Custom coordinate system with a scale factor

[Anonymous]

I am trying to create a custom coordinate system definition that is based on the parameters of Washington State Plane South Zone (NAD83) but with a scale factor included.  The problem is, scale factor is not an option for Lambert Conformal Conic Double Standard Parallel projections.  Scale factor is only a parameter with Single Standard Parallel projections.  I can do this in ArcGIS with a custom .prj file that uses two standard parallels and a scale factor and it works beautifully for what I'm trying to accomplish.  Is there any way to circumvent the Create Coordinate System wizard to be able to do this?  Like with ESRI .prj files you can open them in a text editor and edit the WKT.

Thanks,

Ryan

[Pointdump]

Ryan,

I'm not sure why you would want to scale a published coordinate system, but if you scale a State Plane coordinate system, you won't have State Plane coords anymore. And that makes it harder to use with GIS. As for using two standard parallels, consider this from Michael Dennis, RLS, PE:

"In nearly all cases, a two-parallel LCC should not be used for an LDP with the NAD 83 datum definition (but note that some software may not support a one-parallel LCC). A two-parallel LCC should not be used because the reason there are two parallels is to make the projection secant to the ellipsoid (i.e., the central parallel scale is less than 1). This is at odds with the usual objective of scaling the projection so that the developable surface is at the topographic surface, which is typically above the ellipsoid, particularly in areas where reduction in distortion is desired."

If you really need two standard parallels, use a central parallel scale of less than 1.

Dave

[Anonymous]

Thanks for the reply, Dave.  Why?  Because our engineers' drawings use "modified" State Plane coordinates (i.e. not really State Plane coordiantes at all).  All the State Plane coordinates in a .dwg multiplied by a scale factor.  Good, bad, or indifferent, it has been done that way for years and I have no control over that so I'm just trying to get their drawings and our GIS data to be copacetic.  You're right it makes it harder to use with GIS.  But if I could do what I'm asking about here, it would become a non-issue.  If the program knows what coordinate system is being used, even if it is a wonky "modified" State Plane LCC double standard parallel with scale factor coordinate system, then it can properly transform between other coordinate systems.

[Pointdump]

Ryan,

Looks like you need to have a little chat with your engineers about their horrid coordinate system practices. Until then, you could scale the coords in excel first(or Xref in a scaled ACAD drawing) and then use your two-parallel Lambert. Awkward, but perhaps do-able. I haven't used a two-parallel Lambert yet, but it probably won't allow a scale factor because that would change the two standard parallels.

Good luck to you.

Dave

[Anonymous]

As I said in the original post, no AutoCAD doesn't allow a scale factor for a double parallel LCC.  As you understand, there is good reason for that.  I just wanted to try asking here to see if anybody knew a way to get into the nuts and bolts and force a scale factor into a custom LCC 2 parallel system.  Like I can do in ArcGIS.  It is not looking possible though.  Thanks for the ideas.

[fcernst]

We have to use ground not projected coordinates to get true lengths for design. It's standard industry practice.

[Pointdump]

Fred,
"We have to use ground not projected coordinates to get true lengths for design. It's standard industry practice."
OK, so what do you do? How do you customize your Lambert Conformal Conic, Double Standard Parallel Colorado Systems? Even if you move the Parallels together you'll still be way under your ground elevations.
I'm also curious, what possessed you to dig up this 5-year_old thread?

Dave

[fcernst]

I was seeing if there was anyway I could use the scale factor to get Civil 3D's Bing imagery to line up behind the Modified state plane coordinates.

I'm not sure how the surveyors calculate their scale factors and rotation for Modified state plane coordinates.

[Pointdump]

Fred,
"...use the scale factor to get Civil 3D's Bing imagery to line up..."
Well...
Option #1: Transformation Tab. Unfortunately, none of the Map Commands work with it. And it usually breaks Bing. (Not always, but usually.)
Option #2: Custom Low Distortion Projection with a Lambert Conformal Conic, Single Standard Parallel. This will allow you to use an Elevation Scale Factor. (Scaling up the Ellipsoid.)
Option #3: Use a Lambert Conformal Conic, Double Standard Parallel with a Custom Scaled-up Ellipsoid. This will also require a Custom Datum. I think this will work, but not entirely sure.
Option #4: Use something completely different, and custom, like Colorado's TREX Rectified Skew Orthomorphic. I'll bet you'd impress your clients with this one!
Dave

[ChicagoLooper]

A modified state plane coordinate system sounds exactly like ground-to-grid units where the process to go from ground  to grid (or vice versa) entails a scale factor. Do you have a Control Sheet given to you by the surveyor? Or the scale factors provided by your local DOT?

Was your data was collected in  Bentley Microstation or something akin to microstation? Does your drawing have anything to do with road work to be performed for your local DOT? 

If you typically use Washington SPCS then you are correct, your state is wide (the distance west to east is greater than north to south) and does use a NAD83 Lambert projection.

I agree with @Pointdump to stick with a known CS. Using a known CS is an appropriate workflow because you want to tweak an existing CS, not create a new one from scratch.

Watch this >>VIDEO<< by Alan Gilbert. He provides an excellent step-by-step procedure that appears applicable to your post..

The key is the scale factor. To get the scale factor you may have to contact your local DOT to get it if the surveyor is unwilling or unable it to you.

[fcernst]

In our region of work, the surveyor always provides the scale factor and rotation in the notes on the topographic survey so it can be related back to state plane.

[Pointdump]

Fred,
>>>That Video<<< by Alan Gilbert that @ChicagoLooper mentioned might just work to get you Bing in a Ground System. Can you post a drawing and Surveyor Notes? I'd like to test it.
Dave

[Pointdump]

More reading on this interesting Affine Post-Processor Definition:

@georg.grebenyuk posted this well-detailed AKN Article >>>Here<<<.
Ken Martinez held an Autodesk University Class, archived >>>Here<<<.

(Attached is the Ken Martinez Class PDF)

[Pointdump]

Fred,
Scaled to Ground Distances, Translated, Rotated, and Bing Live Maps works. Even better is the easy back-n-forth from Grid to Ground and GIS Data Import is no problem. All Map Commands work. The Affine Post Processor Transformation kicks the Transformation Tab up one side of the street and down the other.
I had to make up my own "project" by grabbing some control points from City and County of Broomfield, Colorado >>>Here<<<:

1,1213828.578,3121140.553,5342.041,GPS1
2,1214905.232,3133290.135,5230.303,GPS2
3,1229681.391,3128027.335,5387.329,GPS3
4,1217166.133,3127439.219,5310.008,GPS6
5,1219688.469,3125278.788,5336.150,GPS7
6,1209293.652,3120186.565,5354.244,GPS8
7,1215836.576,3114930.796,5390.831,GPS9
8,1217194.231,3109158.504,5448.484,GPS10

I Translated and Truncated those points with an Affine Post Processor like >>>This Video<<< and >>>That AU Class<<<. Then I Rotated everything about 33 1/2 degrees and used those new points to do some Matrix Calcs in Excel, with help from Egor @georg.grebenyuk and his AKN >>>Here<<< to determine my A0, A1, A2, B0, B1, and B2. Tested and working. Attached, please find 2 drawings, 3 point files, and Excel file.

Dave

[gjeppesen]

Fred,

I'm new to this whole Colorado State Planes thing.  I'm from Florida where we didn't have to worry about it.  So what's the workflow?  Do you get a Modified State Planes survey already at ground before you start designing?  Or do you design in normal Colorado State Planes and then convert prior to construction.

I guess I don't understand the workflow and all the local GIS info, DRCOG aerials, etc. are all in standard State Plane.

Thanks.

[ChicagoLooper]

At a minimum you'll need:

1. the combined scale factor
2. north reduction factor
3. east reduction factor 
4. the coordinates of a control point(s)

Items 1, 2 and 3 vary from site-to-site, this means the factors are not one-size-fits-all. They are typically noted on the survey drawing or provided by the surveyor. (If you don't have these factors, then you'll need to acquire them from the surveyor. Without these factors, then someone else will have to create this 'custom coordinate system' for you.)

You will need to change Lambert Conformal Conic Double Standard Parallel to Lambert Conformal Conic, with affined post-processor. The best way to make this change is to make a copy of the former then edit the copy to create the custom CS. If you have never worked on creating a custom CS then this will be a sophisticated procedure, not a simple one. If done correctly, a new 'custom coordinate system' will be added to the coordinate system library. All coordinate systems in the library have WKTs and this newly created one won't be an exception.

If you're serious about creating a custom CS, watch this >>VIDEO<< by Ken Martinez. It demonstrates the procedure. 

[Pointdump]

Greg,
Attached is a PDF from a much-missed Colorado Surveyor about the subject.
Dave

[ChicagoLooper]

Modified State Plane Sounds like your coordinates are ‘adjusted’ by a combined scale factor to transform the coordinates from ground coordinates to grid coordinates. 

Your survey parameters (scale factors, truncation values, benchmarks, basis of bearing, etc.) should be noted on the survey itself. These parameters are what you use to create a Custom Coordinate System or the Modified Sate Plane. You will add this modified coordinate system to Map3D's coordinate system library, and once it has been added, you can retrieve it in order to assign it to modelspace.  

[Anonymous]

The practice is far from "horrid". A combined scale factor is used to minimize length distortion over large areas. Without the scale factor, actual lengths of pipe needed or quantities of dirt to be moved, for example, could be substantially different from what is calculated using a strict state plane system.