Using the COGO traverse
Release 9.3 Last modified May 27, 2009	Print all topics in : "Managing the cadastral fabric dataset"

About using the COGO traverse

The coordinate geometry (COGO) traverse is used to compute a sequence of survey point locations starting from an initial known point. Each new survey point is defined by a traverse course and is used as the take-off point for the next course in the sequence. Two methods are used:

• The Bearing method uses directions for the orientation of each course.
  
• The Angle Right method uses an angle measured clockwise from the line of the previous course.
  

A traverse course can be defined using various combinations of directions, distances, angles, and circular arc parameters.

The COGO traverse is primarily used for defining coordinates based on values taken from subdivision plans. The TPS Traverse computation, by contrast, should be used for processing field traverse data. A side-shot course is used to compute a coordinate that is not part of the main traverse course sequence.
The graphic below demonstrates an open traverse.




There are two primary categories for traverses: open or closed. An open traverse ends its sequence with a new survey point, whereas in a closed traverse, the final course ends at an existing survey point.

When a closed traverse starts and ends with the same survey point, it is called a loop traverse or a closed loop traverse. Since the final point of a closed traverse has known coordinates and the final course of the traverse computes coordinates for the same survey point, there is often a discrepancy between the existing coordinates and the computed coordinates—this discrepancy is called a misclosure or a closure error.
The graphic below demonstrates a closed traverse.



The closure error in a COGO traverse may be the result of any of the following:

• Precision
  
• Mistyped values
  
• Incorrect values in the source
  
• Measurement error
  

A closure error that is identified as being caused by incorrect source values indicates a potential problem in the original survey data. Closure error caused by a typing error can be handled by reentering the value.




Misclosure caused by numerical precision or measurement error can be solved by applying an adjustment to distribute the error through each of the courses.

You can define the allowable limits of closure error on the General tab of the COGO traverse computation. There are three adjustment methods available:

• Compass rule
  
• Transit rule
  
• Crandall rule
  

The Compass rule, also known as the Bowditch rule, distributes the misclosure in the northings and eastings in proportion to the distance along all the courses from the first point to each of the unadjusted coordinate locations.

The Transit Rule was developed for cases where the precision of angle or direction measurements exceeds that of distances. Adjustments are applied in the X and Y directions in proportion to the "latitude" and "departure" of each measured line, where the latitude is the Y-component of the line and the departure is its X-component. Thus the longer the component, the larger the correction. After the adjustments are made, revised lengths and bearings for the various traverse lines are computed trigonometrically.

The Crandall method distributes the error in the distances only, assuming directions and angles have no measurement error.

For more information about defining circular curves, see Using circular curve computations.

How to use the COGO traverse

Computing coordinate locations using a Bearing COGO traverse

1. On the Editor toolbar, click the Editor menu and click Snapping. Click Survey Points in the Snapping Environment window.
   
2. Click the Project drop–down arrow on the Survey Editor toolbar and click the target survey project.
   
3. Click the Computation tool palette drop–down arrow, point the mouse to the fourth row on the palette, and click the COGO Traverse button .
   
4. Type a name for the computation and press Tab.
   
5. Type a comment for the COGO traverse computation and press Tab.
   
6. Click Bearing.
   
7. Type the maximum allowable limit for the misclosure in easting and northing.
   
8. Type the maximum allowable relative error as a percentage.
   
9. Click the Parameters tab.
   
10. Snap to and click the start point on the map.
    

The point name appears in the Start Point field. If the point is not visible on the map, type its name and press Tab.


11. If you are doing an open traverse, press Tab and skip to step 13.
    
12. Repeat step 10 for the endpoint of the traverse.
    
13. Double-click the TR/SS field, click SS to choose the next course as a side shot, or choose TR if the next course is part of the main traverse.
    
14. Press Tab.
    
15. Press Tab to accept the default to point name. If you are entering the final course for a closed traverse, double-click the To Point field and type the endpoint name. Press Tab.
    
16. If the course you are entering is a circular curve, skip to step 18. Otherwise, continue to step 17.
    
17. Double-click the Direction field, click ST, press Tab, then skip to step 19.
    
18. Double-click the Direction field and click the type of direction for the circular curve. Press Tab.
    
19. Type the direction value and press Tab.
    
20. Skip to step 23 if the course you are entering is a circular curve. Otherwise, continue with steps 21 and 22.
    
21. Accept the distance parameter type (DI) and press Tab.
    
22. Type the distance value for the course and skip to step 28.
    
23. Double-click the Curve Parameter 1 (CP1) field and click the first parameter type. Press Tab.
    
24. Type the value of the first parameter. Press Tab.
    
25. Double-click the Curve Parameter 2 (CP2) field and click the first parameter type. Press Tab.
    
26. Type the value of the second parameter. Press Tab.
    
27. Double-click the solution field and click the direction in which the circular curve bends.
    
28. Press Enter.
    
29. Repeat steps 13 through 28 for each of the courses in the traverse.
    

Tip

To choose a curve parameter from a dropdown field, press the first letter of the parameter's name.

Adjusting the closure error in a closed COGO traverse

1. Click the Adjustment tab.
   
2. Click the Method drop–down arrow and click the method you want to use to adjust the error.
   
3. Click the Compute button .
   

The adjusted values and their residuals are displayed.

Tip

When you compute a traverse that is a closed loop, an area is reported on the Adjustment tab.

Computing coordinates using an open Angle Right COGO traverse with straight line courses

1. Click the Computation tool palette drop–down arrow, move the mouse to the fourth row on the palette, and click the COGO Traverse button .
   
2. Type a name for the computation and press Tab.
   
3. Type a comment for the COGO traverse computation and press Tab.
   
4. Click Angle Right as the Angle Format.
   

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