This topic documents the concepts used in forming three dimensional digital terrain models.
The modelling process incorporates a number of important concepts which enable rigorous three dimensional models of virtually unlimited size to be formed faster and more efficiently than previously possible.
Geographical Indexing
A specially developed geographical indexing process is used whereby the points to be modelled can be divided into efficiently sized regions based on their geographical position. This enables the model to be formed region by region much faster than would otherwise be possible and also allows models of virtually unlimited size to be formed.
Region Size
Because the region size is user selectable, it can be set to a value which maximises the performance of your particular hardware configuration. Typically, a region size of between 250 and 1000 points will give the fastest modelling results, however, the best region size to select will depend on the hardware being used and the amount of available memory.
Regardless of the region size selected, the model formed will be identical.
Search Distance
The concept of a user selectable search distance has been introduced to ensure that the user has control over whether or not triangles are formed across areas of sparsely spaced data.
It is used to govern the maximum search distance used by the system in locating apex points during triangulation. For example, a higher search distance will tend to form long triangles across areas which have no data, whereas a lower distance factor will tend not to triangulate across regions of sparse data points.
Validation
The validation process checks the data for conditions which would result in erroneous model formation and provides a report containing any errors that are found. Such errors should be rectified before forming a model.
If there are any boundary lines, the validation process only considers objects within the minimum bounding rectangle of the boundary lines. Therefore, if a boundary does not close, it is probable that objects which you require to be validated would not have been considered in the validation process. Hence, if you encounter errors associated with incomplete boundaries, you should validate the data set again for all conditions after the boundary errors are rectified.
Note that the model may be formed without first successfully validating the data but the resulting model may then be erroneous.
To prevent the possibility of erroneous model formation, follow these steps:
1. Validate the data base for all possible errors.
2. Edit the data base to fix all errors reported from the validation process.
3. Repeat steps 1 and 2 until no validation errors are reported and only then form the model.
Block and Group Sensitivity
Model formation is block and group sensitive. That is, only points and lines defined by a currently active block and on the currently displayed groups are used in the modelling process.
Therefore, if you wish to use only a certain region to form the model, you may define a block to include only those points and lines in that region. In addition, you can exclude points and lines on particular groups by ensuring that those groups are not displayed.
Note that if a breakline crosses the block perimeter, the segment of the line that crosses the perimeter is not used in the model.
Refer also to Groups concepts and the Block command.
Model formation
The modelling process constructs a rigorous triangular model over the surface. Up to 10 different models may be created for any particular data base.
All points and lines which have a contourable attribute are used in the model. The contourable points are used as triangle apexes. The contourable lines, also known as break lines or change of grade lines, are imposed as triangle edges. Lines with a boundary attribute that form a closed figure define a boundary of the model.
Points and lines which do not have a contourable attribute are not used in the model formation.
Any unclosed boundary lines in the data base when it is modelled will be treated only as break lines and not used to form a boundary for the triangulation. The resulting model may therefore be erroneous.
If errors are discovered in the modelling process, or if the data set has not been validated to your satisfaction, you should edit the data set to fix the errors and ensure that a successful validation has been carried out before forming the model again.
If the file has not been validated, you may still proceed to form the model but the resulting model may be incorrect.
If the number of points in the data base exceeds the selected Region Size, then a quad tree index is drawn over the region to be modelled. The system will then proceed to draw the triangles as they are formed. At the completion of the model formation, the quad tree index is erased. If the display is updated, the triangles will not be redrawn unless the Triangles check box is active in the Display Features command.
If certain error conditions are encountered in the modelling process, they are written to an error file which you may view or print using the Terrain Modelling/Errors commands. The following conditions are checked for and reported on:
Contourable Points with the same XY position.
During model formation one of the points is ignored, possibly resulting in an incorrect model being formed. You should edit the data base to delete one of the points or change its attribute to non contourable.
Unclosed boundaries.
During model formation, any unclosed boundaries are ignored for boundary purposes and treated only as contourable lines, possibly resulting in an incorrect model being formed. You should edit the data base to ensure that the boundaries close and reflect the required limits of the model.
Contourable points which lie on a contourable line but are not part of the contourable line.
During model formation, the contourable line is treated as a planimetric line possibly resulting in an incorrect model being formed. You should edit the data base to include such points in the contourable line, or change the points or lines where appropriate to non contourable.
Crossing break lines can only be detected by using the Modelling/Validate command. The modelling process does not report on crossing break lines. You should therefore ensure that you use the Modelling/Validation command to find and rectify any occurrences of this condition before forming the model.
For any pair of crossing break lines, the modelling process uses the last crossing break line, possibly resulting in an incorrect model being formed. You should edit the data base to ensure that the break lines do not cross and that they reflect the changes of grade.
As it is possible to model the same data set using a variety of region sizes, it may appear that different models could be created. However, the system employs a unique modelling technique that ensures you always get the same result from the same data set no matter which region size is used provided you use the same distance factor. Note however that varying the distance factor may result in more or less triangles being formed as the search distance governs whether certain areas are triangulated.
If there are any boundary lines, the modelling process only considers objects within the minimum bounding rectangle of the boundary lines. Therefore, if a boundary does not close, it is probable that objects which you require to be validated and modelled would not have been considered in the modelling process. Hence, if you encounter errors associated with incomplete boundaries, you should validate the data set again for all conditions after the boundary errors are rectified and then form the model.
See Also