STAR*NET V6 - Circe

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Chapter 8 Analysis of Adjustment Output 8.16 Error Ellipse Interpretation The error ellipse represents the area of uncertainty around a given point. For example, assume that a survey network point is computed from the intersection of three lines from known points, each line being of a measured distance and bearing. The measured distances and bearings all contain some uncertainty, which were expressed as standard errors in the input data. The resulting computed point then has standard deviations in Northing and Easting (or in X and Y), which are computed and listed by STAR*NET. The total effect of all the random measurement errors results in an error ellipse of a specific size, shape, and orientation. The ellipse itself is computed by STAR*NET from the standard errors in Northing and Easting and the correlation between those standard errors. The size of the ellipse is a measure of the reliability of the computed point position. In a particular survey, a smaller ellipse means the point has greater reliability than one with a larger ellipse. Values of the semi-major and semi-minor parameters, as printed in the listing and on the screen, are the actual ground dimensions across half the ellipse, in the long and short directions respectively. The lengths of the axes depend on the standard errors and correlations of the computed coordinates. If N and E are of equal precision, then the ellipse will be a circle. The ellipse azimuth is the azimuth of the semi-major axis, and gives the orientation of the ellipse with respect to the ground coordinate system. The azimuth depends on the correlation between the adjusted coordinates of the point. If they are uncorrelated, the ellipse axes will be parallel to the control axes. STAR*NET allows you to define your own confidence level. A commonly used confidence level is 95% which produces a 95% error ellipse. That means that there is a 95% probability that the actual point position lies somewhere within the extents of the reported dimensions of the ellipse. The plotted error ellipses provide a means of analyzing the strength of the survey network, and the reliability of the computed points. At a first glance, the absolute dimensions of the ellipses provide a measurement of positional accuracy. The relative sizes of the ellipses indicate which points are weaker than others within a given network. The shapes and orientations of the ellipses indicate how the network can be strengthened. An elongated ellipse shows that there is a larger uncertainty in one of the coordinates, and that perhaps an additional distance or angle measurement is required to that station. A large but circular ellipse means that the point is balanced in Northing and Easting (or X and Y), but that more accurate measurement techniques are required, or additional measurements from several other stations are required. If many ellipses are elongated and point in the same direction, then the network is unbalanced along that direction. Typically, there may be an azimuth deficiency, and one side of the network has little resistance to rotation. An additional control point or observed azimuth will help to stabilize the survey. 132
Chapter 8 Analysis of Adjustment Output Relative ellipses express the precision of the relative position of two points. Two points with large station ellipses may actually have a small relative ellipse between them, meaning that their absolute positions in the survey are weak, but that they are correlated with each other and “move together”. That information can be important if you are planning to use the distance or bearing between the two stations as the basis for additional work. As shown in the figure below, the length of the relative ellipse projected onto the line between the two points is a measure of the precision of the distance between the two points. The length of the relative ellipse perpendicular to the line is a measure of the precision of the bearing between the two stations. A table of actual relative errors in distance and bearing can be generated as explained earlier in this chapter. Semi-Major Axis Semi-Minor Axis Relative Error in Bearing Relative Error in Distance ERROR ELLIPSE A RELATIVE ELLIPSE A-B Ellipses 133 Semi-Major Axis Semi-Minor Axis ERROR ELLIPSE B
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STAR*NET V6 Least Squares Survey Ad
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TABLE OF CONTENTS Chapter 1 OVERVIE
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10.9 Entering a Geoid Height and Ve
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Chapter 1 Overview 4. Survey planni
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Chapter 2 Installation 2.3 Starting
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2.7 The Registry Chapter 2 Installa
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3.3 The Menu System Chapter 3 Using
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3.8 Checking the Error File Chapter
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Chapter 3 Using STAR*NET 3.12 Displ
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Chapter 4 Options 4.2 Changing Proj
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Option Group: Units Chapter 4 Optio
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General Options Chapter 4 Options T
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Chapter 4 Options Option Group: Dis
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Chapter 4 Options What standard err
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Chapter 4 Options The default metho
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Option Group: Adjusted Contents Cha
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Other Files Options Chapter 4 Optio
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Chapter 4 Options Creating a Ground
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Special Options Chapter 4 Options T
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4.5 Company Options Chapter 4 Optio
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Chapter 4 Options Note that any dat
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Chapter 4 Options When the Instrume
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Using the Input Data Files Dialog C
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Chapter 5 Preparing Input Data 5.5
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Station Names Chapter 5 Preparing I
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Chapter 5 Preparing Input Data Zeni
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Point Descriptors Chapter 5 Prepari
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Example 1: (2D Data) Chapter 5 Prep
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Chapter 5 Preparing Input Data The
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Chapter 5 Preparing Input Data SING
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The “V” Code: Vertical Observat
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Chapter 5 Preparing Input Data Samp
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Example 2: (2D Data) Chapter 5 Prep
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Chapter 5 Preparing Input Data The
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Chapter 5 Preparing Input Data TRAV
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Chapter 5 Preparing Input Data Exam
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The “TE” Code: Ending a Travers
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Chapter 5 Preparing Input Data LEVE
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Chapter 5 Preparing Input Data SIDE
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Chapter 5 Preparing Input Data Usin
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INLINE OPTION: .DELTA ON/OFF Chapte
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Page 106 and 107: How do I weight observations? Chapt
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Page 110 and 111: Chapter 6 Running Adjustments The S
Page 112 and 113: 6.5 Preanalysis Chapter 6 Running A
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Page 116 and 117: Chapter 7 Viewing and Printing Outp
Page 118 and 119: 7.4 Viewing the Error File Chapter
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Page 146 and 147: Chapter 9 Tools As discussed above,
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Page 150 and 151: Chapter 9 Tools The settings select
Page 152 and 153: Chapter 10 Adjustments in Grid Coor
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Page 170 and 171: Appendix A STAR*NET Tutorial Exampl
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Page 178 and 179: Appendix A STAR*NET Tutorial Take a
Page 186 and 187: Appendix A STAR*NET Tutorial Before
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Example 5: Resection Appendix A STA
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Appendix A STAR*NET Tutorial Exampl
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Example 7: Preanalysis Appendix A S
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Appendix B References 6. Mikhail, E
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Appendix C Additional Technical Inf
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Appendix C Additional Technical Inf
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Appendix D Error and Warning Messag
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Data Lines, General Content Data Ty
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Index Installation Installing, 3 Se
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STARPLUS SOFTWARE, INC.
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STAR*NET V6 - Circe

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