Closure Errors

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This item displays all of the closures and closure errors for the last file processed. The display lists the closing shot and error information for that shot. The error information is listed several different ways:


Closure Errors:


Closing Shot: GYQ3 -> GUG8B

Loop Length:     303.90ft

Shot Count:  14


                 north    east   vert.  vect2D  vect3D    %2D    %3D


Error Values:    -0.08ft -1.86ft -2.89ft  1.86ft  3.44ft  0.61%   1.13%

Expected Values:  1.78ft  2.58ft  2.75ft  3.14ft  4.17ft  1.03%   1.37%

Standard Dev.:    0.05    0.72    1.05    0.45    1.10

Error Per Shot:  -0.01ft -0.13ft -0.21ft  0.13ft  0.25ft

Here is a detailed description of each data item:


Closing Shot. The first item gives the station names for the closing shot. This is a simple way of identifying each individual loop. If you want a list of all the stations in the loop, click on the "Loop Stations" radio button.


Length. This gives the total length of all the shots in the loop.


Shot Count. This gives the total number of shots in the loop.


North, East, & Vertical. These numbers show the size of the error in three dimensions. For example, if you have a 10-foot north error, it means that the closure is off by 10 feet in the north direction.


Vector2d/Vector3d. These errors represent the straight-line length of an error. In essence, it is a single number that gives an indication of the overall error of the loop. The Vector2D shows the combined effect for the north and east errors. The Vector3D shows the combined effect for the north, east and vertical errors. The 2D error is useful for seeing the effects of the compass and tape errors alone or, for certain caves, where there are problems with the inclinations.


Percent (%2D, %3D). Percent error is the amount of error compared with the size of the loop. It is derived by dividing the error by the loop length. This gives a measure of the quality of the loop. It is not the best measure of loop quality, but many people are used to it. The display shows two different percentages, one labeled 2D and the other labeled 3D. The 2D shows the error based on the north and east errors, omitting the vertical component. The 3D version shows the errors based on the north, east and vertical errors. The 2D error is useful for seeing the effects of the compass and tape errors alone or, for certain caves, where there are problems with the inclinations.


Predicted Error And Standard Deviation. Each error is displayed on three different lines. The first line lists the actual errors. The second line shows the predicted errors. The last line displays the ratio of the predicted errors to the actual errors. The ratio of predicted errors to actual error gives the number of Standard Deviations the loop varies from the expected value. Small numbers indicate that the loop closely matches the expected error level. Large numbers indicate that the error level is greater than the predicted level, and indicate a problem with the loop. In simple terms, here's what the numbers mean:


Between 0 and 1: If the value is between 0 and 1, the loop is of very high quality and probably does not have any blunders or systematic errors.


Between 1 and 2: If the value is between 1 and 2, the loop is still probably good, but there is a greater chance the errors are caused by blunders.


Between 2 and 3: If the value is between 2 and 3, there is a high probability the loop is blundered. However, there is still a very small chance the random errors in a good loop have accumulated in a way that gives a high error even though the loop is okay.


Greater than 3. If the value is greater than 3, the loop is almost certainly blundered. There is less than one chance in 1000 that the loop is good.


The predicted error value for the loop is controlled by the uncertainty value of the compass, tape and inclinometer. You can set these values to match the kind of survey instruments you are using.


Error Per Shot. This item gives the average amount of error in each shot. This gives a clearer impression of how large the error is relative to single shot. For example, a 10-foot or 3-meter error would obviously be a very large error per shot.