If you work with DTMs, then you have probably had exterior boundary issues. Here are a few tips to help with creating and using exterior boundaries to your benefit. While most are related to existing DTMs, some can be useful for proposed models as well.
1. Not every DTM needs an exterior. If you’re working with a large existing model where nothing around the perimeter is ever used in plans or for design, then perhaps you can save yourself some time and leave the exterior off. Just remember, any concave areas around the site will have ‘bogus’ or ‘bad’ triangles which could lead to erroneous contours, etc.
2. If you don’t have an exterior, the View Perimeter command will follow the outside of the current triangles. Delete unnecessary triangles using Surface > Edit Surface > Delete Triangles and view the perimeter, then load the perimeter as an exterior.
3. If you do have an exterior, the View Perimeter command will always follow the exterior, even if you have deleted additional triangles. To have the perimeter follow the triangles, you can change the exterior to random points or a breakline using Feature Properties.
However, you will need to re-triangulate before starting to delete the triangles – so if you’ve already ‘fixed’ some areas, you’ll have to delete those triangles again. But, View Perimeter will then follow the triangles that are left, and you can then load the perimeter as an exterior.
4. You can draw your own exterior. Use MicroStation to create a shape or complex shape to use as an exterior. If it is to be triangulated, draw it at the correct elevation or draw it inside the perimeter and use the Drape Surface option when Importing.
5. Exteriors do not have to be triangulated. A non-triangulating exterior does not have to be at the correct elevation. The next feature points to the inside of the exterior will be used for triangulation and could result in some triangles overlapping the exterior. If a non-triangulating exterior is inside some of the feature points, triangles may be formed with a portion extending outside the exterior if a majority of the triangle is inside the exterior.
6. If you create an exterior with the Import of a perimeter or other element that follows an existing feature, the exterior will take precedent and the other feature(s) will not be used for triangulation. Certain commands, such as Merge or Copy Surface will only carry forward triangulated features, so the duplicate points along the exterior will not be in the resulting surface. Subsequent removal of the exterior will result in ‘losing’ the triangulated points along the edge. However, you could change the exterior to a breakline or random feature instead of deleting it, but you would still not have the original feature.
7. The Maximum Triangle Length limitation in Surface Properties is useful if you’re trying to get rid of those long triangles crossing concave areas around your surface. Just don’t set it too small or you’ll leave holes in areas where you don’t want them.
8. There is a new command under Edit Surface called Delete Triangles by Filter that allows you to delete triangles and automatically create an exterior. You can use maximum triangle length, angle tolerances or elevation range to determine what triangles to delete. You also get to preview the results before accepting the exterior.
9. Exteriors are always closed. Partially deleting the exterior feature with Partial Delete feature will not result in an ‘open’ exterior. Instead the gap will be closed with a straight line for triangulation purposes.
10. If you have an exterior that you need to modify, use MicroStation. Display the exterior feature, modify the graphic with MicroStation, and import it from graphics to create the new exterior.
With exteriors, as with many features in MicroStation and InRoads, the more you know the less pesky and more valuable they become!