Form Model Command

(This functionality is available with the Modelling module)

Forms a digital terrain model of the currently open data base.

The modelling process constructs a triangular model over the surface.

Since model formation is group and block sensitive, check that you have the required groups displayed and that the required block is active. Only points and lines defined by an active block and on the displayed groups will be used in the model. If there is no block active, points and lines on all displayed groups are used. Refer also to Group concepts and the Block command

The points and lines meeting the above criteria, which also 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.

Terrain Model Formation dialog box

Item	Used to
Model Name	Enter a new model name into the list, or select an existing model from the list. If the model name has already been used, you will be given the option to overwrite it with the new model when the OK button is selected.
Region Size	Select a region size which will be used in the modelling process to index the data into a number of regions.
Search distance	Enter the search distance to be used in the modelling process. Refer to the Search Distance topic for further details.
Data set validated for:-
Points with duplicate X,Y co-ords	Indicate whether the data base has been successfully validated for this condition. Refer to the Modelling/Validate command. "NO" is displayed if a successful validation for this condition has not been carried out. If there are duplicate points in the data base when it is modelled, one of the points is disregarded and the resulting model may therefore be erroneous.
Break lines crossing	Indicate whether the data base has been successfully validated for this condition. "NO" is displayed if a successful validation for this condition has not been carried out. If there are crossing break lines in the data base when it is modelled, the last break line is used and previous crossing break lines are ignored. The resulting model may therefore be erroneous.
Closed boundary	Indicate whether the data base has been successfully validated for this condition. "NO" is displayed if a successful validation for this condition has not been carried out. 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.
OK	Form a digital terrain model of the currently open data base. The data is indexed according to the Region Size selected and then triangulated with break lines and boundaries being imposed during the process. After the modelling is completed, the Terrain Model Formation Report dialog box enables you to view or print any errors found. The errors may also be viewed or printed using the Terrain Modelling/Errors! commands.
Cancel	Close the dialog box without performing the modelling process.

NOTE:

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 using the Modelling/Validate command before selecting the Form Model command 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 non contourable 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 any occurrences of this condition.

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 search distance. Note however that varying the search distance 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.