Exporting a DWG to State Plane When Utilizing Transformed Local Coordinates From SPC Data

[EVANV4NUZD]

Our survey team collects field data in SPC format and the data in brought into C3D through a transformation to scale the drawing to ground distance and to replace the SPC rotation with geodetic north. Our drawing settings always have the correct SPC system and zone selected and the drawings are georeferenced even though the coordinates are transformed to a local ground system. More and more, the city is requesting that we supply them with DWG files that are in SPC grid format. What is the best method to provide a drawing in SPC grid once it has been transformed to a local ground system?

Thanks!

[Pointdump]

Hi Evan,
There has always been a disconnect between the Civil 3D side and the Map 3D side. Using the Transformation Tab cements that divide.
I guess the best method would be to work in State Plane Grid or use a custom low distortion projection.
Dave

[EVANV4NUZD]

Thanks Pointdump, 

We have been utilizing the transformation workflow for 5 or 6 years now and it has worked well for us, outside of the issue that it is difficult work around to convert the drawing back to SPC. If we are unable to find a real way to do this, we will need to adopt a new workflow. There is absolutely no way that we could operate by drafting in SPC grid in cad due to the difference in scale between grid and ground. We could work with the SPC rotation as I believe we could easily re-align that to geodetic north with a rotated viewport. If our points come into the drawing SPC grid, how do we scale everything to ground without using a transformation? Maybe there is a way to export our data from the data collector as SPC ground, not grid? We commonly have to supply SPC grid coordinates for monuments so if we loose that connection to grid, it would make that aspect more difficult. 

Unless there is a workflow that actually works, Autocad needs to face the reality that surveyors need the ability to toggle our drawings from local ground to SPC grid. 

[Pointdump]

Evan,
"...how do we scale everything to ground without using a transformation?"
You can use Expressions in your label style to deal with this scale.
Every Projected Coordinate System has convergence. That's why most Records of Survey use a Basis of Bearing. If you wanted to use "True North" bearings, C3D labels have that in label styles.
Dave

[EVANV4NUZD]

The convergence angle and the scale factor are very different in how difficult it is to manage. The convergence angle is easily managed and we could even work with SP rotation as long as it was listed as the basis of bearing. Where we run into trouble is the scale. It would tolerable to draft in grid for a boundary survey but for mapping projects that will be used for design, you are opening a huge can of worms by not having true ground distances in the model. Everything drafted in that environment would need to be manually adjusted to grid. Everything from right of way widths to minimum acreage would be affected. For a while, the the department of transportation was filing plats that were in grid, excepting the width of the right of way that was represented in ground. Several years back, the requirements for filing a plat were modified so that grid distance could not be listed, only ground. What a pain to use those surveys that actually list grid distance, or a combination of the two!

Please speak up if you are a surveyor and you are actually drafting in a SPC grid environment. I would really like to learn how you are able to avoid all the obvious pitfalls!

[Neilw_05]

I haven't read through all the details in this thread, but I think the easiest solution is to explode everything down to plain ACAD objects, then use a Map Query to transform the linework into a dwg set up with SP grid coordinates. Assuming it is for integrating the data into a GIS, they wouldn't need the C3D objects to be intact.

[EVANV4NUZD]

I agree that this would be a good way to get the drawing in SPC format for export to the City, County or other agency. We have not been able to get this to work when the drawing utilizes a transformation to convert SPC grid data into a local ground coordinate system. We use the transformation on 99% of our jobs with the other 1% being a local assumed coordinate system. 

[Neilw_05]

I am no expert on this topic but I have dealt with DOT projects that use a grid to ground scale factor. Our workflow is to do all surveying and design in ground coordinates in that scenario. If you need to integrate data that is in grid coordinates, you can set up a custom coordinate system that will automatically make the transformations when the data is imported or queried.

In general we do all our projects in grid coordinates since the difference to ground is minimal. From a construction standpoint, the difference is just adjusted as needed at the tie-ins. It rarely becomes an issue.

[rl_jackson]

I've used the Transform tab quite often to scale data from Grid to Ground. When I do my transformation, I usually make base point = SPC grid value and then everything scales from that point. This way the local is grid @ ground. and the drawing should actually report both coordinates, ground N/E which would be Local and then Grid N/E.

I would assume that in your case you're at high elevation which affects distances quite a bit. For me it's very minor and I'm usually below the grid.

[EVANV4NUZD]

Yes, I am located up in the Rocky Mountains so the difference in scale from grid to ground equates to .01' every 10 feet. You would definitely end up in trouble if you failed to use ground coordinates for anything other than a very small survey. 

[EVANV4NUZD]

So far, I feel that the best solution is to do what I am currently doing when I need to convert our transformed drawing into SPC grid. This is truly a work around which is necessary because C3D lacks the ability to do this, even though the drawing is actively in the correct SPC format and is geo-referenced. Its crazy that C3D will give you SPC grid and local ground coordinates for all points but for some reason is unable to export or depict other drawing data in SPC format.

WORK AROUND TO CONVERT A TRANSFORMED LOCAL GROUND DRAWING TO STATE PLANE GRID:

1. Start a 2nd drawing and select the coordinate system under Units and Zone on the drawing settings tab.

2. In the transformed drawing, draw in a baseline between points that are on the perimeter of the drawing area and record the SPC coordinates of the points for both points using the Edit Points display.

3. In the new drawing, draw in the same baseline using the previously recorded grid coordinates. This is done by Create Line by Grid Northing/Grid Easting.

4. Use the Copybase command to Copy all data from transformed drawing and then paste it into the new drawing making sure to not have it overlying the previously drawn baseline. 

5. Create a block from all the pasted drawing information

6. Change the color of the baseline in the new drawing so that it differs from the color of the baseline in the newly created block.

7. Use the move command with one end of the baseline as the base point and slide the drawing block onto the SPC grid baseline.

8. Use the rotate command to rotate the drawing block so that both base lines are aligned.

9. Use the scale command to scale the block by using the SPC grid baseline as the target length.

10. When the block is moved, rotated and scaled, verify that the data is geo referenced correctly by toggling on Map, Aerial.

11. Explode the block to convert all data back to its normal form. 

12. Delete the baselines and verify that other points in the drawing (outside of your baseline points) match the published SPC coordinates in your original drawing.

I was very hopeful that the coordinate transformation tool which was added in V22 had solved this flaw but we have found that tool to be worthless as it only transforms civil objects, not linework. It also will not let you transform to the same SPC zone, apparently because it is confused by the transformation to local ground.