions for the Texas North Central zone. They're all in the Projected Coordinate System > State Plane folder. You can find definitions there that will be based on which datum/geographic coordinate system that the your data are in and what units are being used (meters or U.S. survey feet).

The second possibility (and more likely) is that the data uses a modified State Plane coordinate system (we are guessign this becuase you used the word "surface" in the name). If this is so, then it is probable that a scale factor has been applied to the data to reduce the distortion due to the projection and due to the fact that the projection is based on the datum's surface "elevation" and not the average elevation of the area in question.

For this latter case, we would need to know the scale factor. Sometimes it's called the "combined scale factor".

Because Texas State Plane zones use the Lambert conformal conic projection, it gets a little awkward. We've included a example here based on NAD 1983 with meters as the linear unit.

Let's say the combined scale factor which converts from State Plane to the local surface is 0.9999354. You're going to modify an existing NAD 1983 State Plane Texas North Central (meters) definition. You can immediately check whether the modified coordinate system is correct by adding the CAD data (with an undefined coordinate system) to ArcMap with some reference data (data for which you already know the correct projection has been defined). The reference data must have a defined coordinate system, but doesn't need to be in the State Plane coordinate system. You will modify the data frame's coordinate system which will caure the reference data to be projected on-the-fly (in-memory) to the modified system. If it lines up with the CAD data, then use the data frame's coordinate system to define the coordinate system of the CAD data.

To do that, right click the data frame in the Table of Contents and click Properties and click the Coordinate System tab. Select the appropriate State Plane zone. You'll need to have a copy of the parameter values (see below).

Modify the definition. Change the projection name from Lambert_Conformal_Conic to something else to identify for yourself that you have modified the original projection. Now change it back. The parameter values will be reset, and you'll have one additional parameter: Scale_Factor.

Projection Name: Lambert_Conformal_Conic

Origin Parameters:

False_Easting = 600000.0

False_Northing = 2000000.0

Central_Meridian = -98.5

Standard_Parallel_1 = 32.13333333333333

Standard_Parallel_2 = 33.96666666666667

Latitude_Of_Origin = 31.66666666666667

Input the same values for all of these parameters, but update the Scale_Factor parameter to 0.9999354. At this point, click OK for all the dialogs. It's possible but not likely that the reference data will overlay the CAD data. If it does, you're done. If it doesn't, modify the data frame's coordinate system again.

Adjust the False Easting and False Northing by multiplying the original values by the combined scale factor:

False_Easting = 599961.24

False_Northing = 1999870.8

Click OK for all the dialogs again and check the overlay of the reference data with the CAD data.

Sometimes the combined scale factor (csf) is given for the opposite conversion---multiply the surface coordinates by the csf to calculate State Plane coordinates. If this is the case, use the inverse of the above steps.

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