As you may know, AutoChart uses the geodetic functionality of AutoCAD MAP/Civil 3D to perform any geodetic calculation (such as drawing Grids and Graticules, calculation Geodetic distances and Kps or for coordinate transformations). AutoCAD has a multitude of pre-defined coordinate systems but sometimes (particularly with some obscure offshore areas) the correct definition will have to be made by the user to suit the project geodetic parameters.
Setting up a Coordinate System is a relatively easy exercise once you understand how AutoCAD defines them, although at first impression the process can appear a little daunting. We are always happy to set up a coordinate system definition for our customers given the appropriate geodetic information and a trial point. It is however a good idea to try and familiarise yourself with the process, even if it is just to check the pre-defined coordinate systems to find the one with the correct parameters.
To Define or Check an AutoCAD MAP Co-ordinate System, use
MAP->Tools->Define Global Coordinate System.
The Global Coordinate System Manager is displayed.
The Category Manager will display the various Categories of Coordinate Systems (in effect the various ‘containers’ dividing the various systems into type/country).
When creating a new CS it can be hard to find the appropriate CS you have made so it can be a good idea to use the Category Manager to Create a New ‘Container’ holding your user defined parameters.
Use the Category drop down list to select the appropriate place to store the CS definitions.
In the Global Coordinate System Manager dialog, clicking on the Define (or Modify) button will bring up a Dialog where the CS parameters can be entered.
On the General Tab, the Code should be entered in such a way as to indicate what the CS represents and a description should be added to give further information on that CS. Again, it can be useful to use an identifier you are familiar with to help locate the correct CS during AutoCAD/AutoChart operations. Select the Units for the CS appropriately.
The Coordinate System Type should be chosen as Geodetic (select or define the Datum) or Non Geodetic (select or define the Ellipsoid). In either case, you can click on Select or Define and view change or create the appropriate parameters.
Clicking on Geodetic ->Define brings up the Datum Manager where you can check the parameters in use for defined datums or create a new Datum definition.
Again, Clicking on Define or Modify will allow you to change an existing Datum definition or Define a new one.
On the General Tab in the Modify/Define Datum Dialog, again enter an identifier Code and Description and select or define the appropriate Ellipsoid (the parameters of which can again be checked or defined using the Select or Define button).
On the Datum Conversion Tab, select the Appropriate Datum Conversion Technique and enter the Conversion Parameters FROM the Datum TO WGS84. Note for pre-defined Datums you can view the parameters they have been set up with.
With the correct Datum defined, you can click ok to get back to the Modify Global Coordinate System Dialog and select the Projection Tab to set up the appropriate Projection parameters.
The Projection Type can be chosen from the Projection drop Down List. According to the Projection Type selected the Parameters section will change so that the appropriate values can be entered.
Note, for a Latitude/Longitude only CS, the Projection Type should be Set to a Unity Projection and the unit type changed accordingly on the General Tab.
With the Projection CS and Geographic CS set up correctly, these can be selected in the various AutoChart commands. The conversion from WGS84 to the Projection in use can be checked with a known point if available and using the AutoChart point ID command with the appropriate Project projection and geographic CS will check the conversion from Easting/Northing to Latitude/Longitude.
Using the MAP->Assign Coordinate System will save the defined CS into a drawing that can be passed to other users. When they open the drawing, if the CS does not exist on their system then they will be prompted to see if they want to Auto-Register it. Doing so, will make that CS available under the Auto-Registered Category.
Note that if you edit an existing Coordinate definition, then you must exit and restart AutoCAD in order for the changes to be used.
No comments:
Post a Comment