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SURTRI - create a DTM from string and-or point data

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SURTRI

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Introduction
Files, Fields and Parameters
Notes
Example
Errors

Introduction

Generates a triangulated digital terrain model from perimeter, string and/or point data subject to optional boundary and string edge constraints.

How to use

The method used is that of a constrained Delaunay tessellation. This process performs all the functions of the TRIANR process with advantages of greatly improved speed performance and several additional features.

Points, strings, perimeters and boundaries may be input to the process. Additional points may be added into strings. Crossing strings are automatically resolved. Points duplicated within a tolerance are removed. Maximum separation of points to be joined may be specified. Boundaries may be 2D or 3D internal or external. The centre of gravity of each triangle may be output. An error trace file containing a dump of data may be produced to help locate bad data.

Note that a limit of 60,000 strings is set for the string file that is input to SURTRI

The points used in the tessellation are output into a wireframe points file &WIREPT with fields XP, YP, ZP and an additional field PID which is the point number allocated by SURTRI. The triangle definitions are output into a wireframe triangle file &WIRETR with fields PID1, PID2 and PID3 which identify the points at each vertex. Additional fields XBAR, YBAR and ZBAR defining the centre of gravity of each triangle may optionally be added to the triangle file. These output files may be used as input to other wireframing processes such as ADDTRI, TRIFIL, PLOTWS, ISOTRI, PLOTTR, TRICON, TRIVOL, WIREPE.

Boundaries

The tessellation may be constrained by a number of bounding perimeters. These perimeters are input via the &PERIMIN file. Bounding perimeters may form either internal or external boundaries. External boundaries will restrict tessellation to within the perimeter while internal boundaries will restrict tessellation to outside the perimeter.

A combination of internal and external boundaries may be used at one time subject to the following rules:

Boundaries must not cross each other.
If internal and external boundaries are both present then each internal boundary must be fully contained within an external boundary.

Boundary perimeters may be treated as 2D cookie cutters (@SYSTEM=2) or as 3D edges to be included in the tessellation (@SYSTEM=3). If 2D boundaries are used then the tessellation will be confined to points within the perimeter. If no external boundaries are used then the tessellation will be confined outside any internal boundaries and within the convex hull of all points.

Perimeters boundary type may be set using an optional *BOUNDARY field on the &PERIMIN file or via the @BOUNDARY parameter.

The BOUNDARY field/parameter may have the values:

-	missing (use @BOUNDARY),
0	edge constraint only,
1	external boundary or
2	internal boundary.

All perimeters in the &PERIMIN file are treated as closed polygons. They may or may not be explicitly closed with the first and last points being the same.

Edges

Edge constraints may be input to the tessellation as open or closed strings via the &STRING file or as closed perimeters from the &PERIMIN file. All strings should be 3D as they are included in the tessellation as points. Perimeters from &PERIMIN are included as edges if @SYSTEM = 3.

The resultant tessellation will honor all string and perimeter edges provided edges do not cross. Where an edge crosses another edge priority will be given arbitrarily to one edge. Where an edge crosses a bounding edge an additional point will be inserted into the string just inside the boundary. The distance from this point to the boundary is determined by the height to base ratio of the triangle formed by the 2 boundary edge points and the new point. This ratio may be controlled using the @HBRATIO parameter. In the vast majority of cases the default value (0.05) should suffice.

Strings and perimeters therefore may cross over each other and any boundaries.

Additional points may be inserted into strings to improve the tessellation. The maximum separation of points inserted is defined by the @DMAX parameter.

Points

All points input to the tessellation including those from &POINTIN as well as string points, 3D boundary points and additional points created using @DMAX are filtered for duplicates using a finite tolerance. The default value for this (0.001) may be overwritten using the @TOL parameter however the actual tolerance used may be increased by the program. The minimum accepted tolerance is set to the maximum range of the data (XMAX-XMIN or YMAX-YMIN) divided by 500000. The tolerance used is written to the screen.

Partitions

The maximum number of points in total that may be passed to the Delaunay algorithm is limited by the size of the real array compiled into your application. Typically this is 500000 bytes on a workstation or 100000 bytes on a PC. The maximum number of points is ( bytes/20 -3 ). For 500000 bytes this equates to 24297 points. If this value is exceeded SURTRI will automatically partition the data into multiple passes. The maximum number of points per pass may be reduced using the @MAXPTS parameter (this should only be required for testing the process).

SURTRI partitions according to sub-ranges of X or Y depending on which has the largest overall range. Each partition will have a Delaunay optimum tessellation, however the process of knitting together partitions may not be optimum. Strings crossing partitions will be honored. If SURTRI goes into partitioning some points may be duplicated in the output points file although this will not cause any problems with the model.

Trace file

SURTRI has the option of outputting a file, SURTRI.TRC containing a dump of the point and edge data passed to the tessellation algorithm for each pass. This file may also contain a set of plot commands that may be interpreted by the MINSURV program. The amount of trace output is controlled by the @ERRTRACE parameter.

The tessellation program will return an error status for each pass, = 0 no error = 1 warning = 2 for a serious error. Normally if a serious error is encountered the process will stop and no points or triangles will be output. Setting @ERRTRACE will force the output of data despite a serious error. Triangles produced in this way should not be used in subsequent processing.

The SURTRI.TRC file may be used to interpret error messages from the tessellation program. It contains a list of points and edges with their co-ordinates. Error messages from the routines DELAUN, EDGE, BCLIP and TCHECK may refer to point numbers and edges in this file.

SURTRI.TRC Example file:

RANG 6328.94 2963.00 9781.19 8255.50
Pass 1 Partition 2963.000 8255.500 761 24997
start of dataset
761 761
8376.730 4501.780 1
8392.978 4518.233 2                                         BLOCK A
8399.307 4520.785 3
...
1
2
3                                                                     BLOCK B
4
...
116 number of bounding edges.
669 total number of edges.
1 2 8376.730 4501.780 8392.978 4518.233
2 3 8392.978 4518.233 8399.307 4520.788         BLOCK C
3 4 8399.307 4520.785 8405.979 4525.500
4 5 8405.979 4525.500 8411.548 4531.695
...
end of dataset

Explanation

Line 1:	Range of data in the format RANG XMIN YMIN XMAX YMAX.
Line 2:	Pass N Partition PMIN PMAX NPTS MAXPTS where PMIN,PMAX range of partition in X or Y. NPTS number of points passed and MAXPTS the maximum number of points per partition.
Line 3:	start of dataset
Line 4:	NPTS NLST where NPTS is total number of points and NLST is number of points to triangulate.
Block A:	X Y I : Coordinates and point number.
Block B:	I : List of NLST point numbers. NCB : number of bounding edges. NCE : total number of edges.
Block C:	IE1,IE2,X(IE1),Y(IE1),X(IE2),Y(IE2) Edges defined by point numbers and their coordinates.
Last:	end of dataset

Files, Fields and Parameters

Input Files

Name	Description	I/O Status	Required	Type
PERIMIN	Input perimeter file containing XP,YP,ZP,PTN, PVALUE and optional BOUNDARY fields. Perimeters may be included in the triangulation and/or used as boundaries.ALL are assumed closed. The BOUNDARY field may have the values : - missing (use BOUNDARY) , 0 edge constraint, 1 external boundary or 2 internal boundary.	Input	No	String
STRING	Input string file containing XP,YP,ZP,PTN and PVALUE fields. String segments are included in the triangulation as 3D edge constraints, breaklines. Strings may be open or closed. Note that a limit of 60,000 strings is set for this input file.	Input	No	String
POINTIN	Input point file containing XPT,YPT,ZPT fields.	Input	No	Point Data

Output Files

Name	I/O Status	Required	Type	Description
WIRETR	Output	Yes	Wireframe Triangle	Output wireframe triangle file. May include additional XBAR,YBAR and ZBAR fields.
WIREPT	Output	Yes	Wireframe Points	Output wireframe point file.

Fields

Name	Description	Source	Required	Type	Default
XPT	X field in input point file.	POINTIN	No	Numeric	Undefined
YPT	Y field in input point file.	POINTIN	No	Numeric	Undefined
ZPT	Z field in input point file.	POINTIN	No	Numeric	Undefined

Parameters

Name

Description

Required

Default

Range

Values

SURFACE

Optional surface identifier, +1 for upper surface, -1 for lower (1).

-1, 1

-1,1

BOUNDARY

Default boundary specifier for perimeters. Used if the BOUNDARY field does not exist in PERIMIN or has a missing value (0). 0 edge constraints, must be 3D, 1 external boundary or 2 internal boundary.

0,2

0,1,2

SYSTEM

Defines the treatment of boundary perimeters BOUNDARY= 1 or 2 from PERIMIN (3). 2 perimeters are 2D and used only as constraints. 3 perimeters are 3D and are included in the triangulation.

2,3

DMAX

The maximum separation of additional points interpolated into long string segments to improve the triangulation.

Undefined

MAXLINK

Maximum separation of points that will be joined by a triangle.

Undefined

TOL

Tolerance distance below which points are considered to be duplicated. If too small this value may automatically be increased by the program.

Undefined

COG

Used to include extra fields containing the coordinates of the centre of gravity of each triangle in output triangle file (0):

Option	Description
0	- do not include XBAR,YBAR or ZBAR .
1	- include ZBAR only.
2	- include XBAR,YBAR

0,2

0,1,2

ERRTRACE

Used to control the amount of error messages and the creation of a system file SURTRI.TRC containing a dump of points and edges for tracing errors. Also may be used to force the creation of output points and triangle records despite a serious error(0).

Option	Description
0	- no error trace, reduced error messages.
1	- create SURTRI.TRC, force output, full error reporting.
2	- as 1 plus include MINSURV plot records.

0,2

0,1,2

HBRATIO

Height to base ratio of triangles created to resolve crossing strings (0.05).

0.05

Undefined

MAXPTS

Overwrite the program limitation on maximum points per partition with a smaller value. May be used to force partitioning for testing (-).

Undefined

CHECK

0,1

Notes

No additional notes.

Example

Example 1:

!SURTRI

&WIREPT(PT),&WIRETR(TR),&PERIMIN(BOUND),
&STRING(STRINGS),&POINTIN(POINTS),*XPT(X),*YPT(Y),
*ZPT(Z),@SURFACE=1,@BOUNDARY=1,@SYSTEM=3,@DMAX=40,
@MAXLINK=200.0,@COG=1,@ERRTRACE=1

Output:

    SURTRI TIME >14:54:58

    Maximum points per partition. 24997

    TOLERANCE = 2.6462500E-02

    Read point data for perimeter 201.0000

    NUMBER OF POINTS LOADED = 500

    Performing triangulation... PASS 1

    Perimeter 201.0000

    TRIANGULATION COMPLETE. ERROR LEVEL = 0
    (0 no error, 1 warning, 2 serious.)

    Removing border triangles and long edges...

    Output points and triangles...

    >>> 761 RECORDS IN FILE PT <<<
    >>> 1404 RECORDS IN FILE TR <<<

Example 2:

This example demonstrates the honoring of strings in a DTM of a pit design. The input perimeters and resultant triangles are displayed using PLOTPE and PLOTTR respectively.

Plot the design pit perimeters (Figure 1)

!PLOTPE &PROTO(PROTO),&IN(PITDESN),&PLOT(PLOT1)

Triangulate the perimeters

!SURTRI

&WIREPT(PT),&WIRETR(TR),&PERIMIN(PITDESN),@SURFACE=1,
@BOUNDARY=1, @SYSTEM=3

Plot the triangle (Figure 2)

!PLOTTR &PROTO(PROTO),&WIREPT(PT),&WIRETR(TR),&PLOT(PLOT2)

Figure 1: Pit design perimeters.

Figure 2: DTM Triangles.

Error and Warning Messages

Message	Description	Solution
>>> ERROR IN POINTIN FIELDS <<<		Check field names in &POINTIN.

>>> ERROR IN PERIMIN FIELDS <<<		Check field names in &PERIMIN.

>>> ERROR IN STRING FIELDS <<<		Check field names in &STRING.

>>> THERE ARE NO FILES OPEN <<<	At least one of &POINTIN, &STRING or &PERIMIN must be specified.
>>> ERROR IN WIREPT FIELDS <<< or >>> ERROR IN WIRETR FIELDS <<<	Problem creating output files

>>> Too many perimeters and strings	Datamine string limit exceeded.

>>> There are no points to be triangulated	No points found on or in boundary.	Check points file and boundaries.

>>> Error - Points are collinear		Check points.

>>> Error - Further partitioning impossible <<<	Cannot partition points within a boundary. There may be too many points in the boundary perimeter.

>>> Error -There are not enough points.	Less than 3 points to triangulate.

Errors in triangulation

During a triangulation pass a number of error messages may appear from the following routines.

***ERROR IN DELAUN***
***ERROR IN BCLIP***
***ERROR IN EDGE***
***ERROR IN TCHECK***

At the end of each triangulation pass an error status flag will be reported. This flag has the values:

    0     no error.
    1     warning.
    2     serious.

The error messages will usually give a reference to point or edge numbers. The coordinates of these may be checked by running SURTRI with @ERRTRACE=1 and reading the output SURTRI.TRC file.

Errors with a status of 1 usually refer to crossing strings and may be ignored.

Serious errors are likely caused by duplicate points that have not been filtered. try increasing the tolerance using the @TOL parameter.