Monday, 23 July 2012

Using Civil 3D to Convert Drawing Coordinates

Thanks to Mark Green of Amicus Technology for the following post....

The example used here converts a drawing from Irish National grid (IG75) to the newer ITM grid.

Setting the coordinate system of the Irish Grid drawing
1. Open the drawing you wish to convert from IG75 grid to ITM grid
2. At the command prompt type ADESETCRDSYS and Enter
3. In the Assign Global Coordinate System dialog box click Select Coordinate System
4. In the Select Global Coordinate System dialog box select Eire-Ireland from the Category drop-down list
5. Under the Coordinate Systems in Category list, select Ireland1965.ING Irish National Grid referenced to Ireland 1965 (note that this is IG75 grid)
6. Click OK and OK again
7. Save and close the drawing

Setting the coordinate system for the ITM drawing
8. Start a new drawing using the Acadiso.dwt template
9. At the command prompt type ADESETCRDSYS and Enter
10. In the Assign Global Coordinate System dialog box click Select Coordinate System
11. In the Select Global Coordinate System dialog box select Eire-Ireland from the Category drop-down list
12. Under the Coordinate Systems in Category list, select ITM Irish Transverse Mercator 1995 with ETRS89 Datum:Ireland
13. Click OK and OK again
14. Save the drawing with a new name e.g. project123-ITM.dwg

Converting the IG75 drawing to ITM grid
15. At the command prompt type MAPWSPACE and Enter twice. This opens the Map TaskPane
16. In the Taskpane palette select the Map Explorer tab.
17. Under Current drawing, right-click on Drawings and select Attach
18. In the select Drawings to Attach dialog box, click in Look in to navigate to the folder with the
IG75 drawing
19. Select the drawing, click Add and click OK
20. Under Query library, right-click on Current Query and select Define
21. In the Define Query of Attached drawings dialog box, under Query Type, click on Location
22. In the Location Condition dialog box, under Boundary Type, select All and click OK
23. Under Query Mode ensure Draw is selected and click Execute Query
24. The drawing objects from the IG75 drawing should now appear but with ITM coordinates (check Grid Inquest program to verify)
25. Save the drawing.

Footnote: There is a common misconception that the Irish national grid system in common use is based on the 1965 adjustment. In fact this was only used for a short time and the readjusted 1975 network, referred to as IG75, is the basis for the pre ITM grid system. This misnaming appears in Autodesk MAP 3D and Civil 3D and MapInfo software among others. The difference between the 1965 and 1975 grids is up to approx 13 metres.

In MAP 3D and Civil 3D it is essential to select the coordinate system named
Ireland1965.ING as this is the correct 1975 grid while the coordinate systems named TM1965Irishgrid and TM1965Irishnationalgrid are the 1965 grid.

Wednesday, 18 July 2012

Grading Between Corridors

In the same project described in my last post there were two access roads that were close to each other but at very different levels. The embankments between the roads will grade from the back of the verge of one to the back of the verge of the other and vary in slope. See below:
This project is in its early stages and the design is bound to change. We need to 'tie' the two corridors together dynamically so that a change to either road will be reflected in the grading between them without having to manually make these changes each time.

The first step is to create the corridors for both roads. Then extract a dynamic featureline from one of the corridors for the relevant tie in point. In my case I am extracting the back of the verge for the higher road. See below:
Make sure to tick the 'Create Dynamic Link to the Corridor' box and then select the featureline you want from the corridor. This creates a featureline representing the back of the verge which we can now use as a target for our lower corridor earthworks.

Modify the lower corridor assembly to include a LinkwidthandSlope subassembly (find it on the generic tab of the toolpalettes - CTRL+3).
No in our lower corridor we can use this subassembly to target the extracted featureline from the higher corridor.
This creates dynamic grading between the two corridors.
If the design changes for either access road the grading between the two will update automatically.

Creating Profiles using Transparent Commands

I have been giving some customised Civil 3D training recently for site development works - mainly on creating access roads and grading for car parks on an extension to an existing factory.
The project data consisted of a topo survey from which we generated our existing ground surface. The  proposed access road layout drawing is in AutoCAD format with some proposed levels entered as plain AutoCAD text, see below:
The proposed access road centrelines were in polyline format which we could quickly turn in to C3D alignments using the 'Create Alignment from Object' command. We then created a profile along this alignment and sampled the existing ground.
We then needed to draw the proposed road profiles using the levels from plan. This is where the transparent commands came into play. Transparent commands are commands that you use while in another command to easily provide the first command with the information it needs. For example when we are drawing our profile we will be picking IP points on profile that will have chainage and elevation values. In our situation the level values are noted in plan alongside our alignment. We can use the 'Profile Station from Plan' transparent command to allow us to pick these points from plan and enter the elevation when we are drawing our profile. 

Start to draw the profile as normal using the profile creation tools. When prompted to pick your start point select the transparent command from the toolbar on the right of your screen as shown below:
The command prompts you to select a chainage from plan....
Then it allows the user to enter an elevation manually which we have in text format and is corrected here to provide a crossfall...
Continuing with this procedure along the entire road allows us to very quickly create our proposed profile.

There are many more useful transparent commands on this toolbar which I use regularly, definitely worth checking out.