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Contouring at or below elevation zero
Posted by Jason Poitras on 05 September 2007 03:28 PM

Q) I need to create contours for the bottom of a lake. I already know how to do normal contours for most jobs but if the lake elevation is at zero elevation, how do I do the contours below zero?

A) By default the MSCAD program does not extract any data below elevation 2 when creating a surface. This can be changed to allow you to extract information below the elevation of 2, to allow you to do the bottom of your lake. Go under the Modeling Menu -> Configuration Settings -> Data Extraction -> then set the Z Range Minimum to something lower than your lowest elevation (or set it to -5000 to make sure). This will now allow you to extract any data you have down to the elevation you specified. The minor problem that arises with this change is the text in the drawing. The text is inserted at elevation zero and if selected, will extract to a surface as well. This means we have to be careful and not select any text when extracting data to a surface.

We can control this in two ways.

1) Simple layer control during the extraction process. This means you would need to freeze the layers that have the text on them so you don¢‚¬„¢t accidentally select them. The problem that can come with this method is that there may be text on the same layer as the data you do need to extract. This would make it very difficult to select only the valid data and not the text.

2) Another tool has been provided to help make the selection process easier. Go under the Modeling Menu -> Configuration Settings -> Data Extraction -> and turn the Filter By Entity option on. By turning this check mark on, during the extraction process, you will now see an additional dialog box. This dialog will allow you to be very specific about what type of data you will allow to be extracted to your surface. Now that we have the settings required, follow the steps below.

A) Go under the Modeling Menu -> Extract From Drawing -> Extract to Surface. When the dialog comes up, enter in the surface name (the default name is Ground). Pick OK to continue

B) The next dialog is the new one, called Entity Filter. On the left side you need to highlight the types of data you need to extract to your surface. Pick on 1 or more types of valid data (example: Point, Line and Polyline) ¢‚¬€œ then pick the Select button on the right side. This shortens the list on the left to what you had picked. Now you can pick OK and then select the data to extract ¢‚¬€œ pressing Enter grabs everything in the drawing ¢‚¬€œ then it filters through all of the selected entities to retain only the types of data you had chosen to filter.

C) If required, you can now add the breaklines to your surface. Go under the Modeling Menu -> Extract from Drawing -> Extract Breaks. The same dialog comes up as in step A above. Pick OK to continue.

D) The next dialog that comes up is the same as in step B above. You need to pick on the Reset button on the right, then in the same manner as in step B above, select only what will be used to extract the 3D breaklines to a surface. Then continue to complete the selection. Now run the TIN command and continue to generate and complete the contours, as you have done in other survey jobs with elevations above 2.

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