Ground to Grid Coordinate Conversion

using AutoCAD Civil 3D



Why do I need to convert 'Ground' to 'Grid.'


When a surveyor goes to a site to take measurements, he might use a number of different methods to find 'North.' His north can be Magnetic North, True North, Grid North, or an assumed basis of bearing. Additionally, Grid data are converted for Earth's curvature, while Ground measurements are planar. If your survey data need to interface with georeferenced data -- often when you're interfacing to a GIS -- then you will need to convert from the Ground measurements to Grid data.

How to do Ground to Grid conversions with AutoCAD Civil 3D

Before you can apply the Ground to Grid conversion factors, you must assign a coordinate system to the drawing. Edit Drawing Settings and open the Units and Zone tab.

Under Categories, all US state plane categories will be found under USA, <>.

Available Coordinate Systems, make a selection. For example, Calif State Plane Zone 2, US Foot.




After setting the coordinate system, you can define your ground to grid conversion factors. Move to the Transformation tab.

Here you first turn on the dialog by putting a check next to Apply Tranform Settings and then enter your conversion factors.

When you go to import or exports points, notice the options to Do elevation adjustment if possible and Do coordinate transformation if possible. If you check these boxes, your data will be transformed to your assigned coordinate system using the factors you defined.

When you list points in Civil 3D, there are columns for Grid Northing and Grid Easting. These data columns can be also be accessed in the Point Editor, as content within Point Label Styles, and as point export columns.

Reducing memory usage with Civil 3D

Using Civil 3D, memory management is very important. There are techniques you can utilize that will optimize your system resources by minimizing your memory use.

Indexing a reference drawing


Turn on Demand Loading using XREFCTL. Either set it to 1 or 2. 1 means that your Xref's are loaded and locked in their home path; 2 makes a copy of the Xref on your local workstation and that copy is loaded and locked, still allowing others to open and edit the Xref drawing.


INDEXCTL (stands for Index Control) is an AutoCAD command, used with the reference drawing open, that will index your drawing objects by layer and/or spatially. The advantage is that when you reference this drawing into another, objects that are on frozen layers will not be loaded into memory. Set INDEXCTL to 3 in the reference drawing to turn on both layer and spatial indexing.





XCLIP is the command you use, inside the container drawing, that will not only clip your Xref visually, but will also reduce its memory footprint by not loading those objects that lie outside the XCLIP boundary. To utilize layer indexing, simply freeze Xref layers in the container drawing. Be sure you leave VISRETAIN set to 1.


Civil 3D Surface Data Clip Boundaries



To reduce the memory footprint of a Civil 3d Surface, use the 2009 version's new Data Clip Boundary feature. If your surface data area is larger than the area of concern for your project, you can benefit. Just draw a polyline to delineate the extents of your project.


The Data Clip Boundary must be the first data operation in your surface properties, so you would normally apply this boundary before adding other data. You CAN go back and move the data clip boundary to the top of the build sequence later, but it has to be on top or it won't be applied to the surface.
















To add the boundary to a surface that has just been created, on the Prospector tab, go to Surfaces. Find and expand the newly created surface, expand Definition, right click Boundaries and select Add...

















Set the boundary Type to Data Clip, press OK, and select the polyline that represents your data clip.

From here, create your surface as you normally would. The only difference is that any data outside the data clip boundary will be completely ignored and will not be loaded into memory when Civil 3D loads the surface. This feature will save you time and reduces crashes.



Saving memory when using Feature Lines



If you extensively use feature lines, knowing that they are continually looking at one another to detect crossings will help you understand why it can be helpful to place feature lines in different Civil 3D Sites.

When you go to edit, offset, move, or otherwise change a feature line, all the possible crossings of all feature line segments in that site must be checked by the program and that takes a lot of memory. Moving a feature line to its own site, can make functions that take many minutes take only seconds.



To move a feature line to another site, in the drawing window, select the feature line, right click, and select Move to Site

That's the end of Tim's first Civil 3D Blog entry. More to come...