Clip DTM by boundary string

You can use this function to trim a DTM to a boundary string before using the DTM in other processes. For example, to create pillars in the backs of some underground development.

To run this function: Choose Surfaces > DTM file functions > Clip DTM by boundary string, or...

• In the Function Chooser, type GRAPHICS DTM CLIP, and press ENTER.

The outer boundary segment must be clockwise, but can have any number of anticlockwise and clockwise segments within it. Using a boundary string with a mixture of clockwise and anti-clockwise segments produces a "doughnut effect" in the resultant DTM. Producing this nested style of data from topography that defines especially rugged terrain may require that you separate certain contours and repeat this function several times to achieve the desired results.

Because you can use this function to produce DTMs that include voids, you should not process the DTMs you create with GRAPHICS DTM CLIP using DTM functions that require a DTM that does not contain voids. For example:

• SECTION DTM
• DTM BOUNDARY
• DTM VOLUMES
• STRINGS OVER DTMs

You can use DTMs produced by this function to create 3DMs. For example, to create a 3DM, you can stitch a clipped DTM that represents the top of a seam deposit to a clipped DTMs that represents the bottom of a seam deposit using their boundary strings. The new boundary strings created by the clipping process are important when you create a 3DM because the new strings include any new points created when the triangles of the DTM were clipped against the boundary. If these new points did not exist in the string, the 3DM created would not be closed.

Fields on the Apply a boundary to a DTM form

Field	Description
Define the DTM file
Location	The name and location of the DTM to be clipped.
[Refresh]	Loads the Object ID and Trisolation ID drop-down lists with the objects and trisolations of the DTM. In Surpac 6.6.1 and earlier versions, when you selected a DTM in the Location field, Surpac would load the DTM, validate it, and then load the Object ID and Trisolation ID drop-down lists. You could not interact with the form until the process was complete, which could be a long time for some large DTMs. Now, Surpac does not load the drop-down lists until you click the Refresh button, and validation does not occur until you click Apply in the form. Any validation errors are reported in the Message window.
Object ID	The number of the object that you want to clip. You can type the Object ID or select it from the drop-down list, if the list is loaded.
Trisolation ID	The number of the trisolation that you want to clip.
Define the new DTM to create
Location	The name and location to save the DTM to be created.
Object ID	The object number for the DTM to be created.
Object name	The name of the object to be created.
Define the boundary string
Location	The name and location of the boundary string file to use for clipping.
String	The number of the string in the string file to use for clipping. Note: The boundary string can consist of any number of nested segments. The direction of these segments (clockwise or anticlockwise) is not important. More... Imagine that you are viewing a surface along the normal to the plane used during surface creation (that is, looking directly at the plane), the rules for clipping are: For a single segment string, "inside" the segment refers to triangles physically inside the segment. For a string with a single level of nested segments, "inside" refers to triangles physically inside the outermost segment but outside any inner segments. Multiple levels of nesting are handled by alternating "inside" and "outside" for each level of nesting. For example, if the black lines are segments of a string, the red areas are considered "inside" the string and the white areas are "outside":
Retain triangles	Defines whether you want to keep the data inside or outside the boundary string. Inside the boundary: The triangles of the DTM that are inside the boundary string are saved to the new DTM. Outside the boundary: The triangles of the DTM that are outside the boundary string are saved to the new DTM.
Define the file for the new boundary string
Location	The name and location to save the new boundary string file.
String	The string number in which to save the new boundary string.

Output

The DTM and boundary string are loaded. The boundary string is then applied, one segment at a time, to the DTM splitting triangles where necessary to ensure that there are no triangles that cross the boundary segments. After all segments of the boundary string have been processed, the triangles that are not required are deleted from the DTM.

The clipped DTM and the new boundary string, which contains the new points created while splitting the triangles, are saved to the files specified.

When clipping a DTM that was created on a non-horizontal plane, the process is:

1. The surface and input string data are transformed so that the plane is horizontal.
2. The clipping process proceeds as for a horizontal plane surface.
3. The results are back-transformed to sit on the original surface location.

Troubleshooting

Message	Description
Boundary point is outside DTM Y=yyy X=xxx	A point on the boundary string is outside the DTM at the given coordinates.