Editing DTM Triangles
The Edit option from the DTM menu generates a pop-up menu that allows you to edit the triangles of the current DTM. You should note that when editing triangles with a 3D view of the DTM on screen, the 3D view will also update itself.
The Delete options from the Edit pop-up menu allow you to remove triangles from the model using eight different methods.
Triangles can be deleted from the model one by one. The mouse is used to indicate which triangles to be removed.
All triangles using a common apex can be deleted. The mouse is used to indicate the required apex.
All triangles intersecting a line can be deleted. The mouse is used to indicate the start and finish of the line. The triangles indicated by the start and finish points will also be removed.
Triangles that are within a rectangular region can be deleted. The mouse is used to indicate the corners of the rectangle. Only triangles that are wholly within the rectangle will be deleted.
Triangles that are within a polygonal region can be deleted. The mouse is used to indicate the vertexes of the polygon. Only those triangles that are wholly within the polygon will be deleted.
Triangles that are either inside or outside a feature can be deleted. The mouse is used to indicate the feature. When this option is selected, a short pop-up menu allowing you to specify whether the triangles inside or outside the feature will be displayed. Only those triangles wholly inside or outside of the feature will be deleted.
Triangles that are not attached to a group can be deleted.
When forming the initial triangulation, the DTM Settings allow you to filter long thin triangles around the DTM edge. This option allows you to filter the DTM edges if this has not been carried out or you wish to change the filter parameters.
Inserting New Triangles
The Insert options from the Edit pop-up menu allow you to create new triangles using three points using three different methods.
The Insert Centre option allows you to create new triangles that radiate out from a central point. Firstly, you will be asked to indicate the centre point and then a series of points surrounding it. A new triangle will be created using the centre point and tow of the other indicated points. In the example below, the points have been indicated in numerical order and by indicating nine points, seven triangles have been created.
The Insert Stitch option allows you to create new triangles between two lines of points. Effectively you zigzag between the two lines of points and stitch them up. .In the example below, the points have been indicated in numerical order and by indicating 8 points, six new triangles have been created.
The Insert Side option allows you to create new triangles by indicating an existing triangle side and another point. In the example below, the first triangle was created by indicating the side between E1 and E2 and point number 1, the second triangle was created by indicating the side between E1 and 1and point number 2 and so on.
Forcing Triangle Sides
There are times when the triangle sides do not follow topographical features. This may be due to a coding problem where two separate line features should be joined. It could also be due to the basic triangulation algorithm itself. The Force options allow you to correct these anomalies. They also allow you to introduce new data into a model. There are three methods of forcing triangles sides.
Two adjacent triangles form a quadrilateral in which one of the diagonals is the common side. It is often the case that the common side is the wrong diagonal. The Force Switch option allows you to change it. Obviously, the internal angles of the bounding quadrilateral must all be less than 180° for the change to be valid. You will be asked to indicate a common side in the triangle and, assuming the switch can take place, the triangles will be adjusted accordingly.
Where the formation of triangles gives rise to a problem larger than a simple quadrilateral, you can use multiple instances the previous option to correct the anomaly. However, this could be time-consuming. The Force Line option allows you to carry out any such corrections in one go. You will be asked to indicate the start and endpoints of the triangle edge that needs to be forced. The polygon surrounding the triangles which intersect this indicated line will then be calculated. The intersecting triangles will then be rearranged so that the required line becomes an edge in the triangulation.
There are situations where the forcing of all the segments of a feature is required. This can be due to a coding error or where a new feature has been added to the model. The Force Feature option allows you to do this and you will be asked to indicate the feature to be forced. Checks are then made to ensure that there are DTM apexes at each point of the indicated feature. If a DTM apex is missing, the appropriate feature point will be included in
Editing DTM Apexesthe DTM. After these checks, each feature segment will be forced into the DTM triangles.
The Apex options allow you to edit the individual apexes of a DTM.
This option allows you to raise or lower an apex of the DTM and its associated point. Indicate the apex and you will be asked to enter an offset for the height. With this option, you can only change the height of a single apex. If you wish to use the current pick mode, moving the point height has the same effect whilst allowing for the pick mode setting.
This option allows you to remove one or more apexes from the current DTM. It obeys the current pick mode so you will be asked to either indicate the apex to remove, sketch a rectangle enclosing those apexes to remove or confirm that the points on the select list show which apexes to remove. The triangles around each point will be removed and the hole will be stitched up with new triangles.
This option allows you to include additional points in the DTM triangulation. It the current pick mode so you will be asked to either indicate the point to include, sketch a rectangle surrounding those points to include or confirm that the points on the select list should be included. The triangulation around each point will be adjusted whilst retaining any forced triangle sides.