surveying iii (topographic and geodetic surveys) - Modern Prepper

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5-15. Number of Pseudorange Observations Needed. Four pseudorange observations are needed toresolve a GPS 3D position. Only three pseudorange observations are needed for a two-dimensional (2D)(horizontal) location. There are often more than four pseudorange observations due to the need toresolve the clock biases contained in both the satellite and the ground-based receiver. In computing theX-Y-Z coordinates of a point, a fourth unknown (clock bias) must also be included in the solution. Thesolution of a 3D position point is simply the solution of four pseudorange observations containing fourunknowns (X, Y, Z, and time).5-16. Absolute-Positioning Error Sources. There are numerous sources of measurement errors thatinfluence GPS performance. The sum of all systematic errors or biases contributing to the measurementerror is referred to as a range bias. The observed GPS range, without removal of biases, is referred to asa biased range or pseudorange. Principal contributors to errors in the final range and overall GPSreadings include errors in the ephemeris, satellite clock and electronics accuracies, tropospheric andionospheric refraction, atmospheric absorption, receiver noise, and multipath effects. Other errorsinclude those induced by DOD S/A and antispoofing (AS). In addition to these major errors, GPS alsocontains random observation errors (such as unexplainable and unpredictable time variations). Theseerrors are impossible to model and correct. The following paragraphs discuss errors associated withabsolute GPS positioning modes. Many of these errors are either eliminated or significantly minimizedwhen GPS is used in a differential mode because the same errors are common to both receivers duringsimultaneous observing sessions.a. Ephemeris Errors and Orbit Perturbations. Satellite ephemeris errors are errors in the predictionof a satellite's position, which may then be transmitted to the user in the satellite data message.Ephemeris errors are satellite dependent and are very difficult to completely correct or compensate forbecause the many forces acting on the predicted orbit of a satellite are difficult to measure directly.Because direct measurement of all forces acting on a satellite orbit is difficult, it is nearly impossible tocompensate or accurately account for those error sources when modeling the orbit of a satellite. Thepreviously stated accuracy levels are subject to performance of equipment and conditions. Ephemeriserrors produce equal error shifts in the calculated absolute-point positions.b. Clock Stability. GPS relies heavily on accurate time measurements. GPS satellites carryrubidium and cesium time standards that are usually accurate to 1 part in 10 trillion and 1 part in 100trillion, respectively. Most receiver clocks are actuated by a quartz standard accurate to 1 part in 100million. A time offset is the difference between the time recorded by the satellite clock and the timerecorded by the receiver. Range error observed by the user as the result of time offsets between thesatellite and receiver clock is a linear relationship and can be approximated.c. Ionospheric Delays. GPS signals are electromagnetic signals and are nonlinearly dispersed andrefracted when transmitted through a highly charged environment like the ionosphere. Dispersion andrefraction of the GPS signal is referred to as an ionospheric range effect because dispersion andrefraction of theEN0593 5-6
signal result in error to the GPS range value. Ionospheric range effects are frequency dependent.d. Tropospheric Delays. GPS signals in the L-band level are not dispersed by the troposphere, butare refracted. The tropospheric conditions causing refraction of the GPS signal can be modeled bymeasuring the dry and wet components.e. Multipath Effects. Multipath describes an error in positioning which occurs when a receiverreceives a signal from more than one path. This is caused by the reflection of the GPS signal by a nearbyobject, which produces a false signal at the GPS antenna. Multipath normally occurs near largereflective surfaces, such as buildings with reflective surfaces, chain-link fences, and antenna arrays.GPS signals received as a result of multipath give inaccurate GPS positions when processed. Newerreceiver and antenna designs and thorough mission planning can aid in minimizing these errors.Averaging GPS signals over a period of time can also reduce multipath effects.f. Receiver Noise. Receiver noise includes a variety of errors associated with the ability of the GPSreceiver to measure a finite time difference. These errors include signal processing, clock and signalsynchronization and correlation methods, receiver resolution, and signal noise.g. Selective Availability and Antispoofing. S/A purposely degrades a satellite signal to createposition errors by dithering the satellite clock and offsetting the satellite orbits. The effects of S/A canbe eliminated by using differential techniques. AS is implemented by interchanging the P-code with aclassified, encrypted P-code called a Y-code. This code denies access to users who do not possess anauthorized decryption device. Manufacturers of civil GPS equipment have developed methods such assquaring or cross correlation to make use of the P-code when it is encrypted. The absolute value ofrange accuracies obtainable from GPS is largely dependent on which code is used to determine positions.The range accuracy (for example, user-equivalent range error [UERE]), when coupled with thegeometrical relationships of the satellite during the position determination (for example, DOP), result ina 3D ellipsoid that depicts uncertainties in all three coordinates. Given the changing satellite geometryand other factors, GPS accuracy is time and location dependent. Error propagation techniques are usedto define nominal accuracy statistics for a GPS user.h. Root-Mean-Square (RMS) Error Measures. Two-dimensional GPS positional accuracies arenormally estimated using an RMS radial error statistic. A 1-sigma RMS error equates to the radius of acircle in which the position has a 63 percent probability of falling. A circle of twice this radius (2sigmas) represents about a 97 percent probability that the position is within the circle. This 97 percentprobability circle, or 2D RMS, is the most common positional-accuracy statistic used in GPS surveying.In some instances, a 3D RMS or 99 percent or more probability is used. This RMS error statistic is alsorelated to the positional-covariance matrix. An RMS error statistic represents the radius of a circle andtherefore is not preceded by a ± sign.5-7 EN0593
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SUBCOURSEEN0593EDITIONAUNITED STATE
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TERMINAL LEARNING OBJECTIVE:ACTION:
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Part G: Adjusting Precise-Positioni
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PART A - FIGURES OF THE EARTH1-1. T
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Figure 1-2. Flattening Fractiona. T
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1-3. Geoid. In geodesy, precise com
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point in the survey to provide the
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one side of a triangle and all of t
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Figure 1-10. Simple Triangulation N
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observation, contains a light sourc
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of a horizontal rod with a small sp
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The Airy theory, announced by G. B.
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Figure 1-15. Products of the Gravim
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a. If the shape of the earth was ex
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Figure 1-16. Single Astronomic Stat
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Figure 1-17. Astro-Geodetic Deflect
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Figure 1-19. Astro-Geodetic Datum O
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(c) The significance of a gravity-o
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absolute deflections of the vertica
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Figure 1-23. Preferred Datumsa. The
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1-17. The World Geodetic System. Be
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Figure 1-25. Using the Same Referen
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LESSON 1PRACTICE EXERCISEThe follow
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12. Which of the following is assum
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LESSON 1PRACTICE EXERCISEANSWER KEY
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THIS PAGE IS INTENTIONALLY LEFT BLA
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for all military and civilian activ
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i. Additional support for other top
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2-7. Status. At present, topographi
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in both. (North American Datum of 1
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water can impede leveling operation
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A field recording booklet. A single
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PART I - OFFICE WORK2-24. General.
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PART J - SURVEY COMMUNICATION2-28.
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length or side is known, and all th
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second-order, Class I net. The leng
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The number of observations required
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to perform the reconnaissance. Fail
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and cultivated, or fields may have
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e an alphanumeric symbol that is st
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2-53. Intervisibility of Stations.
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station is (K/2) 2 x 0.0676, or one
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Figure 2-8. Similar Triangles Solut
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2-54. Height of Obstructions. It is
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authorized to change the name, it s
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Figure 2-12. Cells Connected in Par
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added as the cells weaken. Three un
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Figure 2-16. Construction Details f
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2-60. Wooden Towers. When it become
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11. The face of the 5-inch signal l
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average line in a triangulation sys
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3-4. Slope Measurements. The slope
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strip as close as possible to its f
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. Sight along the tops of both of t
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Column 1. Record the station number
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stake to steady it. With the back o
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Figure 3-3. DA Form 4446 (Sample of
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micrometer in this optical system i
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laterally across the front of the e
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Figure 3-7. Horizontal Circle and M
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out of adjustment. If the index mar
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(1) In order to avoid instrumental
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location of the vertical circle on
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Figure 3-13. Open Traversec. Closed
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designate the exact point of refere
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example, a line with an azimuth of
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When adjusting a traverse that star
Page 142 and 143: LESSON 3PRACTICE EXERCISEThe follow
Page 144 and 145: 11. The unit of graduation on a hor
Page 146 and 147: LESSON 3PRACTICE EXERCISEANSWER KEY
Page 148 and 149: THIS PAGE IS INTENTIONALLY LEFT BLA
Page 150 and 151: differential leveling. The theodoli
Page 152 and 153: additional runs are made due to exc
Page 154 and 155: Figure 4-2. Wild N-3 Precision Leve
Page 156 and 157: (8) Loosen the azimuth clamp (13),
Page 158 and 159: 4.7. Miscellaneous Equipment. In ad
Page 160 and 161: (2) To check for gross errors in th
Page 162 and 163: (a) (1) Begin by completing the inf
Page 164 and 165: Set up and level the instrument. Re
Page 166 and 167: Figure 4-8. Sun and Wind Code(5) Th
Page 168 and 169: (10) Enter the interval between the
Page 170 and 171: Figure 4-9. Closed-Circuit River Cr
Page 172 and 173: Figure 4-11. Sample DA Form 5820 (R
Page 174 and 175: is 0. When the telescope is pointed
Page 176 and 177: stations. However, since all statio
Page 178 and 179: a. Complete the heading, as shown.b
Page 180 and 181: a. (1) Enter the date, as in the sa
Page 184 and 185: 11. The maximum length of site to o
Page 186 and 187: LESSON 4PRACTICE EXERCISEANSWER KEY
Page 188 and 189: PART A - GLOBAL POSITIONING SYSTEM
Page 190 and 191: 5-7. Pseudorange. A pseudorange is
Page 194 and 195: i. Probable Error Measures. The acc
Page 196 and 197: Table 5-1. Representative GPS Error
Page 198 and 199: Table 5-2. Carrier Phase Trackinga.
Page 200 and 201: exercise extreme caution in applyin
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Page 204 and 205: Table 5-3. GPS-S Design, Geometry,
Page 206 and 207: f. Multiple/Repeat Baseline Connect
Page 208 and 209: possible ionospheric and tropospher
Page 210 and 211: Figures 5-4. PDOP Versus Time PlotP
Page 212 and 213: e. Field Processing and Verificatio
Page 214 and 215: All carrier phase relative-surveyin
Page 216 and 217: time spent and the cost of conducti
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Page 220 and 221: 5-37. General. GPS baseline solutio
Page 222 and 223: (1) Receiver Time. This technique u
Page 224 and 225: NOTE: Repeated baselines should be
Page 226 and 227: 5-43. Loop Closure Checks. Postproc
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Page 230 and 231: This will ensure that future work w
Page 232 and 233: may (and usually does) far exceed t
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Page 236 and 237: i. Relative GPS baseline standard e
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THIS PAGE IS INTENTIONALLY LEFT BLA
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FAR-77 obstructions. Aircraft-movem
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Figure 6-1. Imaginary SurfacesEN059
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Figure 6-3. Approach Surfacesa. Fed
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. Federal Aviation Regulation-77, S
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Table 6-4. Airport Obstruction Accu
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A horizontal scale of 1:12,000 and
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Figure 6-4. Sample DA Form 5821 (Ai
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Figure 6-5. Sample DA Form 5822 (PA
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